ISO 10924-2:2014

Road vehicles — Circuit breakers — Part 2: User's guide

ISO 10924-2:2014 gives guidance for the choice and application of automotive circuit breakers. It describes the various parameters which have to be taken into account when selecting circuit breakers.

Véhicules routiers — Coupe-circuits — Partie 2: Guide de l'utilisateur

General Information

• Status: Published
• Publication Date: 17-Feb-2014
• ICS: 43.040.10 - Electrical and electronic equipment
• Technical Committee: ISO/TC 22/SC 32 - Electrical and electronic components and general system aspects
• Drafting Committee: ISO/TC 22/SC 32/WG 5 - Fuses and circuit breakers
• Current Stage: 9092 - International Standard to be revised
• Completion Date: 24-Feb-2021

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 10924-2
First edition
2014-02-15
Road vehicles — Circuit breakers —
Part 2:
User’s guide
Véhicules routiers — Coupe-circuits —
Partie 2: Guide de l’utilisateur
Reference number
ISO 10924-2:2014(E)
©
ISO 2014

---------------------- Page: 1 ----------------------
ISO 10924-2:2014(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2014
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO 2014 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 10924-2:2014(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General . 1
4.1 Circuit breaker nominal voltage. 2
4.2 Supply voltage maximum (U ) . 2
smax
4.3 Rated current (I ) and continuous current . 2
R
5 Current and conductors (cables) . 3
6 Current and contact resistance . 4
7 Current and ambient temperature . 4
8 Cable protection: temperature versus current characteristics . 5
9 Selectivity . 7
10 Replacement of circuit breakers . 8
11 Voltage peaks during opening of circuit breakers . 8
12 Inrush withstand characteristics of circuit breakers . 8
13 Electromagnetic compatibility (EMC)
. 9
Annex A (informative) Selection procedure for circuit breakers and cables .10
Annex B (informative) Selection of circuit breakers .24
Annex C (informative) Ambient temperature influence.25
© ISO 2014 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 10924-2:2014(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information/TC 22, Road vehicles,
Subcommittee SC 3, Electric and electronic equipment.
ISO 10924 consists of the following parts, under the general title Road vehicles — Circuit breakers:
— Part 1: Definitions and general test requirements
— Part 2: User’s guide
— Part 4: Medium circuit breakers with tabs (Blade type), Form CB15
The following parts are under preparation:
— Part 3: Miniature circuit breakers
— Part 5: Circuit breakers with tabs with rated voltage of 450 V
iv © ISO 2014 – All rights reserved

---------------------- Page: 4 ----------------------
INTERNATIONAL STANDARD ISO 10924-2:2014(E)
Road vehicles — Circuit breakers —
Part 2:
User’s guide
1 Scope
This part of ISO 10924 gives guidance for the choice and application of automotive circuit breakers. It
describes the various parameters which have to be taken into account when selecting circuit breakers.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 8820-1, Road vehicles — Fuse-links — Part 1: Definitions and general test requirements
ISO 10924-1, Road vehicles — Circuit breakers — Part 1: Definitions and general test requirements
ISO 16750-1, Road vehicles — Environmental conditions and testing for electrical and electronic equipment —
Part 1: General
ISO 16750-2, Road vehicles — Environmental conditions and testing for electrical and electronic
equipment — Part 2: Electrical loads
3 Terms and definitions
For the purposes of this document, the terms and definitions in ISO 8820-1 and ISO 10924-1 and the
following apply.
4 General
The various parts of ISO 10924 define basic requirements and test methods for nominal voltage, rated
current, I , and time/current characteristics to give comparable and reproducible results of circuit
R
breakers.
In practice, however, there are other parameters to be considered for the correct selection of circuit
breakers in road vehicles, such as
— continuous current,
— operating time,
— overload protection of one or more electrical/electronic devices,
— connection resistance,
— types of cables, e.g. different cross section, length, insulation, bundling,
— internal resistances (voltage drop) of the circuit breakers, contacts, cables, and devices,
— power dissipation of the components comprising the system,
© ISO 2014 – All rights reserved 1

---------------------- Page: 5 ----------------------
ISO 10924-2:2014(E)

— short-circuit parameters,
— inrush parameters of devices,
— operating mode of the load,
— operating of one or more electrical/electronic devices,
— orientation and location of the circuit breakers, e.g. engine, passenger or luggage compartment,
— different currents, voltages, and temperatures of the system and surroundings,
— distances or clearances inside circuit breaker boxes or holders,
— different circuit breakers, circuit breaker holders and boxes (see Annex B),
— environmental conditions (mechanical loads, climatically loads, chemical loads), and
— forced cooling of the circuit breakers.
NOTE Users are advised to consult the manufacturers of circuit-breaker, contacts and cables, because not all
of the above points can be addressed in this guide.
The parameters listed are not intended to cover all the possible parameters that need to be taken
into consideration for circuit breaker selection nor is it intended that all parameters will need to be
considered in each vehicle applications.
4.1 Circuit breaker nominal voltage
See ISO 16750-1
4.2 Supply voltage maximum (U )
smax
See ISO 16750-2
4.3 Rated current (I ) and continuous current
R
The rated current (I ) is the current used for identifying the circuit breaker.
R
The continuous current (I ) in Figure 1 is the maximum current which the circuit can continuously
C
carry under specified conditions: ambient temperature (23 °C), duration maximum 1 h, standard test
holder, cross sections of wires. The continuous current can be lower than the rated current, I .
R
See A.2.2.3
2 © ISO 2014 – All rights reserved

---------------------- Page: 6 ----------------------
ISO 10924-2:2014(E)

Key
X current, I
Y operating time, t
I continuous current
C
I rated current
R
Figure 1 — Rated current (I ), continuous current, and time-current characteristic
R
5 Current and conductors (cables)
The temperature rise of a cable is a function of current, conductor cross-section, strands, insolating
materials time duration, and ambient temperature.
See A.2.2.4
Figure 2 shows stabilized temperature rise for various conductor cross sections at RT.
© ISO 2014 – All rights reserved 3

---------------------- Page: 7 ----------------------
ISO 10924-2:2014(E)

Key
X1 current, I
X2 conductor cross section
Y conductor temperature, T
Figure 2 — Conductor temperatures for different conductor cross sections vs. current at RT
6 Current and contact resistance
A higher resistance of mated terminals will result in a temperature rise and reduced thermal conduction
away from the circuit breaker. Hence, the temperature of the circuit breaker terminal will be higher and
the continuous current for the application lower.
A temperature rise test can be conducted using circuit breakers, circuit breaker holders and connections
as specified by the vehicle manufacturer. At a specified test current, the temperature of the connections
shall be measured at the points, either tabs or bolt connection of the circuit breaker that protrude from
the base of the circuit breaker body (specified in the appropriate part of the ISO 10924 according to the
type of the circuit breaker). After thermal equilibrium has been achieved, the temperature rise of the
connection shall not exceed the limits specified for terminals and cable.
7 Current and ambient temperature
All components of a circuit and their parts have their own characteristic curve as shown in Figure 3.
Each component in a circuit has an upper temperature limit. An increase of temperature results in
increased resistance, which can increase the temperature by itself. As a result, the circuit breaker can trip.
It is always recommended to consult with specific manufacturers of circuit breakers for current versus
temperature curves as both design and thermal materials used result in different curve characteristics.
See A.2.2.4 and Annex C
4 © ISO 2014 – All rights reserved

---------------------- Page: 8 ----------------------
ISO 10924-2:2014(E)

Key
X ambient temperature, T
Y current, I
1 application area
2 cable
3 connection
4 insulator
5 circuit breaker
Figure 3 — Maximum continuous currents of circuit components vs. ambient temperature
8 Cable protection: temperature versus current characteristics
To ensure satisfactory cable protection, circuit breakers shall be chosen such that they will always
open before the maximum allowed cable temperature, T exceeds. Figure 4 shows the correct circuit
max,
breaker selection. The maximum allowed temperature never exceeds, because above a certain minimal
operating current (I ), the circuit breaker will trip before the maximum permitted temperature of the
f
cable exceeds.
See Annex A
© ISO 2014 – All rights reserved 5

---------------------- Page: 9 ----------------------
ISO 10924-2:2014(E)

Key
X times rated current
Y cable temperature, T
I trip current
t
T maximum allowed cable temperature
max
Figure 4 — Correct circuit breaker selection
Figure 5 shows incorrect circuit breaker selection. The circuit breaker allows some potentially damaging
current to flow for too long, causing the cable to overheat.
6 © ISO 2014 – All rights reserved

---------------------- Page: 10 ----------------------
ISO 10924-2:2014(E)

Key
X times rated current
Y cable temperature, T
I trip current
t
T maximum allowed cable temperature
max
1 unprotected region
Figure 5 — Incorrect circuit breaker selection
9 Selectivity
It shall be ensured that higher level circuit breakers do not trip when lower level circuit breakers are
opening (see Figure 6).
© ISO 2014 – All rights reserved 7

---------------------- Page: 11 ----------------------
ISO 10924-2:2014(E)

Key
1 battery
2 circuit breaker level 1
3 circuit breaker level 2
4 circuit breaker level n
5 load
Figure 6 — Example for a structure hierarchy
10 Replacement of circuit breakers
The replacement of circuit breakers in a circuit shall be performed with the circuit de-energised.
11 Voltage peaks during opening of circuit breakers
During the opening process of the circuit breaker, voltage peaks can occur. The peaks can achieve six
times the rated voltage, depending on the load and the supply.
12 Inrush withstand characteristics of circuit breakers
In selecting a circuit breaker, not only the continuous current and the rated current, I , are to be
R
considered, but also the inrush characteristics of electrical devices.
The inrush characteristic describes the time-current behaviour of electrical devices until the stabilized
continuous current has been attained.
It is important to consider the inrush withstand characteristics as there are different requirements on
the circuit breaker depending on the type of load. The circuit breaker shall withstand the inrush energy
without opening. If the inrush energy is either too high or too long, or a combination thereof, it can be
necessary to select a higher rated circuit breaker to eliminate nuisance openings.
See A.2.2.5
8 © ISO 2014 – All rights reserved

---------------------- Page: 12 ----------------------
ISO 10924-2:2014(E)

13 Electromagnetic compatibility (EMC)
EMC test for circuit breakers are not required by this International Standard.
© ISO 2014 – All rights reserved 9

---------------------- Page: 13 ----------------------
ISO 10924-2:2014(E)

Annex A
(informative)

Selection procedure for circuit breakers and cables
A.1 Introduction
In any given application, the characteristics of load, connecting cable, and circuit breaker should be
carefully matched. This is necessary if the circuit breaker is to provide the expected degree of protection
in the event of an overcurrent in the circuit and to maintain that level of protection throughout the
lifetime of the vehicle.
Key
U nominal voltage
N
1 circuit breaker
2 cable
3 load
Figure A.1 — Scheme of a generic circuit with a circuit breaker
The protection of a load in a vehicle electrical system is typically performed by a protection element
close to the load. But there are loads where the protection of the cable is also a sufficient protection for
the load itself at the same time.
Conventional cables consist in general of a copper-core and an insulation-layer. The copper-core heats
up when the cable is exposed to current. If an overload occurs, the insulation-layer can be damaged or
can even start to melt.
There are three main reasons that can lead to this failure:
— the cross-section of the cable is too small to carry the current, which means voltage drop of the
cable is too high;
— not intended overcurrent leads to critical overheating of the copper-core and the insulation-layer,
although it was dimensioned correctly;
— wire breakage, loose contact, or other damages of the cables lead to an arc.
10 © ISO 2014 – All rights reserved

---------------------- Page: 14 ----------------------
ISO 10924-2:2014(E)

To reliably master the first and second cause, it’s mandatory to evaluate the cross section of the cable
and the rated current, I , of the circuit breaker. After the evaluation of the cable cross section and the
R
rated current, I , of the circuit breaker, these figures shall be adjusted to the cable insulation class. If
R
possible, the behaviour of the load should be considered.
Regarding the third point, there are solutions available in means of arc tracking to detect low current flow
between two conductors. This is not part of this user guide. For further information, the manufacturer
shall be contacted.
There are various factors that should be taken into account (see Clause 4) when determining the value
of rated current, I to be used for selection of circuit breakers and cables. In the example below, the
R,
following factors are considered:
— continuous current;
— ambient temperature [set to room temperature (RT) and 60 °C];
— operating mode of the load (shall be continuous duty);
— inrush withstand characteristics of circuit breakers (see Clause 12).
Other factors should be discussed between the circuit breaker manufacturer and the user.
The procedure that follows in A.2.1 and A.2.2 gives guidelines for
...

DRAFT INTERNATIONAL STANDARD ISO/DIS 10924-2
ISO/TC 22/SC 3 Secretariat: DIN
Voting begins on Voting terminates on

2011-12-16 2012-05-16
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION  •  МЕЖДУНАРОДНАЯ ОРГАНИЗАЦИЯ ПО СТАНДАРТИЗАЦИИ  •  ORGANISATION INTERNATIONALE DE NORMALISATION


Road vehicles — Circuit breakers —
Part 2:
User's guide
Véhicules routiers — Coupe-circuits —
Partie 2: Guide de l'utilisateur
ICS 43.040.10








In accordance with the provisions of Council Resolution 15/1993 this document is circulated in
the English language only.
Conformément aux dispositions de la Résolution du Conseil 15/1993, ce document est
distribué en version anglaise seulement.

To expedite distribution, this document is circulated as received from the committee
secretariat. ISO Central Secretariat work of editing and text composition will be undertaken at
publication stage.
Pour accélérer la distribution, le présent document est distribué tel qu'il est parvenu du
secrétariat du comité. Le travail de rédaction et de composition de texte sera effectué au
Secrétariat central de l'ISO au stade de publication.



THIS DOCUMENT IS A DRAFT CIRCULATED FOR COMMENT AND APPROVAL. IT IS THEREFORE SUBJECT TO CHANGE AND MAY NOT BE
REFERRED TO AS AN INTERNATIONAL STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS BEING ACCEPTABLE FOR INDUSTRIAL, TECHNOLOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT, WITH THEIR COMMENTS, NOTIFICATION OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPORTING DOCUMENTATION.
©  International Organization for Standardization, 2011

---------------------- Page: 1 ----------------------
ISO/DIS 10924-2

Copyright notice
This ISO document is a Draft International Standard and is copyright-protected by ISO. Except as permitted
under the applicable laws of the user’s country, neither this ISO draft nor any extract from it may be
reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic,
photocopying, recording or otherwise, without prior written permission being secured.
Requests for permission to reproduce should be addressed to either ISO at the address below or ISO’s
member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Reproduction may be subject to royalty payments or a licensing agreement.
Violators may be prosecuted.

ii © ISO 2011 – All rights reserved

---------------------- Page: 2 ----------------------
ISO/DIS 10924-2
Contents Page
Foreword . iv
1  Scope . 1
2  Normative references . 1
3  Terms and definitions . 1
4  General . 1
4.1  Circuit breaker nominal voltage . 2
4.2  Supply voltage maximum U . 2
smax
4.3  Rated current I and continuous current . 2
R
5  Current and conductors (cables) . 3
6  Current and contact resistance . 4
7  Current and ambient temperature . 4
8  Cable protection: temperature versus current characteristics . 4
9  Selectivity . 5
10  Replacement of circuit breakers . 6
11  Voltage peaks during opening of circuit breakers . 6
12  Inrush withstand characteristics of circuit breakers . 6
13  Electromagnetic compatibility (EMC) . 6
Annex A (informative) Selection procedure for circuit breakers and cables . 7
Annex B (informative) Selection of circuit breakers . 21
Annex C (informative) Ambient temperature influence . 22

© ISO 2011 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO/DIS 10924-2
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 10924-2 was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 3, Electric
and electronic equipment.
ISO 10924 consists of the following parts, under the general title Road vehicles — Circuit breakers:
⎯ Part 1: Definitions and general test requirements
⎯ Part 2: User's guide
⎯ Part 4: Medium circuit breakers with tabs (Blade type), Form CB 15
The following parts are under preparation:
⎯ Part 3: Miniature circuit breakers with tabs (Blade type), Form CB 11
⎯ Part 5: High current circuit breakers with tabs (Blade type), Form CB 29
⎯ Part 6: Circuit breakers with bolt-in contacts

iv © ISO 2011 – All rights reserved

---------------------- Page: 4 ----------------------
DRAFT INTERNATIONAL STANDARD ISO/DIS 10924-2

Road vehicles — Circuit breakers —
Part 2:
User's guide
1 Scope
This part of ISO 10924 gives guidance for the choice and application of automotive circuit breakers. It
describes the various parameters which have to be taken into account when selecting circuit breakers.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 8820-1, Road vehicles -- Fuse-links -- Part 1: Definitions and general test requirements
ISO 10924-1, Road vehicles – Circuit breakers - Part 1: Definitions and general test requirements
ISO 16750-1, Road vehicles - Environmental conditions and testing for electrical and electronic equipment --
Part 1: General
ISO 16750-2, Road vehicles - Environmental conditions and testing for electrical and electronic equipment --
Part 2: Electrical loads
3 Terms and definitions
For the purposes of this part of ISO 10924 the terms and definitions given in ISO 8820-1 and ISO 10924-1
shall apply.
4 General
The various Parts of ISO 10924 define basic requirements and test methods for nominal voltage, rated current
I and time/current characteristics to give comparable and reproducible results of circuit breakers.
R
In practice however there are other parameters to be considered for the correct selection of circuit breakers in
road vehicles, such as
⎯ continuous current,
⎯ operating time,
⎯ overload protection of one or more electrical/electronic devices,
⎯ connection resistance,
⎯ types of cables: e.g. different cross section, length, insulation, bundeling,
© ISO 2011 – All rights reserved 1

---------------------- Page: 5 ----------------------
ISO/DIS 10924-2
⎯ internal resistances (voltage drop) of the circuit breakers, contacts, cables and devices,
⎯ power dissipation of the components comprising the system,
⎯ short-circuit parameters,
⎯ inrush parameters of devices,
⎯ operating mode of the load,
⎯ operating of one or more electrical/electronical devices
⎯ orientation and location of the circuit breakers: e.g. engine, passenger or luggage compartment,
⎯ different currents, voltages and temperatures of the system and surroundings,
⎯ fixtures and boxes used with circuit breakers and/or circuit breakers,
⎯ orientation and location of the circuit breakers: e.g. engine, passenger or luggage compartment,
⎯ distances or clearances inside circuit breaker boxes or holders,
⎯ different circuit breaker holders and boxes
⎯ environmental conditions (mechanical loads, climatically loads, chemical loads
⎯ forced cooling of the circuit breakers.
NOTE Users are advised to consult the manufacturers of circuit-breaker, contacts and cables, because not all of the
above points can be addressed in this guide.
The parameters listed are not intended to cover all the possible parameters that need to be taken into
consideration for circuit breaker selection nor is it intended that all parameters will need to be considered in
each vehicle applications.
4.1 Circuit breaker nominal voltage
The nominal voltage identifying the voltage of the system shelf (see ISO 16750-1).
4.2 Supply voltage maximum U
smax
The circuit breaker has to withstand the maximum supply voltage of the electrical system of the vehicle (see
ISO 16750-2).
4.3 Rated current I and continuous current
R
The rated current (I ) is the current used for identifying the circuit breaker.
R
The continuous current (I ) in Figure 1 is the maximum current witch the circuit can continuously carry under
C
specified conditions: ambient temperature (23°C), duration maximum 1 h, standard test holder, cross sections
of wires. The continuous current can be lower than the rated current I .
R
2 © ISO 2011 – All rights reserved

---------------------- Page: 6 ----------------------
ISO/DIS 10924-2

Figure 1 — Rated current I , continuous current and time-current characteristic
R
5 Current and conductors (cables)
The temperature rise of a cable is a function of current, conductor cross-section, strands, insolating materials
time duration and ambient temperature.
Figure 2 shows stabilised temperature rise for various conductor cross sections at RT.

Figure 2 — Conductor temperatures for different conductor cross sections vs. current at RT
© ISO 2011 – All rights reserved 3

---------------------- Page: 7 ----------------------
ISO/DIS 10924-2
6 Current and contact resistance
A higher resistance of mated terminals will result in a temperature rise and reduced thermal conduction away
from the circuit breaker. Hence, the temperature of the circuit breaker terminal will be higher and the
continuous current for the application lower.
A temperature rise test may be conducted using circuit breakers, circuit breaker holders and connections as
specified by the vehicle manufacturer. At a specified test current the temperature of the connections shall be
measured at the points at the tabs of the circuit breaker where protrude from the base of the circuit breaker
body (See appropriate part of ISO 8820). After thermal equilibrium has been achieved the temperature rise of
the connection shall not exceed the limits as specified for terminals and cable.
7 Current and ambient temperature
All components of a circuit and their parts have their own characteristic curve as shown in Figure 3.
Each component in a circuit has an upper temperature limit. An increase of temperature result in increased
resistance, which can by itself increases the temperature. As a result, the circuit breaker may trip. It is always
recommended to consult with specific manufacturers of circuit breakers for current versus temperature curves
as both design and thermal materials used result in different curve characteristics.

cable
connection
insulator
Circuit breaker
Application
area

Figure 3 — Maximum continuous currents of circuit components vs. ambient temperature
8 Cable protection: temperature versus current characteristics
To ensure satisfactory cable protection, circuit breakers shall be chosen such that they will always open
before the maximum allowed cable temperature t is exceeded. Figure 4 shows the correct circuit breaker
max
selection. The maximum allowed temperature is never exceeded, because above a certain minimal operating
current (I ), the circuit breaker will trip before the maximum permitted temperature of the cable is exceeded.
f
4 © ISO 2011 – All rights reserved

---------------------- Page: 8 ----------------------
ISO/DIS 10924-2

Figure 4 — Correct circuit breaker selection
Figure 5 shows incorrect circuit breaker selection. The circuit breaker allows some potentially damaging
current to flow for too long, causing the cable to overheat.

Figure 5 — Incorrect circuit breaker selection
9 Selectivity
It shall be ensured that higher level circuit breakers do not trip when lower level circuit breakers are opening
(see Fig. 6).
© ISO 2011 – All rights reserved 5

---------------------- Page: 9 ----------------------
ISO/DIS 10924-2

Circuit breaker level 1
Circuit breaker level 2
Circuit breaker level n

Figure 6 — Example for a structure hierarchy
10 Replacement of circuit breakers
The replacement of circuit breakers in a circuit shall be performed with the circuit de-energised.
11 Voltage peaks during opening of circuit breakers
During the opening process of the circuit breaker voltage peaks can occur. The peaks can achieve 6 times the
rated voltage.
12 Inrush withstand characteristics of circuit breakers
In selecting a circuit breaker, not only the continuous current and the rated current I are to be considered but
R
also the inrush characteristics of electrical devices.
The inrush characteristic describes the time-current behaviour of electrical devices until the stabilised
continuous current has been attained.
It is important to consider the inrush withstand characteristics as there are different requirements on the circuit
breaker depending on the type of load. The circuit breaker shall withstand the inrush energy without opening.
If the inrush energy is either too high or too long, or a combination thereof, it may be necessary to select a
higher rated circuit breaker to eliminate nuisance openings.
13 Electromagnetic compatibility (EMC)
For circuit breakers as passive components EMC tests are not necessary.
6 © ISO 2011 – All rights reserved

---------------------- Page: 10 ----------------------
ISO/DIS 10924-2
Annex A
(informative)

Selection procedure for circuit breakers and cables
A.1 Introduction
In any given application, the characteristics of load, connecting cable and circuit breaker shall be carefully
matched. This is necessary if the circuit breaker is to provide the expected degree of protection in the event of
an overcurrent in the circuit and to maintain that level of protection throughout the lifetime of the vehicle.

Figure A.1 -- Scheme of a generic circuit with a circuit breaker
The protection of a load in a vehicle electrical system is typically performed by a protection element close to
the load. But there are loads, where the protection of the cable is also a sufficient protection for the load itself
at the same time.
Conventional cables consist in general of a copper-core and an insulation-layer. The copper-core heats up
when the cable is exposed to current. If an overload occurs, the insulation-layer can be damaged or may even
start to melt.
There are three main reasons that may lead to this failure:
⎯ the cross-section of the cable is too small to carry the current which means voltage drop of the cable is
too high,
⎯ a not intended overcurrent leads to a critical overheating of the copper-core and the insulation-layer,
although it was dimensioned correctly,
⎯ wire breakage, loose contact or other damages of the cables lead to an arc
To reliably master the first and second cause, it’s mandatory to evaluate the cross section of the cable and the
rated current I of the Circuit Breaker. After the evaluation of the cable cross section and the rated current I
R R
of the Circuit Breaker, these figures must be adjusted to the cable insulation class. If possible, the behaviour
of the load should be considered.
© ISO 2011 – All rights reserved 7

---------------------- Page: 11 ----------------------
ISO/DIS 10924-2
Regarding the third point, there are solutions available in means of arc tracking to detect low current flow
between two conductors. This is not part of this user guide. For further information, Manufacturer shall be
contacted.
There are various factors that should be taken into account (see clause 4) when determining the value of
rated current I to be used for selection of circuit breakers and cables.
...