(PDF) ABB circuit-breakers inside LV switchboards

1SDC007103G0202

December 2006

4Technical Application Papers

ABB circuit-breakers

inside LV switchboards

1

Index

ABB circuit-breakers inside LV switchboards



Introduction ..............................................2

1 Problems of overheating

inside switchboards

1.1 General aspects ......................................3

1.2 The current carrying capacity ................. 3

1.3 Veriﬁcation of temperature-rise

by test (in compliance with

IEC 60439-1) ...........................................4

1.4 Veriﬁcation of temperature-rise by

extrapolation ...........................................7

2 Advices to improve the

current carrying capacity of

the circuit-breakers inside

switchboards

2.1 Power loss inside switchboards .............9

2.1.1 Internal structure .................................................. 9

2.1.2 Tipology of the circuit-breaker installed ............... 9

2.1.3 Cross-section of conductors within

switchboards ......................................................11

2.1.4 Paths of the current ...........................................15

2.2 Dissipation of the heat generated

inside switchboards ..............................16

2.2.1 Switchboard ventilation ......................................16

2.2.2 Side surfaces and positioning of switchboards .....16

2.2.3 Forms of internal separation of switchboards ....17

2.2.4 Switchboard protection degree .......................... 17

2.3 Dissipation of the heat generated in the

terminals ..............................................17

2.3.1 Problems linked to convection ..........................17

2.3.2 Problems linked to conduction ..........................20

2.3.3 Current carrying capacity of circuit-breakers

and busbars ...................................................... 22

3 Problems concerning short-

circuit

3.1 Main deﬁnitions of the parameters

characterizing a switchboard under

short-circuit conditions .........................39

3.1.1 General prescriptions and information about

short-circuit withstand strength .........................39

3.2 Prescriptions concerning the electrical

circuits of a switchboard .......................40

3.2.1 Main busbar systems .........................................40

3.2.2 Distribution busbars and conductors derived by

the main busbars ................................................41

3.3 Reduction of the possibility of short-circuit

events and of the relevant effects ......... 42

3.3.1 Minimum anchor distances for conductors ........ 42

3.3.2 Veriﬁcation of the short-circuit withstand strength

and of the current limiting characteristics of the

circuit-breakers ...................................................45

3.3.3 Problems concerning the installation distances 46

Annex A:

Example of electrical switchboards with ABB

circuit-breakers ..................................................... 48

Annex B:

Forms of internal separation ................................. 50

Annex C:

Degrees of protection (IP code) ............................. 51

Glossary ................................................................ 52

2ABB circuit-breakers inside LV switchboards

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Introduction

An electrical switchboard is the combination of more

protection and switching devices assembled in one or

more adjacent compartments.

A switchboard is formed by the compartment, which

the Standards name “enclosure” (with support and

mechanical protection functions for the different

components enclosed), and the electrical equipment,

constituted by the apparatus, the internal connections

and the incoming and outgoing terminals for the

connection to the installation.

This Technical Paper is intended to deal in detail with the

equipment in the switchboard, providing the reader with

the basic information necessary to choose the circuit-

breakers to be installed inside low voltage switchboards

in the easiest and most correct way, paying particular

attention to ABB SACE range of products.

After a quick survey of the main product Standards

concerning switchboards and circuit-breakers, IEC

60439-1 and IEC 60947-2 respectively, the main

problems which a manufacturer has to face when

designing a switchboard are analyzed.

This Technical Paper is divided into three main parts

dealing with the problems of overheating in switchboards,

general prescriptions to improve the current carrying

capacity of the circuit-breakers inside enclosures and the

problems caused by short-circuit in switchboards

1 Introduction

1 The product Std. IEC 60439-1 applies to low voltage and controlgear assemblies, the

rated voltage of which does not exceed 1000 Va.c. at frequencies not exceeding 1000 Hz,

or 1500 Vd.c.; the product Std. IEC 60947-2 applies to circuit-breakers, the main contacts

of which are intended to be connected to circuits, the rated voltage of which does not

exceed 1000 Va.c. or 1500 Vd.c.

3

ABB circuit-breakers inside LV switchboards

1 Problems of overheating inside switchboards

1.1 General aspects

One of the main problems which makes difﬁcult the

identiﬁcation of the correct typology of circuit-breakers to

be installed inside a switchgear or controlgear assembly

is calculating the maximum continuous current which the

circuit-breaker can carry without damages or premature

ageing according to the service temperature.

The total freedom of the manufacturer in designing

switchboards using components different for number,

position and dimensions makes the installation conditions

of the same circuit-breaker so different that it results

impossible to determine exactly its “maximum current

carrying capacity” which, affected by peculiar operating

conditions, results different from that deﬁned by the

manufacturer and referred to standard conditions.

1.2 The current carrying capacity

Now we shall take into consideration how the concept

of current carrying capacity is dealt with in the different

Standards, in particular, in the product Standard

concerning circuit-breakers and in that one regarding

low-voltage switchgear and controlgear assemblies.

The circuit-breakers, according to the prescriptions of the

European Low Voltage Directive 2006/95/CE (ex 73/23

CE std.), are manufactured and tested in compliance with

the product Std. IEC 60947-2 “Low-voltage switchgear

and controlgear – Part 2: Circuit-breakers”.

As regards the veriﬁcation of the current carrying capacity

in uninterrupted duty (Iu), the Std. IEC 60947-2 states the

conditions of the test performance. Here are the main

requirements to be met :

- the current carrying capacity shall be veriﬁed in free-air

the Std. IEC 60947-1 “Low-voltage switchgear and

controlgear - Part 1: General rules” speciﬁes in detail

what is meant by “free air”:

“Free air is understood to be air under normal indoor

conditions (indoor conditions are understood to be

not the conditions inside switchgear or controlgear

assemblies or enclosures, but the conditions inside

buildings or similar environments), reasonably free from

draughts and external radiation”

therefore, no external radiations (e.g. those due to the

sun’s rays-Figure 1) or draughts which are not caused

simply by the natural convective motion originated by

heating (Figure 1a) are admitted.

Figure 1 Figure 1a

- the carrying capacity is veriﬁed by connecting

the circuit-breakers with conductors having size

(maximum) and length (minimum) as speciﬁed in

the relevant Standard

this means that the standard conditions are referred also

to the connection modalities of the circuit-breaker

- the carrying capacity is veriﬁed by ensuring that

during the test, the maximum temperature-rise

limits admitted on the different parts of the circuit-

breaker are not exceeded

such temperature-rise, not meant as absolute temperature,

but as a temperature difference expressed in Kelvin, are

referred to an ambient air temperature of 40°C.

The circuit-breakers are generally installed inside

enclosures which have different functions; among these

the following :

- making inaccessible to people the connections of the

different apparatus (if not for voluntary actions);

- giving a place to house the circuit-breakers where

steady positioning is guaranteed;

- ensuring an adequate protection against ingress of

solid foreign objects and ingress of water.

These enclosures are called controlgear and switchgear

assemblies (hereafter referred to as assemblies) and

comply with the speciﬁc product Std. IEC 60439-1 “Low-

voltage switchgear and controlgear assemblies – Part 1:

Type-tested (TTA) and partially type-tested assemblies

(PTTA)”

However, the installation conditions inside an assembly

differ from the conditions speciﬁed by the Std. IEC

60947-2, which are the veriﬁcation conditions of a circuit-

breaker current carrying capacity in free air.

The conditions inside the switchboard (wiring, separations,

arrangement of the different apparatus) force the circuit-

breaker to operate under conditions characterized by the

following aspects:

4ABB circuit-breakers inside LV switchboards

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1 Problems of overheating inside switchboards

- not in free air, but with particular prescriptions re-

garding air circulation

in particular, it is possible to have assemblies with

complex forms of internal separations (Figure 2), or

assemblies with forced ventilation or air-conditioned

assemblies

Figure 2

- the circuit-breakers are connected through

conductors of size and length stated by the

manufacturer

- with an air ambient temperature around the circuit-

breaker depending on the assembly design and on

the devices it houses.

In particular, the different degree of protection and

the modality of arrangement of the assembly in the

environment cause a modiﬁcation of the amount of heat

exchanged towards the outside of the assembly and,

consequently, also of the air temperature inside it.

After these considerations, it is evident that the conditions

leading the manufacturer to deﬁne a rated uninterrupted

current for a single circuit-breaker are different from the

conditions under which the circuit-breaker shall be used

inside an assembly; as a consequence, it is obvious

that the current carrying capacity of circuit-breakers

determined in compliance with the relevant product

Standard cannot be considered equal to their carrying

capacity when they are installed inside an assembly,

without the appropriate evaluations.

This concept is also recalled in the Std. IEC 60947-1, which

in the performance prescriptions regarding temperature-

rise, states that in normal service the current carrying

capacities may differ from the test values, depending,

for example, on the different installation conditions and

on the size of the connected conductors.

Besides, also the Std. IEC 60947-2, relevant to the low

voltage circuit-breakers, as regards the general test

conditions, reminds that the prescribed tests do not

preclude the need for additional tests on circuit-breakers

incorporated in assemblies, for example in accordance

with IEC 60439-1.

1.3 Veriﬁcation of temperature-rise by test

(in compliance with IEC 60439-1)

The product Std. IEC 60439-1 concerning low voltage

controlgear and assemblies does not refer to the indivi-

dual components present, but to the “equipment” meant

as a combination of one or more protection and switching

apparatus equipped with any possible switching, mea-

suring, protection and setting, mounted and wired with

internal electrical and mechanical connections.

As a consequence, making reference to the current

carrying capacity, this Standard deals with the rated

current of the single electrical circuit and not with the

rated current of the individual components, such as

circuit-breakers or conductors. In accordance with the

deﬁnition, the rated current of a circuit is deﬁned by the

switchboard manufacturer as a function of the ratings of

the electrical components of the circuit, of their disposi-

tion and application.

This current shall be carried without the temperature-rise

of the various parts of the assembly exceeding the limits

speciﬁed when the test is performed in accordance with

prescriptions of the Standard itself.

The performance modalities of the temperature-rise test

include two main prescriptions:

- the circuits of the switchboard shall be tested at a cur-

rent which is equal to the rated current multiplied by the

rated diversity factor fn, understood as the ratio between

the maximum value of the sum of the currents ﬂowing

through all the main circuits considered, at any moment,

and the sum of the rated currents of the same circuits

Itest = InC x fn

- if no d etailed information about the external conductors

to be used under normal operating conditions are known,

cross-sections depending on the rated current of the

circuits are imposed by the Standard.

For further information about correlated subjects, re-

ference shall be made to the indications given in the

Standard itself.

From the prescriptions above it results that:

- if a rated diversity factor fn<1 is present (not all the

loads are supplied with 100% of their rated current) the

switchboard circuits are tested at a current value lower

than the rated one at full load; however, the test shall be

carried out on those circuits which allow the heaviest

temperature-rise conditions to be reproduced;

- if the switchboard is cabled with conductors having a

cross-section reduced with respect to that prescribed

5

ABB circuit-breakers inside LV switchboards

1 Problems of overheating inside switchboards

by the Standard and used in the test, temperature-rise

higher than the maximum acceptable values measured

in the test might occur during normal operations.

The following numerical example has the purpose of

making clear what explained above. Reference is made

to the switchboard of Figure 3, whose loads are cabled

with the same conductors through which it is put into

service; the manufacturer assigns the rated current for

the load circuits and assigns a rated diversity factor “fn”

to the enclosure to be tested. Under these conditions, the

switchboard, or a part of the switchboard, is tested by

“applying” simultaneously to all the circuits a test current

equal to the assigned rated current multiplied by “fn”.

I2 = 160A fn=0.8 I2test= 128A

I3 = 400A fn=0.8 I3test = 320A

I4 = 250A fn=0.8 I4test = 200A

I5 = 630A fn=0.8 I5test = 504A

I6 = 160A fn=0.8 I6test = 128A

I7 = 400A fn=0.8 I7test = 320A

Figure 3

Therefore, in a switchboard, the rated current of a circuit

is not that assigned, but it is determined by considering

the assigned diversity factor. In accordance with these

test conditions, the absolute temperature values TT

(expressed in °C) at which the different parts of the

assembly operate are determined and, with reference to

an average ambient temperature TA lower than or equal to

35 °C, the temperature-rise limits ΔT = (TT – TA) imposed

by the Std. IEC 60439-1 must not be exceeded.

For the different components of assemblies, Table 1

below shows the temperature-rise limits and the relevant

remarks of the Std. IEC 60439-1 (updating of Annex A1

dated March 2005) which are valid when the temperature-

rise test is carried out in compliance with the prescriptions

of the Standard itself.

Table 1

Parts of assemblies

Temperature-rise

(values or prescriptions)

Built in components

For example conventional switchgear

and controlgear; electronic sub-

assemblies (e.g. rectiﬁer bridge and

printed circuit); parts of equipment

(e.g. regulator, stabilized power

supply unit, operational ampliﬁer).

In accordance with the relevant

requirements for the individual com-

ponents, or in accordance with the

manufacturer’s instructions, taking

into consideration the temperature

in the assembly.

Terminals for

external insulated conductors

70K

An assembly used or tested under

installation conditions may have

connections, the type, nature and

disposition of which will not be the

same as those adopted for the test

and a different temperature rise of

terminals may result and may be

required or accepted.

When the terminals of the built-in

components are also the terminals

for the external insulated conductors

the lowest temperature-rise limits

shall be applied.

Busbars and conductors,

plug-in contacts of removable

or withdrawable parts which

connect to busbars

Limited by:

- mechanical strength of conducting

material;

- possible effects on adjacent

equipment;

- permissible temperature limit of

the insulating materials in contact

with the conductor;

- the effect of the temperature of

the conductor on the apparatus

connected to it;

- for plug-in contacts, nature and

surface treatment of the contact

material.

By assuming that all the other

mentioned criteria have been

fulﬁlled, a maximum temperature of

105K shall not be exceeded for bare

busbars and copper conductors

so that the mechanical strength of

conducting material is guaranteed.

Manual operating means:

accessible with closed assembly

of metal 15K

of insulating material 25K

accessible with open assembly

of metal 40K

of insulating material 50K

Accessible external enclosures and covers:

which need to be touched during normal operation

of metal 30K

of insulating material 40K

which need to be touched during normal operation

of metal 40K

of insulating material 50K

Discrete arrangements of plug

and socket type connection

Determined by the limits of those

components of the equipment of

which they form part.

Circuit-breakers can be deﬁned as built-in components

and therefore they must comply with the prescriptions

of the product Standards. However, it is evident that

I1

IG I2

I3

I4

I5

I6

I7

C

B

D

E

A

6ABB circuit-breakers inside LV switchboards

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1 Problems of overheating inside switchboards

Table 2

circuit-breakers and in particular some parts of them

(e.g. accessible parts and operating means) can be

also considered in all respects as part of controlgear

or switchgear assemblies. In particular this applies to

the terminals where external insulated conductors are

connected, which, in accordance with the comments

reported in Table 1, must comply with the most

demanding or restricting prescription of the two product

Standards.

To make clear this concept, Table 2 and ﬁgure 4 hereunder

show the indications concerning temperature-rise limits

given in the Std. IEC 60947-2 for circuit-breakers

considered as an individual component in free-air.

Parts of assemblies-

description

Temperature-rise

limits

Temperature limits

(starting from

TA =40°C)

Terminals 80K 120 °C

Manual operating means:

parts of metal 25K 65 °C

parts of insulating material 35K 75 °C

Parts intended to be touched but not gripped:

parts of metal 40K 80 °C

parts of insulating material 50K 90°C

Parts which need not to be touched during normal operation:

parts of metal 50K 90°C

parts of insulating material 60K 100°C

Figure 4

From Table 2 it results how for a circuit-breaker in free-

air the accepted temperature-rise on the terminals is

∆T=80K; therefore, taking as reference an ambient

temperature TA = 40°C, it can be deduced that the

maximum permissible temperature is TT = (∆T + TA) =

120°C.

The prescriptions regarding temperature-rise deﬁned

by the switchboard Standard instead refer to an

average ambient temperature TA = 35°C; the maximum

temperature-rise limit of the switchboard terminals for

insulated external connections is 70K and consequently

the maximum operating temperature is 105°C.

If the circuit-breaker is installed inside a switchboard,

a reference ambient temperature of 35°C shall be

considered, and Table 1, with the comments reported for

the built-in components (circuit-breakers are components

of the switchboard), allows the manufacturer to state for

the circuit-breaker terminals a maximum temperature of

120°C; thus, by difference, it can be obtained that the

maximum temperature-rise limit is equal to 85K.

When the connection to the terminals is realized through

PVC insulated conductors, it is the temperature of the

cable component to determine the maximum acceptable

temperature on the terminals, in this case 70°C . On

the contrary, if the connection to the circuit-breaker is

constituted by bare copper busbars, whose maximum

operating temperature is 105°C, it is the prescription

for the terminals of the circuit-breaker component

which determines the maximum operating temperature,

consequently equal to 85°C.

As a summary of the above, Table 3 and Figure 5 show the

maximum acceptable temperature-rise and temperature

limits for the different parts of the assemblies as stated

by the switchboard Standard, and the temperature-

rise limits for a circuit-breaker installed inside a LV

switchboard recalculated for a reference ambient

temperature TA = 35°C.

Parts of assemblies-

description

Temperature-rise

limits

Temperature limits

(starting from

TA =35°C)

Terminal for external

insulated connections (IEC

60439-1)

70K 105 °C

Terminals for external con-

nections (IEC 60947-2) 85K 120 °C

Manual operating means:

Accessible with enclosed assembly

of metal 15K 50 °C

of insulating material 25K 60 °C

Accessible only with open assembly

of metal 30K 65 °C

of insulating material 40K 75 °C

Parts intended to be touched but not gripped: (CEI EN 60439-1)

of metal 30K 65 °C

of insulating material 40K 75°C

Accessible parts which need not to be touched during normal

operation (IEC 60439-1):

of metal 40K 75°C

of insulating material 50K 85°C

Non-accessible parts which need not to be touched during normal

operation (IEC 60947-2):

of metal 55K 90°C

of insulating material 65K 100°C

Table 3

Figure 5 Connection with

PVC-insulated cable

Connection

with busbar

7

ABB circuit-breakers inside LV switchboards

1 Problems of overheating inside switchboards

1.4 Veriﬁcation of temperature-rise by

extrapolation

The Standard regarding low voltage controlgear and

switchgear assemblies provides that temperature-

rise veriﬁcation for the PTTA type can be carried out

also by extrapolation making specific reference to

the prescriptions given in IEC/TR 60890 “A method

of temperature-rise assessment by extrapolation for

partially type-tested assemblies (PTTA) of low-voltage

switchgear and controlgear”.

The method proposed allows temperature-rise to be

determined inside PTTA enclosures without forced

ventilation.

The calculation validity is limited by a series of initial

assumptions:

- there is an approximately even distribution of power

losses inside the enclosure;

- the installed equipment is so arranged that air

circulation is but little impeded;

- the equipment installed is designed for direct current

or alternating current up to and including 60 Hz, with

the total of supply currents not exceeding 3150 A;

- conductors carrying high currents and structural

parts are so arranged that eddy current losses are

negligible;

- for enclosures with ventilation openings, the cross-

section of the air outlet openings is at least 1.1 times

the cross-section of the air inlet openings;

- there are no more than three horizontal partitions in

the assembly or in a section of it;

- where the enclosures with external ventilating

openings have compartments, the surface of the

ventilating openings in each horizontal partition shall

be at least 50% of the horizontal cross-section of the

compartment.

The following data are needed to calculate the

temperature-rise of the air inside an enclosure:

- geometric dimensions (height/width/depth);

- effective power loss of equipment, busbars, cables

and connections;

- type of installation of the enclosure (exposed, covered,

etc....);

- presence and dimensions of ventilation openings;

- number of internal horizontal partitions.

As regards the analysis of the suggested calculation

methods, the reader is required to consult the Standard

itself.

To carry out an analysis of temperature-rise in accordance

with this calculation method, ABB SACE offers its free

software program OTC. Starting from the required input

data, this program calculates the temperature of the air

at different heights of the enclosure through a dedicated

interface which appears as in the ﬁgure below.

Once the temperature of the air at the different heights

of the enclosure is known, it is possible to verify if the

components which are in a certain position are suitable to

operate at that temperature or if they need to be replaced

by other components.

To this purpose, with reference to circuit-breakers, ABB

SACE gives a derating of the current carrying capacity

as a function of the temperature of the air around the

circuit-breaker: thus it becomes possible to calculate if

the carrying capacity admitted for the circuit-breaker at

the temperature calculated at its installation point results

to be higher than the current of the supplied load.

As regards the above, the mere knowledge of the

temperature of the air around the circuit-breakers

would not allow the calculation of the current carrying

capacity.

However, it shall be taken into account that the calculation

method suggested by IEC/TR 60890 is a conservative

one which generally results into values higher than those

which can be veriﬁed in reality. As a consequence it is

possible to state that, if the minimum dimensions of the

connections suggested by ABB (see Tables 16 and 17

at page 21) are complied with, the power losses of all

the components are calculated correctly and the results

thus obtained are integrated with the manufacturer’s

experience, then the suggested calculation method can

be used without running into errors.

OTC interface

8ABB circuit-breakers inside LV switchboards

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2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

2 Advices to improve the current carrying capacity

of the circuit-breakers inside switchboards

In order to give the necessary indications on the methods

intended to improve the current carrying capacity of

the circuit-breakers inside switchboards, ﬁrst of all it is

necessary to analyze an assembly from a thermodynamic

point of view.

A switchboard can be considered as an enclosure

housing a series of elements generating heat and able

to dissipate heat towards the outside.

The elements generating heat inside the enclosure

exchange heat between them (conduction), with the air

inside the switchboard (convection) and with the walls of

the switchboard itself (radiation) as shown in Figure 6.

Figure 6

In its turn, the enclosure exchanges heat towards the

external environment. Also this heat exchange occurs

by conduction (through the cables connected to the

assembly), convection and radiation, as shown in

Figure 7. In enclosures with a degree of protection not

very high or with ventilation openings, part of the heat

is exchanged through a real air circulation between the

assembly and the external environment.

Figure 7

All these phenomena of circulation and exchange of

internal and external air, together with the structure of

the enclosure, affect temperature at each point of the

enclosure itself and of each component installed inside it.

This chapter analyses the main elements which contribute

to generate and influence the temperature inside a

switchboard and tries to give some useful information

for their optimization with the purpose of decreasing the

temperature and consequently of reducing the derating

of the current carrying capacity of circuit-breakers.

These elements are:

- the power loss inside the enclosure;

- the dissipation of the heat produced inside the enclosure;

- the dissipation of the heat produced by the terminals.

Conduction

Convection

Radiation

Heat

Circuit-breaker

Connection busbar

Circuit-breaker

Switchboard wall

Air outside

the

enclosure

Air inside

the

enclosure

9

ABB circuit-breakers inside LV switchboards

Support in amagnetic

material

Bars

Insulator

One pole bars

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

2.1 Power loss inside switchboards

As known, a modiﬁcation of the temperature may be

caused by a power loss due to the current ﬂow. Now, the

different components which constitute the main power

sources and which consequently represent also heat

sources inside a switchboard shall be considered in detail,

together with the measures to be taken in order to reduce

the power loss and limit its effects. These elements are:

the internal structure, the typology of the circuit-breaker

installed, the cross-sectional area of the internal con-

ductors of the switchboard, and the current paths.

2.1.1 Internal structure

The material used to realize structure and partitions inside

switchboards is often ferromagnetic and conductive. If

the system structure is such as to create a closed conﬁ-

guration embracing the conductors, Joule-effect losses

due to eddy currents and hysteresis losses are induced,

with consequent local heating of remarkable importance.

The same phenomenon occurs in the bus ducts between

the enclosure and the conductor bars.

As an example to illustrate the inﬂuence of this pheno-

menon, Table 4 shows the percentage value representing

the part of losses developing inside the enclosure related

to the power loss inside the conductor bars.

Table 4

No. of

phases

No.

of

busbars

in parallel

per

phase

Cross-

section

[mm] In

[A]

Material of

the encasement

(of the bus duct)

Losses inside the

enclosure

(% referred to the

t o t a l l o s s i n s i d e

the conductor bars)

3 1 100x10 1000 ferromagnetic 35% - 45%

3 3 100x10 3000 ferromagnetic 55% - 65%

3 3 100x10 3000 amagnetic

(aluminum) 15% - 20%

From these data, it results that the increase of the rated

current and consequently the number of busbars in

parallel per phase and the material used for the separation

of the conductor bars may considerably affect heating.

For a correct assessment of the power losses it is

necessary to take into consideration also the conﬁguration

of the separation form: in fact, if a ferromagnetic ring

embraces all the three conductors of a three-phase

system, as Figure 8 shows (or all the four conductors

in a system with the neutral conductor), the sum of the

Also the mechanical ﬁxing of conductors could cause this

inconvenient; therefore it is important that the formation

of close rings is prevented by the insertion of insulators

or anchor clamps made of amagnetic and/or insulating

material (see Figure 9).

currents shall result into null induction; on the contrary,

if each conductor is enclosed by a single ring (Figure

8a), the total induction is not null, with the consequent

circulation of induced current, power loss and therefore

heat generation.

Figure 8 Figure 8a

Figure 9

2.1.2 Typology of the circuit-breaker installed

Circuit-breakers are components of switchboards which

cannot be disregarded when calculating total power

loss.

To make this evaluation easier, ABB SACE offers some

tables which are reported below and refer to molded-

case circuit-breakers of Tmax series (Table 5) and air

circuit-breakers type Emax (Table 6) respectively.

As the tables below show, the power loss of the same

circuit-breaker varies depending both on its version as

well as on the type of protective release installed.

Ferromagnetic meterial of

the separation form

Terminals

Current inductionNo induction

10 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Taking reference to these two variables, it is possible to

observe that :

- the power losses of withdrawable circuit-breakers are

higher than those of the ﬁxed ones

- the power losses of the circuit-breakers equipped with

thermo-magnetic releases are higher than those of the

circuit-breakers with electronic releases.

Under heavy conditions from a thermal point of view, it

is advisable to use circuit-breakers in ﬁxed version and

equipped with electronic type releases.

The difference between the power loss of a circuit-breaker

in three-pole version compared with a four-pole version is

not considered, since in a normal circuit the current ﬂowing

in the neutral conductor is assumed to be null.

Total (3/4 poles)

power loss In T11P T1 T2 T3 T4 T5 T6 T7 S,H,L T7 V

[W] [A] F F F P F P F P/W F P/W F W F W F W

Releases 1 4.5 5.1

1.6 6.3 7.5

2 7.5 8.7

2.5 7.8 9

3.2 8.7 10.2

4 7.8 9

5 8.7 10.5

6.3 10.5 12.3

8 8.1 9.6

10 9.3 10.8

12.5 3.3 3.9

16 1.5 4.5 4.2 4.8

TMF 20 1.8 5.4 5.1 6 10.8 10.8

TMD 25 2 6 6.9 8.4

TMA 32 2.1 6.3 8.1 9.6 11.1 11.1

MF 40 2.6 7.8 11.7 13.8

MA 50 3.7 11.1 12.9 15 11.7 12.3

63 4.3 12.9 15.3 18 12.9 15.3

80 4.8 14.4 18.3 21.6 14.4 17.4 13.8 15

100 7 21 25.5 30 16.8 20.4 15.6 17.4

125 10.7 32.1 36 44.1 19.8 23.7 18.6 21.6

160 15 45 51 60 23.7 28.5 22.2 27

200 39.6 47.4 29.7 37.2

250 53.4 64.2 41.1 52.8

320 40.8 62.7

400 58.5 93

500 86.4 110.1

630 92 117

800 93 119

10 1.5 1.8

25 3 3.6

63 10.5 12

100 24 27.2 5.1 6.9

160 51 60 13.2 18

250 32.1 43.8

320 52.8 72 31.8 53.7

PR221 400 49.5 84 15 27 24 36

PR222 630 123 160.8 90 115 36 66 60 90

PR223 800 96 125 57,9 105,9 96 144

1000 150 90 165 150 225

1250 141 258 234,9 351,9

1600 231 423

Table 5

F: ﬁxed W: withdrawable P: plug-in

Total (3/4 poles)

power loss X1B-N X1L E1B-N E2B-N-S E2L E3N-S-H-V E3L E4S-H-V E6H-V

[W] FWFWFWFWFWFWFWFWFW

In=630 41 63 50 87

In=800 65 100 80 140 65 95 29 53 22 36

In=1000 102 157 125 219 96 147 45 83 38 58

In=1250 159 257 196 342 150 230 70 130 105 165 60 90

In=1600 260 400 253 378 115 215 170 265 85 150

In=2000 180 330 130 225 215 330

In=2500 205 350 335 515

In=3200 330 570 235 425 170 290

In=4000 360 660 265 445

In=5000 415 700

In=6300 650 1100

Table 6

F: ﬁxed W: withdrawable

11

ABB circuit-breakers inside LV switchboards

1) dd dd

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

2.1.3 Cross-section of the conductors within

switchboards

In primary distribution switchboards, the power loss of the

connection systems (busbars/cables) is usually from 20%

to 40% of the total power loss of the switchboard.

The Std. IEC/TR 60890 includes a series of tables

which give the power loss of cables and busbars inside

switchboards per unit length, making reference to the

current carrying capacity.

By applying these tables (here deﬁned as Tables 7 – 8 – 9)

it is possible to point out how a reduction in the power

loss corresponds to an increased cross-section.

In addition, it is important to remark how the cables

entering the enclosure give a contribution not negligible

to power loss, whereas they are often not considered

since they are not “strictly” part of the switchboard.

Here is an example to show how the contribution of

the connection cables is fundamental for a correct

assessment of the total power loss of the components

inside the switchboard.

Table 7: Operating current and power losses of insulated conductors

Section

(Cu)

Maximum permissible conductor temperature 70 °C

Air temperature inside the enclosure around the conductors

35 °C 55 °C 55 °C 35 °C 55 °C

operating current

power losses 2)

operating current

power losses 2)

operating current

power losses 2)

operating current

power losses 2)

operating current

power losses 2)

operating current

power losses 2)

mm2A W/m A W/m A W/m A W/m A W/m A W/m

1.5

2.5

4

12

17

22

2.1

2.5

2.6

8

11

14

0.9

1.1

12

20

25

2.1

3.5

3.4

8

12

18

0.9

1.3

1.8

12

20

25

2.1

3.5

3.4

8

12

20

0.9

1.3

2.2

6

10

16

28

38

52

2.8

3.0

3.7

18

25

34

1.2

1.3

1.6

32

48

64

3.7

4.8

5.6

23

31

42

1.9

2.0

2.4

32

50

65

3.7

5.2

5.8

25

32

50

2.3

2.1

3.4

25

35

50

85

104

130

6.3

7.5

7.9

55

67

85

2.6

3.1

3.4

85

115

150

6.3

7.9

10.5

65

85

115

3.7

5.0

6.2

70

95

120

161

192

226

8.4

8.7

9.6

105

125

147

3.6

3.7

4.1

175

225

250

9.9

11.9

11.7

149

175

210

7.2

8.3

150

185

240

275

295

347

11.7

10.9

12.0

167

191

225

4.3

4.6

5.0

275

350

400

11.7

15.4

15.9

239

273

322

8.8

9.4

10.3

300 400 13.2 260 5.6 460 17.5 371 11.4

Conductors for auxiliary circuits

mm2A W/m A W/m Diam.

0.12

0.14

2.6

2.9

1.2

1.3

1.7

1.9

0.5

0.6

0.4

-

0.20

0.22

3.2

3.6

1.1

1.3

2.1

2.3

0.5

-

0.5

0.30

0.34

4.4

4.7

1.4

2.9

3.1

0.6

0.50

0.56

6.4 1.8

1.6 4.2 0.8

0.7

0.8

-

0.75

1.00

8.2

9.3

1.9

1.8

5.4

6.1

0.8

1.0

-

1) Any arrangement desired with the values speciﬁed referring to six cores in a multi-core bundle with a simultaneous load 100%

2) single length

35 °C

12 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Width

x

Thickness

Cross-

section

(Cu)

Maximum permissible conductor temperature 85 °C

Air temperature inside the enclosure

around the conductors 35 °C

Air temperature inside the enclosure

around the conductors 55 °C

50 Hz 60 Hz ac dc and ac to 16 2/3 Hz 50 Hz 60 Hz ac dc and ac to 16 2/3 Hz

operating current

power losses 1)

operating current

power losses 1)

operating current

power losses 1)

operating current

power losses 1)

operating current

power losses 1)

operating current

power losses 1)

operating current

power losses 1)

operating current

power losses 1)

mm x mm mm2A* W/m A** W/m A* W/m A** W/m A* W/m A** W/m A* W/m A** W/m

12 x 2 23.5 144 19.5 242 27.5 144 19.5 242 27.5 105 10.4 177 14.7 105 10.4 177 14.7

15 x 2

15 x 3

29.5

44.5

170

215

21.7

23.1

282

375

29.9

35.2

170

215

21.7

23.1

282

375

29.9

35.2

124

157

11.6

12.3

206

274

16.0

18.8

124

157

11.6

12.3

206

274

16.0

18.8

20 x 2

20 x 3

20 x 5

20 x 10

39.5

59.5

99.1

199

215

271

364

568

26.1

27.6

29.9

36.9

351

463

665

1097

34.8

40.2

49.8

69.2

215

271

364

569

26.1

27.6

29.9

36.7

354

463

668

1107

35.4

40.2

50.3

69.6

157

198

266

414

13.9

14.7

16.0

19.6

256

338

485

800

18.5

21.4

26.5

36.8

157

198

266

415

12.3

14.7

16.0

19.5

258

338

487

807

18.8

21.4

26.7

37.0

25 x 5 124 435 34.1 779 55.4 435 34.1 78 55.6 317 18.1 568 29.5 317 18.1 572 29.5

30 x 5

30 x 10

149

299

504

762

38.4

44.4

894

1410

60.6

77.9

505

770

38.2

44.8

899

1436

60.7

77.8

368

556

20.5

27.7

652

1028

32.3

41.4

369

562

20.4

23.9

656

1048

32.3

41.5

40 x 5

40 x 10

199

399

641

951

47.0

52.7

1112

1716

72.5

88.9

644

968

47.0

52.6

1128

1796

72.3

90.5

468

694

25.0

28.1

811

1251

38.5

47.3

469

706

24.9

28.0

586

1310

38.5

48.1

50 x 5

50 x 10

249

499

775

1133

55.7

60.9

1322

2008

82.9

102.9

782

1164

55.4

61.4

1357

2141

83.4

103.8

566

826

29.7

32.3

964

1465

44.1

54.8

570

849

29.4

32.7

989

1562

44.3

55.3

60 x 5

60 x 10

299

599

915

1310

64.1

68.5

1530

2288

94.2

116.2

926

1357

64.7

69.5

1583

2487

94.6

117.8

667

955

34.1

36.4

1116

1668

50.1

62.0

675

989

34.4

36.9

1154

1814

50.3

62.7

80 x 5

80 x 10

399

799

1170

1649

80.7

85.0

1929

2806

116.4

138.7

1200

1742

80.8

85.1

2035

3165

116.1

140.4

858

1203

42.9

45.3

1407

2047

61.9

73.8

875

1271

42.9

45.3

1484

1756

61.8

74.8

100 x 5

100 x 10

499

999

1436

1982

100.1

101.7

2301

3298

137.0

164.2

1476

2128

98.7

102.6

2407

3844

121.2

169.9

1048

1445

53.3

54.0

1678

2406

72.9

84.4

1077

1552

52.5

54.6

1756

2803

69.8

90.4

120 x 10 1200 2314 115.5 3804 187.3 2514 115.9 4509 189.9 1688 61.5 2774 99.6 1833 61.6 3288 101.0

*) one conductor per phase **) two conductors per phase 1) single length

Table 8: Operating current and power losses of bare conductors, in vertical arrangement, without direct connections to the apparatus

Table 9: Operating current and power losses of bare conductors used as connections between the apparatus and the main busbars

Width

x

Thickness

Cross-

section

(Cu)

Maximum permissible conductor temperature 65 °C

Air temperature inside the enclosure

around the conductors 35 °C

Air temperature inside the enclosure

around the conductors 55 °C

50 Hz to 60 Hz ac and dc 50 Hz to 60 Hz ac and dc

operating current

power losses 1)

operating current

power losses 1)

operating current

power losses 1)

operating current

power losses 1)

mm x mm mm2A* W/m A** W/m A* W/m A** W/m

12 x 2 23.5 82 5.9 130 7.4 69 4.2 105 4.9

15 x 2

15 x 3

29.5

44.5

96

124

6.4

7.1

150

202

7.8

9.5

88

102

5.4

4.8

124

162

5.4

6.1

20 x 2

20 x 3

20 x 5

20 x 10

39.5

59.5

99.1

199

115

152

218

348

6.9

8.0

9.9

12.8

184

249

348

648

8.9

10.8

12.7

22.3

93

125

174

284

4.5

5.4

6.3

8.6

172

198

284

532

7.7

6.8

8.4

15.0

25 x 5 124 253 10.7 413 14.2 204 7.0 338 9.5

30 x 5

30 x 10

149

299

288

482

11.6

17.2

492

960

16.9

32.7

233

402

7.6

11.5

402

780

11.3

21.6

40 x 5

40 x 10

199

399

348

648

12.8

22.7

648

1245

22.3

41.9

284

532

8.6

15.3

532

1032

15.0

28.8

50 x 5

50 x 10

249

499

413

805

14.7

28.5

805

1560

27.9

53.5

338

660

9.8

19.2

655

1280

18,5

36.0

60 x 5

60 x 10

299

599

492

960

17.2

34.1

960

1848

32.7

63.2

402

780

11.5

22.5

780

1524

21.6

43.0

80 x 5

80 x 10

399

799

648

1256

22.7

45.8

1256

2432

42.6

85.8

532

1032

15.3

30.9

1032

1920

28.8

53.5

100 x 5

100 x 10

499

999

805

1560

29.2

58.4

1560

2680

54.8

86.2

660

1280

19.6

39.3

1280

2180

36.9

57.0

120 x 10 1200 1848 68.3 2928 85.7 1524 46.5 2400 57.6

*) one conductor per phase **) two conductors per phase 1) single length

13

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Example

This example has the purpose of evaluating – as ﬁ rst approximation – the total power loss inside the switchboard of

which Figure 10 shows the arrangement of the components, the dimensions, the structure and the relevant single-

wire diagram.

Figure 10

The components which form the switchboard are circuit-breakers, busbars and cables.

The power loss is calculated for each component and then the total power loss is determined.

Switchboard front

Single-wire diagram

H

L

P

IG

I1 I2 I3 I4 I5

H

[mm] W

[mm] D

[mm]

2000 1440 840

Dimensions

Number of horizontal partitions = 0

Wall-mounted separated enclosure

IG

B

D

F

A

E

C

I1

I2

I3

I4

I5

Table 10

Circuit-breaker

In CB

[A]

Ib

[A]

Power loss

[W]

IG E2 1600 EL 1600 1340 80.7

I1 T5 400 EL 400 330 33.7

I2 T5 400 EL 400 330 33.7

I3 T5 400 EL 400 330 33.7

I4 T3 250 TMD 250 175 26.2

I5 T3 250 TMD 250 175 26.2

Total power loss of the circuit-breakers [W] 234

Circuit-breakers

As regards circuit-breakers, the power loss can be determined on the basis of the dissipated power “Pn ” at the

rated current “InCB” (see previous Tables 5 and 6) referred to the current which really ﬂ ows through the circuit-

breaker “Ib” (full load current of the circuit).

The formula linking these three quantities is the following :

PCB = PnCB x (Ib / InCB)2

Then, according to the type of apparatus installed inside the switchboard, the contribution to the load current in

terms of power loss of the individual circuit-breaker and the total power loss are reported in the following table:

14 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Busbars

As regards main busbars, distribution busbars and the busbars connecting circuit-breakers and cables, the ef-

fective power loss can be determined from the dissipated powers, at the nominal current and per unit length, as

shown in the previous Tables 8 and 9.

The formula to relate the data in the table to the characteristics (load current and length) of the busbars installed

in the switchboard is the following:

PSB = PnSB (Ib/InSB)2 x 3 x LSB

Therefore, with reference to the typology, the length “L” and the load current of the busbars installed inside the

switchboard, the contribution in terms of power loss of the single length and the total power loss are reported in

Table 11 below:

Connection busbar

Cross-sectional area

nx[mm]x[mm]

Length

[m]

Ib

[A]

Power loss

[W]

IG 2x60x10 0.450 1340 54

I1 30x10 0.150 330 3.8

I2 30x10 0.150 330 3.8

I3 30x10 0.150 330 3.8

I4 20x10 0.150 175 1.6

I5 20x10 0.150 175 1.6

Total power loss of the connection busbars [W] 68

Table 11

Cables

As regards cables, taking reference to Table 8 above, the same method used for the busbars can be applied and

the relevant results are reported in Table 12.

Then, the total power dissipated inside the switchboard is given by the sum of the three contributions already de-

termined above, therefore:

PTQ = 234+68+332=784W

It is important to note how the total power loss would be equal to 452W and therefore the estimated temperature

would be much lower than the effective one if the cable contribution (332W) were not taken into account.

Tabella 12

Cable

Cross-sectional area

[n]xmm2

Length

[m]

Ib

[A]

Power loss

[W]

IG 4x240 1.0 1340 133.8

I1 240 2.0 330 64.9

I2 240 1.7 330 55.2

I3 240 1.4 330 45.4

I4 120 1.1 175 19

I5 120 0.8 175 13.8

Total power loss of the connection busbars [W] 332

15

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

2.1.4 Paths of the current

The positioning of apparatus and conductors may

result into a different power loss inside the switchboard.

It is a good rule to position the circuit-breakers as

shown in Figure 11, so that the paths of the highest

currents are as short as possible.

Thus, contrary to what

occurs in a type of installation as that of Figure 11a, the

dissipated power inside the switchboard is reduced and

unquestionable advantages from the thermal point of

view are achieved.

Figure 11

Suggested positioning:

The highest current (500 A) ﬂows through the shortest path

Figure 11a

NOT suggested positioning:

The highest current (500 A) ﬂows through the longest path

In case of switchboards with many columns, whenever

possible, it is advisable that the main circuit-breaker is

installed in the middle column or, however, in barycentric

position with respect to the load distribution, as shown in

Figure 12. Thus, by dividing the current into the two bran-

ches of the switchboard busbar system, a remarkable

reduction in the power loss is obtained – with the same

cross-section – in comparison with a conﬁguration having

the incoming feeder at both ends of the switchboard as

in Figure 12a, which is a solution implying the circulation

of highest currents.

Figure 12

Advisable solution from a thermal point of view

Figure 12a

Heavier solution from a thermal point of view

50 A

100 A

300 A

500 A

INCOMING FEEDER OUTGOING FEEDER

50 A

100 A

300 A

500 A

INCOMING FEEDER OUTGOING FEEDER

2000 A 1200 A

3200 A

Busbar

Cables

3200 A

Busbar

Cables

16 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

2.2 Dissipation of the heat generated

inside switchboards

After an analysis of the main heat sources and of the

measures to limit heat generation, the modalities through

which switchboards can dissipate the heat towards the

outside are described now.

Many of these considerations derive from the Std.

IEC/TR 60890, which gives formulas and tables where

constructional characteristics and installation modalities

are in relation with the temperature-rise at the same

power loss. In particular, this chapter shall take into

consideration the switchboard ventilation, the surfaces

of the switchboard and their positioning, the form of

internal separation of the switchboard and the degree

of protection of the switchboard.

2.2.1 Switchboard ventilation

To increase the switchboard cooling, it is important

that a good circulation of air inside the switchboard is

realized (see Figure 13) and maintained. To this purpose,

for example, the possible ventilation openings are to be

properly dimensioned and positioned.

As regards dimensioning, IEC/TR 60890 for temperature-

rise assessment inside low voltage switchgear and

controlgear assemblies prescribes for the enclosures with

ventilation openings that the cross section for air outlets

is 1.1 times the cross section of the inlets.

This requirement is due to the greater volume of hot air

(going out of the switchboard) in comparison with the

cold air (going into the switchboard).

When disregarding this prescription, the air inlet surface

of the switchboard is not fully exploited.

As regards the positioning of the ventilation openings,

these are to be located so that the “draught chimney”

effect is achieved: an opening shall be positioned at the

bottom of the switchboard, on the front part, the other

one shall be positioned at the top, on the rear part, or

on the “roof” of the switchboard. It is important to re-

mind that any openings at mid height could reduce the

“draught chimney” effect, thus causing a reduction of

the air “draught”.

The equipment inside the switchboard shall be positioned

so that the circulation of the air is not excessively impeded

by a reduction of the section for the air ﬂow.

In case of withdrawable circuit-breakers, particular

attention shall be paid to prevent the obstruction of the

ventilation openings in the ﬁxed part of the circuit-breaker

(Figure 14).

Figure 13

Figure 14

2.2.2 Side surfaces and positioning of

switchboards

It is necessary to take into account that a switchboard

exchanges heat with the surrounding environment through

its surfaces (top, bottom and side walls) and therefore,

at the same power dissipation level by the internal

components, the larger the exchange surface towards

the outside and the better the exchange conditions

depending on the installation modality are, the greater

amount of heat is released. For example, the switchboard

should be positioned so that the air circulation around

its external surface is facilitated or, however, is little

impeded, thus improving heat exchange.

IEC/TR 60890, which, as already said, suggests a method

for the temperature-rise assessment inside switchboards,

does not consider the real external geometric surface

of the switchboard, but introduces the concept of

effective cooling surface “Ae”, intended as the sum of

the individual surface areas (top, front, side, ....) “A0“

multiplied by the surface factor “b”. This factor takes into

account the heat dissipation of the individual surfaces

according to the type of installation of the enclosure, that

is the different capacity of dissipating heat according to

17

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

the surface positions and to their being either exposed or

covered. The values of the parameter “b” related to the

different surface types are shown in Table 13.

Ae =∑ (A0 x b)

Table 13

Type of installation

Surface factor

“b”

Exposed top surface 1.4

Covered top surface, e.g. of built-in enclosures 0.7

Exposed side faces, e.g. front, rear and side walls 0.9

Covered side faces, e.g. rear side of wall-mounted

enclosures 0.5

Side faces of central enclosures 0.5

Floor surface

Not taken into

account

2.2.3 Forms of internal separation of

switchboards

With separation form it is meant the type of division provided

for the different circuits inside the switchboard.

Separation is obtained by means of metallic or insulating

barriers or partitions.

For further information about the different forms of

separation reference shall be made to the content of the

Annex B or to the prescriptions of Std. IEC 60439-1.

As evident, remarkable separation forms tend to limit

air circulation inside the switchboard, thus affecting

the temperature inside the switchboard itself. To take

into consideration this phenomenon, Tables 14 and 15

show the multiplying factor “d” which IEC/TR 60890

suggests to use under particular conditions to increase

the temperature-rise of the air inside the switchboard as

a function of the number of horizontal partitions in the

column under examination.

Number of horizontal partitions n Factor d

0 1

1 1.05

2 1.15

3 1.3

Table 14: For enclosures without ventilation openings and effective

cooling surface >1.25m2

Table 15: For enclosures with ventilation openings and effective

cooling surface >1.25m2

Number of horizontal partitions n Factor d

0 1

1 1.05

2 1.10

3 1.15

From the tables above, it results how horizontal partitions

can cause air temperature rises up to 30% (3 partitions

without ventilation openings).

2.2.4 Switchboard protection degree

The degree of protection IP shows the protection of the

enclosure against access to hazardous parts, against

ingress of solid foreign objects and ingress of water.

The code IP is the identiﬁcation system of the degrees

of protection based on the prescriptions of the Std. IEC

60529.

The degree of protection of a switchboard affects its

capacity of dissipating heat: the higher the degree

of protection is, the less heat is dissipated by the

switchboard. Therefore, the choice of high degrees

of protection is not recommended when they are

unnecessary. Besides, it should be kept in mind that a

deﬁned degree of protection may be reached through

different modalities.

For example, the protection against the vertical fall of

drops of water (IPX1) can be realized by such modalities

so as not to affect heat dissipation and so as to keep the

“chimney effect” inside the switchboard.

2.3 Dissipation of the heat generated in the

terminals

After a study of the main power sources inside the

switchboard and of the modalities through which

switchboards can dissipate the generated heat, an

analysis of how the circuit-breaker current carrying

capacity can be improved by reducing local heating

phenomena near the terminals shall be carried out.

In practice, when heat dissipation has not been

optimized, the presence of localized heating phenomena

limiting the maximum service current of the circuit is quite

frequent, even with low average air temperatures inside

the switchboard.

The phenomena affecting heat dissipation by the circuit-

breaker terminals are mainly convection (through the air

moving inside the switchboard) and conduction (through

the bars connected to the terminals); these phenomena

are to be related with the typology of terminals used

and to the version (ﬁxed, withdrawable or plug-in) of the

circuit-breaker installed.

2.3.1 Problems linked to convection

As a general principle linked to the phenomenon of

convection, based on the convective motion of the air

which, while heating, tends to rise to the top, the busbar

arrangement should be such as to present the minimum

cross-sectional area to the air ﬂow and to be licked

by the air ﬂow on its maximum surface, therefore in a

“comb-like” arrangement. The circuit-breaker typology

which is most suitable for this conﬁguration is the version

providing vertical rear terminals.

18 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Here are some practical considerations regarding use

and installation modalities of vertical rear terminals for

Emax series circuit-breakers. The use of these terminals

allows a better heat dissipation since, compared with the

horizontal ones, they oppose a smaller cross-section

to the natural motion of the air and a greater surface to

thermal exchange. Yet, one of the main problems to be

faced when using vertical terminals is their complicated

connection to the main busbar system when this runs

horizontally along the switchboard with the busbar

section arranged vertically. This problem is not present

with the same busbar system when the circuit-breaker

terminals are horizontal, since both busbars and terminals

are oriented according to two simple connection planes.

This concept is deﬁnitely more evident when making

reference to Figure 15.

Figure 15

Circuit-breaker with vertical terminals and vertical main busbars

Circuit-breaker with horizontal terminals and vertical main busbars

For example, in the case of E4 and E6 circuit-breakers,

to facilitate the connection between the vertical terminals

and the vertical connection busbars, it is possible to use

bars suitably bent as shown in Figure 16.

Figure 16

Emax E6

Connection bars to

the main busbars

Bars properly bent

Connection bars to

the main busbars

Vertical

terminals

Bars properly bent

Top view

Vertical terminals for E4 and E6 CBs

(detail referred to one pole)

Connection

busbars

Main busbars running horizontally along the

switchboard and vertically arranged

Circuit-breaker with

horizontal terminals

Detail of the horizontal

connection busbar with

air ﬂow direction

Main busbars running horizontally along the

switchboard and vertically arranged

Circuit-breaker with

vertical terminals

Detail of the vertical con-

nection busbar with air ﬂow

direction

Connection

busbars

19

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

As further example, Figure 17 shows two other pictures

representing an hypothesis of solution for the connection

of the vertical terminals to the vertical connection busbar

system for Emax E3 circuit-breakers.

Figure 17

On the contrary, when in the presence of upper vertical

terminals and lower terminals of different type or, howe-

ver, when in the presence of different upper and lower

terminals, the solutions to be adopted must not impede

the circulation of air in the upper terminals. For example,

as Figure 18 shows, the lower terminals shall not divert

too much the air ﬂow and prevent it from reaching the

upper terminals causing the loss of the beneﬁts of cooling

by convection.

Figure 18

Lower connection with rear horizontal terminals.

Air circulation, near the upper terminals (vertical) is impeded.

Lower connection with front terminals.

Air circulation, near the upper terminals (vertical), is only partially reduced.

Generally speaking, for the improvement of the heating

condition of busbars and terminals, the positioning of the

busbars acquires a remarkable importance; here is an

example of the solutions which can be adopted.

It should be kept into consideration that, the more the

clearance between the busbars, the more heat they dis-

sipate and that the upper middle terminal is usually that

with the most problems from the thermal point of view.

Therefore it is advisable to separate and keep apart

as much as possible the connection busbars (from the

main busbars to the circuit-breaker terminals) so that the

heating condition does not get worse.

Busbars

Connection busbars

Terminal

Busbars

Connection busbars

Terminal

Busbars

Terminal

Connection busbars

20 ABB circuit-breakers inside LV switchboards



Top view

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

For example, as Figure 19 shows, in the case of three-

pole circuit-breakers, the external connections can be

taken out of alignment with the terminals, so that distance

is increased.

Figure 19

In case of circuit-breaker connection through busbar

systems with the three phases vertically arranged, it is

advisable that spacing of the 3 phases starts as near as

possible to the circuit-breaker, with a solution as that

shown in Figure 20.

Figure 20

As already mentioned, the upper terminals and in parti-

cular the upper middle terminal are usually those which,

due to their positions, reach the highest temperatures.

Then, particular measures can be taken to improve

the heat exchange of these terminals, for example by

lengthening the horizontal part of the upper connection

busbars in comparison with the lower ones, as shown

in Figure 21.

Figure 21

Further increases in the carrying capacity of the circuits

may be obtained by mounting some dissipators (see

Figure 22) on the connection conductors – between the

circuit-breaker and the busbar system – for a better heat

dissipation, or by treating busbars and dissipators with

special paints, which allow an increase in the radiated

heat, without creating, on the other hand, a surface

thermal insulation.

Figure 22

2.3.2 Problems linked to conduction

Instead, with reference to the thermal exchange through

the phenomenon of conduction, the terminals of a circuit-

breaker release heat also towards the busbars or the

cables connected to them. In particular, besides carrying

the current, the connection busbars convey heat far from

the terminals. Therefore, their dimensioning and position

shall have to take into account this double function.

There is a rise in the heat exchanged by conduction

both when the cross-section through which heat is

exchanged enlarges (contact section between the

cables or the connection busbars and the circuit-breaker

terminals), as well as when the temperature difference

between the bodies in touch involved by this exchange

increases. From this remark it results that, on their turn,

the connection busbars shall effectively dissipate heat

to keep their temperature low. To obtain a connection

allowing a sufﬁcient heat exchange between the terminals

and the distribution system of the switchboard, ABB

SACE indicates the minimum cross-sectional area of the

cables and busbars to be used.

Main busbars

Connection busbars

Connection

busbars

Main busbars

21

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Table 16

Circuit-breaker type In Cables Bars

Tmax [A] [ n // ] x [ mm2 ] [ n // ] x [ mm x mm ]

T2 <=8 1

T2-T4 10 1,5

T1-T2 16 2,5

T1-T2-T4 20 2,5

T1-T2-T4 25 4

T1-T2-T4 32 6

T1-T2-T4 40 10

T1-T2-T4 50 10

T1-T2-T3-T4 63 16

T1-T2-T3-T4 80 25

T1-T2-T3-T4 100 35

T1-T2-T3-T4 125 50

T1-T2-T3-T4 160 70

T3-T4 200 95 20x5

T3-T4 250 120 25x5

T4-T5 320 185 40x5

T5 400 240 50x5

T5 500 2x150 2x30x5

T5-T6 630 2x185 2x40x5

T6 800 2x240 2x50x5

T6-T7 1000 3x240 2x60x5

T7 1250 4x240 2x80x5

T7 1600 5x240 2x100x5

Here are the indications given for the molded-case

circuit-breakers series Tmax and the air circuit-breakers

series Emax, reported in Table 16 and in Table 17

respectively.

The cross-sectional areas of the cables and busbars

reported in Tables 16 and 17 are those used to determine

the rated current carrying capacity of the circuit-breakers

in free air according to the Std. IEC 60947-2.

Circuit-breaker Vertical terminals Front horizontal terminals

Emax [ n // ] x [ mm x mm ] [ n // ]x[ mm x mm ]

E1B/N 08 1x(60x10) 1x(60x10)

E1B/N 12 1x(80x10) 2x(60x8)

E2B/N 12 1x(60x10) 1x(60x10)

E2B/N 16 2x(60x10) 2x(60x10)

E2B/N 20 3x(60x10) 3x(60x10)

E2L 12 1x(60x10) 1x(60x10)

E2L 16 2x(60x10) 2x(60x10)

E3S/H 12 1x(60x10) 1x(60x10)

E3S/H 16 1x(100x10) 1x(100x10)

E3S/H 20 2x(100x10) 2x(100x10)

E3N/S/H 25 2x(100x10) 2x(100x10)

E3N/S/H 32 3x(100x10) 3x(100x10)

E3L20 2x(100x10) 2x(100x10)

E3L 25 2x(100x10) 2x(100x10)

E4H 32 3x(100x10) 3x(100x10)

E4S/H 40 4x(100x10) 6x(60x10)

E6V 32 3x(100x10) 3x(100x10)

E6V 40 4x(100x10) 4x(100x10)

E6H/V 50 6x(100x10) 6x(100x10)

E6H/V 63 7x(100x10) ----------------------

Table 17

22 ABB circuit-breakers inside LV switchboards

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2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

2.3.3 Current carrying capacity of circuit-breakers

and busbars

To conclude this chapter, Figure 23 shows the curves

reporting the carrying capacity of the Tmax molded-case

circuit-breakers equipable with electronic relays, referred

to the different temperatures and the different types of

terminals and available versions, whereas Table 18 repor-

ts the current carrying capacity of the Tmax molded-case

circuit-breakers equipable with thermomagnetic releases.

As regards the air circuit-breakers of Emax series, Table

19 reports the carrying capacity of the single apparatus

at the different temperatures.

Note: in the plug-in version the

maximum setting is derated

by 10%

Note: in the plug-in version the

maximum setting is derated

by 10%

F = Front flat terminals

FC Cu = Front terminals for

copper cables

EF = Front extended terminals

FC CuAl = Front terminals for CuAl cables ES = Front extended spread terminals

R = Rear terminals

F = Front flat terminals

FC Cu = Front terminals for

copper cables

EF = Front extended terminals

FC CuAl = Front terminals for CuAl cables ES = Front extended spread terminals

R = Rear terminals

T2 160

Fixed

T2 160

Plug-in

Figure 23

23

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

215

220

225

230

235

240

245

250

255

Iu [A]

T [°C]

40 45 50 55 60 65 70

210

205

VR

HR-F-FC

T4 250

215

220

225

230

235

240

245

250

255

Iu [A]

T [°C]

40 45 50 55 60 65 70

HR-F-FC

VR

T4 250

Fixed

FC = Front terminals for cables

VR = Rear flat vertical terminals

F = Front flat terminals HR = Rear flat horizontal terminals

Plug-in / Withdrawable

FC = Front terminals for cables

VR = Rear flat vertical terminals

F = Front flat terminals HR = Rear flat horizontal terminals

24 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

250

260

270

280

290

300

310

320

330

Iu [A]

T [°C]

40 45 50 55 60 65 70

240

VR

HR-F-FC

T4 320

Fixed

250

260

270

280

290

300

310

320

330

Iu [A]

T [°C]

40 45 50 55 60 65 70

240

30 35

FC-HR-F

VR

T4 320

Plug-in / Withdrawable

FC = Front terminals for cables

VR = Rear flat vertical terminals

F = Front flat terminals HR = Rear flat horizontal terminals

FC = Front terminals for cables

VR = Rear flat vertical terminals

F = Front flat terminals HR = Rear flat horizontal terminals

25

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

365

370

375

380

385

390

395

400

405

Iu [A]

T [°C]

40 45 50 55 60 65 70

360

355

350

HR-FC-F

VR

T5 400

365

370

375

380

385

390

395

400

405

Iu [A]

T [°C]

40 45 50 55 60 65 70

360

355

350

35

345

340

335

330

HR-FC-F

VR

T5 400

Fixed

Plug-in / Withdrawable

FC = Front terminals for cables

VR = Rear flat vertical terminals

F = Front flat terminals HR = Rear flat horizontal terminals

FC = Front terminals for cables

VR = Rear flat vertical terminals

F = Front flat terminals HR = Rear flat horizontal terminals

26 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

560

570

580

590

600

610

620

630

640

Iu [A]

T [°C]

40 45 50 55 60 65 70

550

540

530

35

520

510

500

490

30

480

470

VR

HR-FC-F

T5 630

Iu [A]

T [°C]

40 45 50 55 60 65 7035

600

550

500

450

400

VR

EF-HR

T5 630

EF = Front extended terminals VR = Rear flat vertical terminalsHR = Rear flat horizontal terminals

Fixed

Plug-in / Withdrawable

FC = Front terminals for cables

VR = Rear flat vertical terminals

F = Front flat terminals HR = Rear flat horizontal terminals

27

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

4

T6 630

Fixed

VR

EF-HR

T6 630

Withdrawable

R(VR)

F-FC

R(HR)

EF = Front extended terminals HR = Rear flat horizontal terminals VR = Rear flat vertical terminals

FC = Front terminals for cablesF = Front flat terminals R(VR)=Rear(vertical)terminals R(HR) = Rear(horiz.)terminals

28 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

4

VR

EF-HR

T6 800

Withdrawable

R(VR)

T6 800

Fixed

F-FC

R(HR)

EF = Front extended terminals HR = Rear flat horizontal terminals VR = Rear flat vertical terminals

FC = Front terminals for cablesF = Front flat terminals R(VR)=Rear(vertical)terminals R(HR) = Rear(horiz.)terminals

29

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

900

1000

35 40 45 50 55 60 65 70

1100

R(VR)

FC

R (HR)

ES

T6 1000

Fixed

FC = Front terminals for cables R (VR) = Rear flat vertical terminals

R (HR) = Rear flat horizontal terminals

ES = Front extended spread terminals

30 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

950

900

1000

35 40 45 50 55 60 65 70

1050

VR

EF-HR

T7 V 1000

Fixed

950

900

1000

35 40 45 50 55 60 65 70

1050

T7 V 1000

Withdrawable

VR

EF-HR

Note: For ratings below 1000 A Tmax T7 does not undergo any thermal derating.

EF = Extended front terminals VR = Rear flat vertical terminals HR = Rear flat horizontal terminalsEF = Extended front terminals VR = Rear flat vertical terminals HR = Rear flat horizontal terminals

EF = Extended front terminals VR = Rear flat vertical terminals HR = Rear flat horizontal terminals

31

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

1200

1150

800

850

900

950

1000

1050

1100

1250

35 40 45 50 55 60 65 70

1300

VR

EF-HR

T7 S,H,L, 1250

1200

1150

800

850

900

950

1000

1050

1100

1250

35 40 45 50 55 60 65 70

1300

T7 V 1250

VR

EF-HR

Fixed

EF = Extended front terminals VR = Rear flat vertical terminals HR = Rear flat horizontal terminals

32 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

1200

1150

800

850

900

950

1000

1050

1100

1250

35 40 45 50 55 60 65 70

1300

VR

EF-HR

T7 S,H,L, 1250

Withdrawable

1200

1150

800

850

900

950

1000

1050

1100

1250

35 40 45 50 55 60 65 70

1300

T7 V 1250

VR

EF-HR

Withdrawable

EF = Extended front terminals VR = Rear flat vertical terminals HR = Rear flat horizontal terminals

33

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

1200

1000

1100

1400

1500

1600

1700

35 40 45 50 55 60 65 70

1300

VR

EF-HR

T7 S,H,L, 1600

Fixed

1200

1000

1100

1400

1500

1600

1700

35 40 45 50 55 60 65 70

1300

T7 S,H,L, 1600

Withdrawable

VR

EF-HR

EF = Extended front terminals VR = Rear flat vertical terminals HR = Rear flat horizontal terminals

34 ABB circuit-breakers inside LV switchboards



Table 18

Tmax T1 and T1 1P (*)

In [A]

10 °C 20 °C 30 °C 40 °C 50 °C 60 °C 70 °C

MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX

16 13 18 12 18 12 17 11 16 11 15 10 14 9 13

20 16 23 15 22 15 21 14 20 13 19 12 18 11 16

25 20 29 19 28 18 26 18 25 16 23 15 22 14 20

32 26 37 25 35 24 34 22 32 21 30 20 28 18 26

40 32 46 31 44 29 42 28 40 26 38 25 35 23 33

50 40 58 39 55 37 53 35 50 33 47 31 44 28 41

63 51 72 49 69 46 66 44 63 41 59 39 55 36 51

80 64 92 62 88 59 84 56 80 53 75 49 70 46 65

100 81 115 77 110 74 105 70 100 66 94 61 88 57 81

125 101 144 96 138 92 131 88 125 82 117 77 109 71 102

160 129 184 123 176 118 168 112 160 105 150 98 140 91 130

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Tmax T2

In [A]

10 °C 20 °C 30 °C 40 °C 50 °C 60 °C 70 °C

MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX

1,6 1,3 1,8 1,2 1,8 1,2 1,7 1,1 1,6 1 1,5 1 1,4 0,9 1,3

2 1,6 2,3 1,5 2,2 1,5 2,1 1,4 2 1,3 1,9 1,2 1,7 1,1 1,6

2,5 2 2,9 1,9 2,8 1,8 2,6 1,8 2,5 1,6 2,3 1,5 2,2 1,4 2

3,2 2,6 3,7 2,5 3,5 2,4 3,4 2,2 3,2 2,1 3 1,9 2,8 1,8 2,6

4 3,2 4,6 3,1 4,4 2,9 4,2 2,8 4 2,6 3,7 2,4 3,5 2,3 3,2

5 4 5,7 3,9 5,5 3,7 5,3 3,5 5 3,3 4,7 3 4,3 2,8 4

6,3 5,1 7,2 4,9 6,9 4,6 6,6 4,4 6,3 4,1 5,9 3,8 5,5 3,6 5,1

8 6,4 9,2 6,2 8,8 5,9 8,4 5,6 8 5,2 7,5 4,9 7 4,5 6,5

10 8 11,5 7,7 11 7,4 10,5 7 10 6,5 9,3 6,1 8,7 5,6 8,1

12,5 10,1 14,4 9,6 13,8 9,2 13,2 8,8 12,5 8,2 11,7 7,6 10,9 7,1 10,1

16 13 18 12 18 12 17 11 16 10 15 10 14 9 13

20 16 23 15 22 15 21 14 20 13 19 12 17 11 16

25 20 29 19 28 18 26 18 25 16 23 15 22 14 20

32 26 37 25 35 24 34 22 32 21 30 19 28 18 26

40 32 46 31 44 29 42 28 40 26 37 24 35 23 32

50 40 57 39 55 37 53 35 50 33 47 30 43 28 40

63 51 72 49 69 46 66 44 63 41 59 38 55 36 51

80 64 92 62 88 59 84 56 80 52 75 49 70 45 65

100 80 115 77 110 74 105 70 100 65 93 61 87 56 81

125 101 144 96 138 92 132 88 125 82 117 76 109 71 101

160 129 184 123 178 118 168 112 160 105 150 97 139 90 129

(*) For T1 1P circuit-breakers (ﬁtted with TMF themomagnetic trip unit) consider only the column corresponding to the maximum adjustament of the TMD

trip units.

Tmax T3

In [A]

10 °C 20 °C 30 °C 40 °C 50 °C 60 °C 70 °C

MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX

63 51 72 49 69 46 66 44 63 41 59 38 55 35 51

80 64 92 62 88 59 84 56 80 52 75 48 69 45 64

100 80 115 77 110 74 105 70 100 65 93 61 87 56 80

125 101 144 96 138 92 132 88 125 82 116 76 108 70 100

160 129 184 123 176 118 168 112 160 104 149 97 139 90 129

200 161 230 154 220 147 211 140 200 130 186 121 173 112 161

250 201 287 193 278 184 263 175 250 163 233 152 216 141 201

(*) For plug-in circuit-breakers, a derating of 10% is to be considered.

35

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Tmax T4

In [A]

10 °C 20 °C 30 °C 40 °C 50 °C 60 °C 70 °C

MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX

20 19 27 18 24 16 23 14 20 12 17 10 15 8 13

32 26 43 24 39 22 36 19 32 16 27 14 24 11 21

50 37 62 35 58 33 54 30 50 27 46 25 42 22 39

80 59 98 55 92 52 86 48 80 44 74 40 66 32 58

100 83 118 80 113 74 106 70 100 66 95 59 85 49 75

125 103 145 100 140 94 134 88 125 80 115 73 105 63 95

160 130 185 124 176 118 168 112 160 106 150 100 104 90 130

200 162 230 155 220 147 210 140 200 133 190 122 175 107 160

250 200 285 193 275 183 262 175 250 168 240 160 230 150 220

Tmax T5

In [A]

10 °C 20 °C 30 °C 40 °C 50 °C 60 °C 70 °C

MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX

320 260 368 245 350 234 335 224 320 212 305 200 285 182 263

400 325 465 310 442 295 420 280 400 265 380 250 355 230 325

500 435 620 405 580 380 540 350 500 315 450 280 400 240 345

Tmax T6

In [A]

10 °C 20 °C 30 °C 40 °C 50 °C 60 °C 70 °C

MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX MIN MAX

630 520 740 493 705 462 660 441 630 405 580 380 540 350 500

800 685 965 640 905 605 855 560 800 520 740 470 670 420 610

36 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

Table 19

X1 withdrawable – rear horizontal terminals

Temperature

[°C]

X1 630 X1 1800 X1 1000 X1 1250 X1 1600

% [A] % [A] % [A] % [A] % [A]

10 100 630 100 800 100 1000 100 1250 100 1600

20 100 630 100 800 100 1000 100 1250 100 1600

30 100 630 100 800 100 1000 100 1250 100 1600

40 100 630 100 800 100 1000 100 1250 100 1600

45 100 630 100 800 100 1000 100 1250 100 1600

50 100 630 100 800 100 1000 100 1250 97 1550

55 100 630 100 800 100 1000 100 1250 94 1500

60 100 630 100 800 100 1000 100 1250 93 1480

X1 withdrawable – rear vertical terminals

Temperature

[°C]

X1 630 X1 1800 X1 1000 X1 1250 X1 1600

% [A] % [A] % [A] % [A] % [A]

10 100 630 100 800 100 1000 100 1250 100 1600

20 100 630 100 800 100 1000 100 1250 100 1600

30 100 630 100 800 100 1000 100 1250 100 1600

40 100 630 100 800 100 1000 100 1250 100 1600

45 100 630 100 800 100 1000 100 1250 100 1600

50 100 630 100 800 100 1000 100 1250 100 1600

55 100 630 100 800 100 1000 100 1250 98 1570

60 100 630 100 800 100 1000 100 1250 95 1520

SACE Emax E1

Temperature

[°C]

E1 800 E1 1000 E1 1250 E1 1600

% [A] % [A] % [A] % [A]

10 100 800 100 1000 100 1250 100 1600

20 100 800 100 1000 100 1250 100 1600

30 100 800 100 1000 100 1250 100 1600

40 100 800 100 1000 100 1250 100 1600

45 100 800 100 1000 100 1250 98 1570

50 100 800 100 1000 100 1250 96 1530

55 100 800 100 1000 100 1250 94 1500

60 100 800 100 1000 100 1250 92 1470

65 100 800 100 1000 99 1240 89 1430

70 100 800 100 1000 98 1230 87 1400

37

ABB circuit-breakers inside LV switchboards

2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

SACE Emax E2

Temperature

[°C]

E2 800 E2 1000 E2 1250 E2 1600 E2 2000

% [A] % [A] % [A] % [A] % [A]

10 100 800 100 1000 100 1250 100 1600 100 2000

20 100 800 100 1000 100 1250 100 1600 100 2000

30 100 800 100 1000 100 1250 100 1600 100 2000

40 100 800 100 1000 100 1250 100 1600 100 2000

45 100 800 100 1000 100 1250 100 1600 100 2000

50 100 800 100 1000 100 1250 100 1600 97 1945

55 100 800 100 1000 100 1250 100 1600 94 1885

60 100 800 100 1000 100 1250 98 1570 91 1825

65 100 800 100 1000 100 1250 96 1538 88 1765

70 100 800 100 1000 100 1250 94 1510 85 1705

SACE Emax E3

Temperature

[°C]

E3 800 E3 1000 E3 1250 E3 1600 E3 2000 E3 2500 E3 3200

% [A] % [A] % [A] % [A] % [A] % [A] % [A]

10 100 800 100 1000 100 1250 100 1600 100 2000 100 2500 100 3200

20 100 800 100 1000 100 1250 100 1600 100 2000 100 2500 100 3200

30 100 800 100 1000 100 1250 100 1600 100 2000 100 2500 100 3200

40 100 800 100 1000 100 1250 100 1600 100 2000 100 2500 100 3200

45 100 800 100 1000 100 1250 100 1600 100 2000 100 2500 100 3200

50 100 800 100 1000 100 1250 100 1600 100 2000 100 2500 97 3090

55 100 800 100 1000 100 1250 100 1600 100 2000 100 2500 93 2975

60 100 800 100 1000 100 1250 100 1600 100 2000 100 2500 89 2860

65 100 800 100 1000 100 1250 100 1600 100 2000 97 2425 86 2745

70 100 800 100 1000 100 1250 100 1600 100 2000 94 2350 82 2630

SACE Emax E4

Temperature

[°C]

E4 3200 E4 4000

% [A] % [A]

10 100 3200 100 4000

20 100 3200 100 4000

30 100 3200 100 4000

40 100 3200 100 4000

45 100 3200 100 4000

50 100 3200 98 3900

55 100 3200 95 3790

60 100 3200 92 3680

65 98 3120 89 3570

70 95 3040 87 3460

SACE Emax E6

Temperature

[°C]

E6 3200 E6 4000 E6 5000 E6 6300

% [A] % [A] % [A] % [A]

10 100 3200 100 4000 100 5000 100 6300

20 100 3200 100 4000 100 5000 100 6300

30 100 3200 100 4000 100 5000 100 6300

40 100 3200 100 4000 100 5000 100 6300

45 100 3200 100 4000 100 5000 100 6300

50 100 3200 100 4000 100 5000 100 6300

55 100 3200 100 4000 100 5000 98 6190

60 100 3200 100 4000 98 4910 96 6070

65 100 3200 100 4000 96 4815 94 5850

70 100 3200 100 4000 94 4720 92 5600

38 ABB circuit-breakers inside LV switchboards



2 Advices to improve the current carrying capacity of the circuit-breakers inside switchboards

As regards small-size molded-case circuit-breakers there

is no remarkable difference among the various terminal

typologies, whereas for big-size molded-case circuit-

breakers (starting from T4), the rear vertical terminals are

to be preferred to the other terminal typologies when the

circuit-breaker is installed in vertical position. In general,

however, the ﬁxed version is suggested instead of the

withdrawable and the plug-in ones. If the circuit-breaker

were installed in horizontal position, to determine its

current carrying capacity reference should be taken to

the lowest curve in the graphics.

As regards the air circuit-breakers of Emax series, the

behaviour of the rear horizontal terminals is analogous

to that of the front ones.

The greater heat dissipation capacity is deﬁnitely that of

the rear vertical terminals.

In the same way, as an example, Table 20 shows the

different current carrying capacity given by the Std. DIN

43671 for copper conductors of rectangular cross-sec-

tion in indoor installations, where the radiation coefﬁcient

is assumed to be equal to 0.4 for not painted bars and

0.9 for painted bars.

As it can be observed from the table, under the same

conditions in the busbar system, passing from bare

busbars to painted busbars there is an increase in the

current carrying capacity which can reach also 15%.

Table 20

Width

x

Thickness

Carrying capacity in A

a.c. up to 60Hz

for bare copper conductors

Carrying capacity in A

up to 60Hz

for painted copper conductors

[mm] x [mm] I II III II II* I II III II II*

50 x 5 583 994 1240 1920 679 1140 1330 2010

50 x 10 852 1510 2040 2600 1020 1720 2320 2950

60 x 5 688 1150 1440 2210 826 1330 1510 2310

60 x 10 985 1720 2300 2900 1180 1960 2610 3290

80 x 5 885 1450 1750 2720 1070 1680 1830 2830

80 x 10 1240 2110 2790 3450 1500 2410 3170 3930

100 x 5 1080 1730 2050 3190 1300 2010 2150 3300

100 x 10 1490 2480 3260 3980 1810 2850 3720 4530

Validity condition of the table: ambient temperature 35°C, conductor temperature 65°C, conductor width vertical, clearance between conductors in paral-

lel equal to conductor thickness.

(*) minimum clearance between the central conductors: 50mm.

39

ABB circuit-breakers inside LV switchboards

3 Problems concerning short-circuit

In this chapter the problems regarding short-circuit are

analyzed, making speciﬁc reference to the interaction

between the protection circuit-breaker installed in the

switchboard and the switchboard itself. After a short

introduction deﬁning the main electrical parameters

related to short-circuit, an analysis shall be carried out to

illustrate the prescriptions regarding the electrical circuits

of switchboards and the modalities aimed at reducing the

possibility of occurrence of a short-circuit on the circuits

inside the switchboards and to reduce its effects.

3.1 Main deﬁnitions of the parameters

characterizing a switchboard under

short-circuit conditions

As far as the short-circuit withstand strength of a switchboard

is concerned, the main parameters characterizing a

switchboard are:

- rated short-time withstand current;

- rated peak withstand current;

- rated conditional short-circuit current.

The Std. IEC 60439-1 about low voltage controlgear and

switchgear deﬁne the above parameters as follows :

Rated short-time withstand current “Icw”

The rated short-time withstand current of a circuit of

a switchboard is the r.m.s. value of short-time current

assigned to that circuit by the manufacturer which

that circuit can carry without damage under the test

conditions speciﬁed by the Standard. Unless otherwise

stated by the manufacturer, the reference time is 1s. To

this short-time current a determined peak value “Ipk” is

associated and it is assumed that the maximum current

value which can occur and which can be withstand by

the switchboard does not exceed the peak value, related

to the Icw by a coefﬁcient “n”.

Rated peak withstand current “Ipk”

The rated peak withstand current of a switchboard circuit

is the value of peak current assigned to that circuit

by the manufacturer which that circuit can withstand

satisfactorily under the test conditions speciﬁed by

the Standard. The peak current value, which is used

to determine electro-dynamic stresses, is obtained by

multiplying the short-time current by the coefﬁcient “n”.

The values normalized by the multiplying factor “n” are

reported in Table 21.

Values normalized by coefﬁcient

“n”

RMS value of

short-circuit current

kA Cojn

I ≤ 5 0.7 1.5

5 < I ≤10 0.5 1.7

10 < I ≤20 0.3 2

20 < I ≤50 0.25 2.1

50 < I 0.2 2.2

Table 21

Rated conditional short-circuit current “Icc”

The rated conditional short-circuit current characterizing

the circuit of a switchboard is the value of prospective

short-circuit current, stated by the manufacturer, which

that circuit, protected by a short-circuit protective

device speciﬁed by the manufacturer, can withstand

satisfactorily for the operating time of the device under

the test conditions speciﬁed.

With reference to these deﬁnitions, it is possible to say

that the circuit of a switchboard for which a speciﬁc Icw

has been deﬁned, can withstand the electro-dynamic

stresses due to the initial peak value which can reach

a maximum value equal to “Icw x n” and a speciﬁc

thermal energy due to the current and equal to Icw

2

x t

(with t=1s).

On the other hand, the circuit of a switchboard protected

by a suitable device shall have a rated conditional short-

circuit current if it can withstand the electro-dynamic

stresses due to the peak current limited by the protective

device and a speciﬁc thermal energy let through by the

protective device in correspondence with the prospective

short-circuit current Ik.

3.1.1 General prescriptions and information

about short-circuit withstand strength

As regards the short-circuit withstand strength of an

assembly, the Std. IEC 60439-1 prescribes that the user

of the switchboard shall give the manufacturer the data

relevant to the short-circuit currents at the installation

point so that the assembly is protected against short-

circuit by protective devices – for example automatic

circuit-breakers positioned inside or outside the

switchboard – and so that it is manufactured to withstand

the thermal and dynamic stresses occurring under short-

circuit conditions. The information concerning the short-

circuit withstand strength of the switchboard are given

by the manufacturer according to the presence or not of

the protective device.

40 ABB circuit-breakers inside LV switchboards



3 Problems concerning short-circuit

For assemblies with an automatic circuit-breaker

incorporated in the incoming unit, the manufacturer shall

indicate the maximum allowable value of short-circuit

current, expressed as:

- rated conditional short-circuit current Icc, when the

protective device is a circuit-breaker having remarkable

current limiting characteristics;

- allowable short-time withstand current Icw, when the

protective device is a circuit-breaker having a high Icw

value.

For assemblies where a protective circuit-breaker is not

incorporated in the incoming unit, the manufacturer may

indicate:

- a rated conditional short-circuit current (Icc), specifying

the characteristics of the external device which protects

the switchboard (rated current, breaking capacity,

limited current, speciﬁc let-through energy);

-

an allowable value of short-time withstand current (Icw).

If the duration of the current is not speciﬁed, the current

is understood to be equal to 1sec; if the initial peak

value is not speciﬁed, this is understood to be linked to

the assigned conditional short-circuit withstand current

through the factor “n”.

3.2 Prescriptions concerning the electrical

circuits of a switchboard

In addition to the previous general prescriptions

concerning the indication of the short-circuit withstand

strength of a switchboard, the Standard prescribes

how the electrical circuits inside the assembly must be

dimensioned in order to reduce the possibility that a

fault occurs.

The main prescriptions are given with reference to the

internal busbar system, which shall be designed so that

the likelihood of a short-circuit on the busbars is remote.

As regards busbar dimensioning in relation to the short-

circuit withstand strength, different prescriptions must

be complied with if reference is made to a main busbar

system rather than to circuits derived from the busbars.

3.2.1 Main busbar systems

The main busbars (bare or insulated) shall be arranged

in such a manner that an internal short-circuit is not

to be expected under normal operating conditions.

Unless otherwise speciﬁed, the busbars shall be rated

in accordance with the information concerning the short-

circuit withstand strength and designed to withstand the

short-circuit stresses limited by the protective devices

installed on the supply side of the busbars.

In practice, if the limiting characteristics of the protective

device upstream the busbar system are not remarkable

(or if they are not known in advance), the busbar system

shall be rated so that the Icw is higher than the short-

circuit current value at the installation point.

Here is an application example to illustrate this

concept:

In the switchboard of Figure 24, the circuit-breaker

protecting the main busbar system is an Emax E4H with

Icw = 100kA.

If also the busbar system has an Icw value equal to 100kA

or higher, the circuit formed by the circuit-breaker and by the

busbar system shall be considered to have Icw = 100kA.

On the other hand, if the device protecting the main

busbar system is an automatic circuit-breaker with

current limiting characteristics, the busbar system may

be dimensioned to withstand the stresses due to the

limited peak current and to the speciﬁc let-through energy

limited by the circuit-breaker.

The switchboard of Figure 25 to be installed in a plant

with a prospective short-circuit current Ik equal to 100kA

at 400V is now taken as example. A Tmax T6L1000 is

chosen as incoming circuit-breaker.

Figure 24

Icw=100kA

Ik=100kA

SACE

41

ABB circuit-breakers inside LV switchboards

3 Problems concerning short-circuit

In correspondence with the Ik values, downstream the

incoming apparatus, the following parameters are veriﬁed:

- speciﬁc let-through energy lower than 20MA2s

- limited peak current lower than 80kA

The presence of a current limiting apparatus inside the

switchboard allows a busbar system with an Icw value

< 100kA (short-circuit current in the plant) to be installed

on the load side of this apparatus, to which, however,

a peak current value and a speciﬁc let-through energy

exceeding those measured on the load side of the circuit-

breaker shall correspond.

Figure 25

For example, a busbar system characterized by an Icw

value equal to 50kA can withstand the following para-

meters:

- speciﬁc let-through energy

50kA x 50kA x 1s = 2500MA2s

- peak current

50kA x 2.1 = 105kA

As a consequence it is quite easy to verify that the busbar

system (Icw = 50kA, Ipk = 105kA, I2t = 2500MA2s) can

withstand greater stresses than those ones generated

on the load side of T6L circuit-breaker. To conclude: it

is possible to install a busbar system with an Icw value

equal to 50kA on the load side of the circuit-breaker

type T6L; in this case, the circuit formed by the busbars

and by the circuit-breaker shall have Icc = 100kA and

therefore it is suitable for the prospective short-circuit

current of the plant.

3.2.2 Distribution busbars and conductors

derived by the main busbars

Within a section of an assembly, the conductors and

the distribution busbars positioned between the main

busbars and the supply side of functional units, as well

as the components included in these units, may be

rated on the basis of the reduced short-circuit stresses

occurring on the load side of the respective short-circuit

device, provided that these conductors are arranged

so that under normal operating conditions, an internal

short-circuit between phases and/or between phases

and earth is only a remote possibility. Such conductors

are preferably of solid rigid manufacture.

The economical and dimensional advantages which

result from this prescription of the Standard are evident,

above all when there are many circuits derived by a single

main busbar system.

Figure 26

Figure 26 represents a switchboard where the vertical

distribution busbar, constituted by a bare bar of solid

manufacture and provided with spacers, is derived from

the main busbar, so that the possibility that a short-circuit

occurs can be considered as remote.

From this busbar, different horizontally-arranged

conductors (in cable) depart, supplying the main circuit-

breakers of the different outgoing feeders.

For a correct dimensioning of the vertical distribution

busbar, it is possible to consider the outgoing device

having the lowest current limiting performances. Thus,

also in the event of a fault on the load side of the circuit-

Icw=50kA

Ik=100kA

T2 160

T3 250

T7

Main busbar

Distribution busbar

Dimensioned according to the limiting

characteristics of the main circuit-breaker

Dimensioned according to

the limiting characteristics of T2

circuit-breaker

Dimensioned according to

the limiting characteristics

of T3 circuit-breaker

Conductors

Dimensioned according either

to the limiting characteristics

of the biggest circuit-breaker

among the outgoing circuit-

breakers of the single units, or

to the performances of the

main circuit-breaker.

42 ABB circuit-breakers inside LV switchboards



breaker with lower limiting characteristics, the busbar

shall undergo acceptable stresses.

Dimensioning of the distribution busbar carried out

according to the above corresponds to the Standard

prescriptions; in spite of this, the usual procedure for many

switchboard manufacturers is dimensioning distribution

busbars making reference to the performances of the

circuit-breaker on the incoming of the switchboard, in

terms of let-through energy and limited peak current

value.

On the contrary, the different cables which feed the

individual circuit-breakers shall be dimensioned

according to the limiting characteristics of the relevant

device they supply.

3.3 Reduction of the possibility of short-

circuit events and of the relevant

effects

As regards the prescriptions aimed at making unlikely

the occurrence of a short-circuit in live conductors, the

Std. IEC 60439-1 suggests a series of measures which

depend on the conductor typology. As an example here

are the prescriptions intended for:

- bare conductors, or single-core conductors, with basic

insulation, e.g. cables complying with IEC 60227-3, for

which mutual contact or contact with conductive parts

shall be avoided, for example by use of spacers;

- single-core conductors with basic insulation and a

maximum permissible conductor-operating temperature

above 90° C, for example cables complying with IEC

60245-3, or heating resistant PVC insulated cables

according to IEC 60227-3, for which mutual contact

or contact with conductive parts is permitted where

there is no applied external pressure. Contact with

sharp edges must be avoided. There must be no risk

of mechanical damage. These conductors may only

be

loaded such that an operating temperature of 70° C

is

not exceeded:

- conductors with basic insulation (cables complying

with IEC 60227-3), having additional secondary

insulation, for example individual covered cables with

shrink sleeving or individually run cables in plastic

conduits, or conductors insulated with a very high

mechanical strength material (FTFE insulation), for

which there are no additional requirements if there is

no risk of mechanical damage.

3 Problems concerning short-circuit

3.3.1

Minimum anchor distances for conductors

One of the main problems which regard short-circuit

and which is to be faced directly by panel builders is the

maximum anchor distances of the conductors connected

to the circuit-breakers from the circuit-breaker terminals.

As known, inside the switchboards it is necessary that

cables and busbars are fastened to the frame.

In fact, during a short-circuit, the dynamic stresses on the

conductors could affect also the circuit-breaker terminals

causing damages. Figure 27 shows some diagrams

relevant to Tmax and Emax series circuit-breakers

allowing to determine, as a function of the maximum

peak current under short-circuit conditions and of the

circuit-breaker typology, the maximum distances from

the circuit-breaker terminals to the ﬁrst anchor element

of the conductors.

The term conductor is used for busbars when the

current exceeds or is equal to 400A, whereas cables are

referred to when the current is lower than this value. This

distinction is made complying with the Tables 8 and 9 of

the Std. IEC 60439-1.

When speciﬁc requirements demand or prescribe the

use of busbars also for currents lower than 400A, the

distances which can be obtained by the diagrams shall

not undergo any variations, whereas the distances

referred to the use of busbars are not valid when using

cables.

Distance suggested for the ﬁrst anchor element of busbars as a

function of the maximum prospective short-circuit current peak.

Figure 27

Tmax T1

350

300

250

200

150

100

50

0

10 100

I

pk

[kA]

L [mm]

43

ABB circuit-breakers inside LV switchboards

450

400

350

300

250

200

150

0

10

100

L [mm]

100

50

1000

I

pk

[kA]

I

pk

[kA]

500

400

350

300

250

200

150

0

10 100

L [mm]

100

50

1000

450

Tmax T3

Tmax T4

Tmax T2

Distance suggested for the ﬁrst anchor element of busbars as a

function of the maximum prospective short-circuit current peak.

Circuit-breaker with horizontal and vertical terminals..

Emax

3 Problems concerning short-circuit

Tmax T5

Valid for:

- front and rear terminals

- connection by means of rigid busbars

700

400

500

300

200

0

10 100

L [mm]

100

1000

600

I

pk

[kA]

600

400

500

300

200

0

10 100

L [mm]

100

1000

I

pk

[kA]

0

40

I

pk

[kA]

60 80 100 120 140 160 180 200 220 240 260 280 300

50

100

150

200

250

300

350

400

450

500

L [mm]

E4 S-H

E6 H-V

E2 B-N

E3 N-S-H

E1 B

E2 L

E3 L

44 ABB circuit-breakers inside LV switchboards



3 Problems concerning short-circuit

Figure 28

200200

300300

Tmax T1 Tmax T3

Tmax T5 Tmax T6 Tmax T7

200200

Tmax T2

5050

1÷10mm2

200200

5050

16÷70mm2

6060

2.5÷10mm2

200

Tmax T4

As regards Tmax molded-case circuit-breakers, Figure

28 gives an example of the maximum distance (in mm)

suggested for the positioning of the nearest anchor sup-

port, in relation to the maximum peak current admitted

for the circuit-breaker.

45

ABB circuit-breakers inside LV switchboards

3 Problems concerning short-circuit

As regards Emax air circuit-breakers, Figure 29 gives an

example of the maximum distance (in mm) suggested

for the positioning of the nearest anchor support for the

busbars connecting to the circuit-breaker according to the

different types of terminals available and for the

highest

peak values, as derived from the curves of Figure 28.

Figure 29

3.3.2 Veriﬁcation of the short-circuit withstand

strength and of the current limiting

characteristics of circuit-breakers

In some cases, the Std. IEC 60439-1 allows that the short-

circuit withstand strength of assemblies is not veriﬁed.

In particular, the following switchgear and controlgear

assemblies are free from veriﬁcation:

-

those having rated short-time withstand currents or rated

conditional short-circuit currents lower than 10kA;

- those protected by current limiting devices with a limited

peak current not exceeding 17kA in correspondence

with the maximum prospective short-circuit current

measured at the terminals of the incoming circuit of

the assembly.

As known, the limiting characteristics of a circuit-breaker are a

function of the working voltage of the circuit-breaker itself.

Table 22 below gives - for the different protective circuit-

breakers and the most common voltages of the plant

– the values which approximately represent the maximum

prospective short-circuit current in [kA] which guarantee

a limited peak current not exceeding 17kA, so that the

short-circuit withstand test for the switchboard is not to

be carried out.

Table 22

Circuit-breaker Rated voltage of the plant

Type

Rated

current In

[A] 230Vac 415Vac 500Vac 690Vac

S200 ≤63 20 10 - -

S200M ≤63 25 15 - -

S200P ≤25 40 25 - -

S200P 32-63 25 15 - -

S800 ≤125 50 50 15(In≤80A) 6(In≤80A)

10(In≥80A) 4.5(In≥80A)

S290 ≤125 25 15 - -

T1 <160 50 35 15 6

T1 160 37 33 15 6

T2 ≤32 120 85 50 10

T2 ≤50 120 85 39 10

T2 ≤63 120 65 30 10

T2 80 -160 120 50 29 10

T3 63 37 20 18 8

T3 80 27 18 17 8

T3 100 21 16 15 8

T3 125-160 18 15 14 8

T3 200-250 16 14 13 8

T4 20 200 200 150 80

T4 32-50 200 200 150 55

T4 80 200 100 48 32

T4 100-320 200 24 21 19

T5 T6 T7 320-1600 10 10 10 10

The short-circuit current value reported in the table above must be compared

with the breaking capacity of the circuit-breaker for the different versions

available.

Emax X1

Emax E1÷E6

200 200

200

P

Vertical

P

Horizontal

Front Flat

P

PP

Emax

Horizontal

P

[mm]

Vertical

P

[mm]

Front

P

[mm]

Flat

P

[mm]

E1-E2 250 250 - -

E3-E4-E6 150 150 - -

E1-E6 - - 250 250

46 ABB circuit-breakers inside LV switchboards



3.3.3 Problems concerning the installation

distances

The Std. IEC 60439-1 assigns the circuit-breaker ma-

nufacturers the task of deﬁning the prescriptions for the

installation of these devices inside switchboards.

Hereunder, Figures 30 and 31 give, for ABB SACE cir-

cuit-breakers series Tmax and Emax respectively, the

indications relevant to the distances to be complied with

in the plants up to 690 Va.c.; such distances are already

speciﬁed in the circuit-breaker technical catalogues and

installation manuals.

Figure 30

3 Problems concerning short-circuit

A

B

C

I

HH

H

Insulation distances for installation in metallic cubicle

T1

T2

T3

T4

T5

T6

T7

A

[mm]

25

50

30(*)

35(*)

50(*)

B

[mm]

20

25

20

C

[mm]

20

25(*)

20

10

(*) For Un ≥ 440V and T6L all versions: distances A 100 mm

Note: For the insulation distances of the 1000 V circuit-breakers, please ask ABB SACE.

Distances between two circuit-breakers side by side or superimposed

For assembly side by side or superimposed, check that the connection busbars or cables

do not reduce the air insulation distance

Minimum centre distance for two circuit-breakers side by side

T1

T2

T3

T4

T5

T6

T7

3 poles

76

90

105

140

210

4 poles

102

120

140

184

280

Circuit-breaker width (mm)

3 poles

76.8

90

105

140(*)

210

4 poles

102

120

140

184(*)

280

Centre distance I [mm]

(*) For Un ≥ 500 V minimum centre I (mm) 3 poles 180, minimum centre I (mm) 4 poles 224

Minimum distance between two superimposed circuit-breakers

T1

T2

T3

T4

T5

T6

T7

60

90

140

160

180

H

[mm]

Cable terminal

Insulated cable

Connection -

not insulated

Note: The dimensions shown apply for operating voltage Un up to

690 V. The dimensions to be respected must be added to

the maximum dimensions of the various different versions

of the circuit-breakers, including the terminals. For 1000 V

versions, please ask ABB SACE.

47

ABB circuit-breakers inside LV switchboards

Figure 31

3 Problems concerning short-circuit

Compartment dimensions

Emax

E1

E2

E3

E4

E4f

E6

E6f

A

[mm]

400

500

700

-

1000

-

B

[mm]

490

630

790

880

1130

1260

Emax – withdrawable version

Emax – fixed version

380

500

A

3 poles

B

4 poles

242 min.

282 max

A

3 poles

B

4 poles

500

Nota: For Emax X1 CB, please consider the same indications of

Figure 30 referred to the insulation distance of Tmax T7 CB

48 ABB circuit-breakers inside LV switchboards



Annex A

Example of electrical switchboards with ABB circuit-breakers

This Annex contains considerations about two different typologies of switchboards with ABB circuit-breakers.

First example

Data table

Circuit Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9

Circuit-breaker E6H E3H E2L E2L S7L S7L S6L S5L S4L

Rated current In [A] 5000 2500 1250 1250 1600 1250 630 400 250

Rated diversity factor 0.9 0.8 0.9 0.9 0.85 0.85 0.85 0.85 0.85

Test current [A] 4500 2000 1125 1125 1360 1062.5 535.5 340 212.5

Rated short-time current Icw [kA] 100 100 - - - - - - -

Rated peak current Ipk [kA] 220 220 - - - - - - -

Rated conditional short-circuit current Icc [kA] - - 100 100 100 100 100 100 100

Switchboard characteristics

Switchboard dimensions Heigth: 2300 mm Width: 2900 mm Depth: 1100 mm

Degree of protection IP3X Form of separation 4B

The table above reports the data concerning the circuit-breakers installed in the switchboard and the relevant effective current carrying

capacities obtained by the tests carried out in compliance with IEC 60439-1.

From these results it is evident that, if the switchboard has been designed according to a correct layout and with suitable forms of se-

paration, by positioning the apparatuses properly and with conductors and busbars rated in compliance with cross-sectional areas and

minimum length prescribed by the Standard, the current carrying capacities of the circuit-breakers inside the switchboard shall be very

near to the rated ones.

From this table it results also how the main distribution circuit (orange trace), which is provided with a non-current-limiting circuit-breaker,

shall be rated to withstand the conditioned current for 1 second and its relevant peak; on the other hand, the distribution circuits (brown

trace), which are equipped with a current limiting circuit-breaker, shall be dimensioned according to the conditioned short-circuit current:

thus, for the section C, the circuit-breakers shall be rated to withstand the peak and speciﬁc energy let-through by E2L, whereas, for the

section A, they shall be sized according to the peak and speciﬁc energy let-through by S7L. Dimensioning carried out in compliance with

this criterion is valid only if it can guarantee that the possibility that a fault occurs on the distribution busbar is null. If not, the distribution

busbar too shall be dimensioned as the main busbar.

Annex A

S7L - 1600A

S7L - 1250A

S6L - 630A

S5L - 400A

S4L - 250A

E6 - 5000A E3 - 2500A

E2 - 1250A

Supply

Section A Section B Section C

Configuration of the switchboard front

SACE

Q5

Q6

Q7

Q8

Q9

Q1 Q2

Q3

Q4

49

ABB circuit-breakers inside LV switchboards

Second example

Data table

Circuit Q1 Q2 Q3 Q4 Q5

Circuit-breaker E6H S7H T5H T5H T4H

Rated current In [A] 5000 1250 630 400 250

Rated diversity factor 11111

Test current [A] 5000 1250 630 400 250

Rated short-time current Icw [kA] 100 - - - -

Rated peak current Ipk [kA] 220 - - - -

Rated conditional short-circuit current Icc

[kA] 100 100 100 100

Switchboard characteristics

Switchboard dimensions Heigth: 2320 mm Width: 1800 mm Depth: 1240 mm

Degree of protection IP30 Form of separation 4

The conductors and busbars used for the realization of the circuits of this switchboard are of bigger cross-sections than those suggested

by the Standard. Under these conditions, as it results from an analysis of the data reported in the table, the effective current carrying

capacities of the circuits inside the switchboard turn out to coincide with the rated carrying capacities of the circuit-breakers.

The same remarks of the previous case are valid also for the veriﬁcation of the busbar protection.

Annex A

Configuration of the switchboard front

Section A Section B

ChimneyChimney

SACE

E6H

withdrawable

5000A

S7

withdraw.

1250A

T5

withdraw.

630A

T5

withdraw.

400A

T4

fixed

250A

Q1

Q2

Q3

Q4

Q5

50 ABB circuit-breakers inside LV switchboards



Forms of internal separation

By dividing assemblies by means of barriers or partitions (metallic or non-metallic) into separate compartments or

enclosed protected spaces, protection against contact with hazardous live parts belonging to the adjacent functional

units and protection against the passage of solid foreign bodies from one unit of the assembly to an adjacent one

can be attained.

The following are typical forms of separation by barriers or partitions:

Annex B

Form 1

No internal separation

Form 2

Separation of busbars from the functional

units

Form 2a

Terminals for external conductors not

separated from busbars

Form 2b

Terminals for external conductors separated

from busbars

Form 3

Separation of busbars from the functional units

and separation of all functional units from one

another. Separation of terminals for external

conductors from the functional units, but not

from those of other functional units

Form 3a

Terminals for external conductors not separated

from busbars

Form 3b

Terminals for external conductors separated

from busbars

Form 4

Separation of busbars from all functional units

and separation of all functional units from one

another. Separation of terminals for external

conductors associated with a functional unit

from those of any other functional unit and

the busbars

Form 4a

Terminals for external conductors in the same

compartment as the associated functional

unit

Form 4b

Terminals for external conductors not in

the same compartment as the associated

functional unit, but in individual, separate,

enclosed protected spaces or compartments

Caption

aEnclosure

b Internal separation

cFunctional units including terminals for associated external conductors

dBusbars, including distribution busbars

b

a

d

c

Symbols

51

ABB circuit-breakers inside LV switchboards

Annex C

Degrees of protection (IP code)

As example, the table below reports the minimum degrees of protection for a switchgear and controlgear assembly

to be installed in the speciﬁed environments in compliance with the above mentioned Standards.

The degree of protection prescribed for an apparatus against access to hazardous live parts and against ingress of

solid foreign objects and liquids is indicated by the Code IP... in accordance with the Std. IEC 60529.

A brief description of the IP Code elements is given hereunder. For full details reference shall be made to the Std.

IEC 60529.

When a characteristic numeral is not required to be speciﬁed, it shall be replaced by the letter “X” (“XX” if both numerals are omitted).

Code letters (International Protection) IP

First characteristic numeral (numerals 0 to 6, or letter X) Against ingress of solid foreign objects

Second characteristic numeral (numerals 0 to 8, or letter X)

Against ingress of water with harmful effects

Additional letter (optional) (letters A, B, C, D): Against access to hazardous parts

Supplementary letter (optional) (letters H, M, S, W): Supplementary information

Type of assemblies / Type of environment Standards and sub-clause Minimum degree of protection

Switchgear and controlgear assembly: enclosed switchboard IEC 60439-1 sub-clause 2.3.3 Not deﬁned

Assemblies for outdoor installation IEC 60439-1 sub-clause 7.2.1.3 IPX3

Assemblies with protection by total insulation IEC 60439-1 sub-clause 7.4.3.2.2 IP2XC

Installations in normal environments

Live parts which are not be touched intentionally IEC 60364-4 sub-clause 412.2.1 IPXXB (IP2X)

Live parts which are readily accessible (horizontal top) IEC 60364-4 sub-clause 412.2.2 IPXXD (IP4X)

Installations in locations containing a bath tube or shower basin

Zones 1 and 2 IEC 60364-7 sub-clause 701.512.2 IPX4

Zone 3 IEC 60364-7 sub-clause 701.512.2 IPX1

Zones 1–2–3 public baths where water jets are used for

cleaning purposes IEC 60364-7 sub-clause 701.512.2 IPX5

Installations for swimming-pools

Zone 0 IEC 60364-7 sub-clause 702.512.2 IPX8

Zone 1 IEC 60364-7 sub-clause 702.512.2 IPX5

Zone 2 for indoor locations IEC 60364-7 sub-clause 702.512.2 IPX2

Zone 2 for outdoor location IEC 60364-7 sub-clause 702.512.2 IPX4

Zone 2 where water jets are used for cleaning purposes IEC 60364-7 sub-clause 702.512.2 IPX5

Installations for rooms and cabins containing sauna heaters IEC 60364-7 sub-clause 703.512.2 IP24

Assemblies for construction sites (ACS) IEC 60439-4 sub-clause 7.2.1.1 IP44

Annex C

52 ABB circuit-breakers inside LV switchboards



Glossary

fn rated diversity factor

Incrated current of the circuit

I2test, I3test.... test current of the circuit “2”, test current of the circuit “3”, etc.

TTabsolute temperature [°C]

TAair ambient temperature [°C]

∆Ttemperature-rise [K]

LV low voltage

PTTA partially type-tested low-voltage switchgear and controlgear assembly

d.c. direct current

a.c. alternating current

Ib full load current

PCB power loss of the circuit-breaker at Ib

PnCB power loss of the circuit-breaker at InCB

InCB rated current of the circuit-breaker

PSB power loss of the busbar at Ib

PnSB power loss of the busbar at InSB

InSB busbar rated current

LSB busbar length

PTQ total power dissipated inside the switchboard

AEeffective cooling surface

bsurface factor

A0sum of the individual surface areas

dtemperature coefﬁcient

IP degree of protection

Icw rated short-time current

Ipk rated peak withstand current

Icc rated conditional short-circuit current

Ik prospective short-circuit current

npeak factor

Technical Application Paper

QT1

Low voltage selectivity with ABB circuit-breakers

QT2

MV/LV trasformer substations: theory and exam-

ples of short-circuit calculation

QT3

Distribution systems and protection against indi-

rect contact and earth fault

QT4

ABB circuit-breakers inside LV switchboards

QT5

ABB circuit-breakers for direct current

applications

QT6

Arc-proof low voltage switchgear and controlge-

ar assemblies

1SDC007103G0202 May ’08

Printed in Italy

6.000 - CAL

December 2006

4Technical Application Papers

http://www.abb.com

Due to possible developments of standards as well as of

materials, the characteristics and dimensions speciﬁed in this

document may only be considered binding after conﬁrmation

by ABB SACE.

ABB SACE

A division of ABB S.p.A.

L.V. Breakers

Via Baioni, 35

24123 Bergamo - Italy

Tel.: +39 035.395.111 - Telefax: +39 035.395.306-433

ABB circuit-breakers inside LV switchboards

Figures

Recommended publications

Dcs in power station

Chapter Matlab Optimization

Gis insulated switchgear

Power station ISCS