ISO 10924-2:2025

International
Standard
ISO 10924-2
Second edition
Road vehicles — Circuit breakers —
2025-03
Part 2:
Guidance for users
Véhicules routiers — Coupe-circuits —
Partie 2: Recommandations pour les utilisateurs
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General . 1
4.1 Overview .1
4.2 Circuit breaker nominal voltage .2
4.3 Supply voltage maximum, U .2
Smax
4.4 Rated current, I , and continuous current .2
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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
14 Raw material . 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
Bibliography .26

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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
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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 document 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).
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patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
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this may not represent the latest information, which may be obtained from the patent database available at
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constitute an endorsement.
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related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 32,
Electrical and electronic components and general system aspects.
This second edition cancels and replaces the first edition (ISO 10924-2:2014), which has been technically
revised.
The main changes are as follows:
— added Clause 14.
A list of all parts in the ISO 10924 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
International Standard ISO 10924-2:2025(en)
Road vehicles — Circuit breakers —
Part 2:
Guidance for users
1 Scope
This document provides guidance for the choice and application of automotive circuit breakers. It describes
the various parameters that are taken into account when selecting circuit breakers.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8820-1 and ISO 10924-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 General
4.1 Overview
ISO 10924-1, ISO 10924-3, ISO 10924-4 and ISO 10924-5 define basic requirements and test methods for
nominal voltage, rated current, I , and time/current characteristics to give comparable and reproducible
R
results of circuit breakers.
In practice, however, there are other parameters which shall 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;
— 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);
— forced cooling of the circuit breakers.
NOTE Consult the manufacturers of the circuit-breaker, contacts and cables, because not all of the above points
can be addressed in this document.
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 are considered in each
vehicle application.
4.2 Circuit breaker nominal voltage
See ISO 10924-1:2025, Clause 3.
4.3 Supply voltage maximum, U
Smax
See ISO 10924-1:2025, Clause 3.
4.4 Rated current, I , and continuous current
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The rated current, I , is the current used for identifying the circuit breaker.
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The continuous current, I , in Figure 1 is the maximum current which the circuit can continuously carry
C
under specified conditions: room 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 I-t characteristics of the circuit breaker described in A.2.2.3.

Key
X current, I
Y operating time, t
I continuous current
C
I rated current
R
1 time current characteristic
Figure 1 — Rated current (IR), continuous current and time-current characteristic
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 room temperature.
See I-t - characteristic of the insulated conductor (cable) in A.2.2.4.
Figure 2 shows stabilized temperature rise for various conductor cross sections at room temperature (RT).

Key
X1 current, I
X2 conductor cross section
Y conductor temperature, T
Figure 2 — Conductor temperatures for different conductor cross sections versus current at room
temperature
6 Current and contact resistance
A higher resistance of mated terminals results in a temperature rise and reduced thermal conduction away
from the circuit breaker. Hence, the temperature of the circuit breaker terminal is 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 ISO 10924-1 according to the type of 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 I-t - characteristic of the insulated conductor (cable) A.2.2.4 and rerating factor Table C.1.

Key
X room 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. room 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 trips before reaching the maximum permitted temperature of the cable.
f
See the selection procedure for circuit breakers and cables in Annex A.

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.

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
Higher level circuit breakers shall not trip when lower level circuit breakers are opening (see Figure 6).

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
Circuit breakers in a circuit shall be replaced 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 shall the continuous current and the rated current, I , be considered,
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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.
The inrush withstand characteristics shall be considered 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 the rating based on the I-t - characteristic curves (load, circuit breaker, and cable) in A.2.2.5.

13 Electromagnetic compatibility (EMC)
The EMC test for circuit breakers is not required by this document.
14 Raw material
Alloys for contacts shall not contain heavy metals such as cadmium or lead.
The terminals shall be plated with materials as it is common to wire harness connector or socket plating to
avoid electrochemical corrosion (e.g. tin plating with tin-plated connectors or sockets).

Annex A
(informative)
Selection procedure for circuit breakers and cables
A.1 General
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 a level of protection throughout the lifetime of the vehicle.
See Figure A.1.
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 generally consist of a copper-core and an insulation-layer. The copper-core heats up
when the cable is exposed to a current. If an overload occurs, the insulation-layer can be damaged or can
even start to melt.
There are three main reasons for 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;
— an unintended overcurrent leads to critical overheating of the copper-core and the insulation-layer, even
if it was dimensioned correctly;
— wire breakage, loose contact or other damage to the cables led to an arc.
To reliably master the first and second cause, the cross section of the cable and the rated current, I , of the
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circuit breaker shall be evaluated. After the evaluation of the cable cross section and the rated current, I , of
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the circuit breaker, these figures shall be adjusted to the cable insulation class. If possible, the behaviour of
the load should be considered.
Regarding the third point, there are solutions available by means of arc tracking to detect low current flow
between two conductors. This is not part of this document. 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 following clause, the listed
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factors are considered:
— continuous current;
— ambient temperature (set to room temperature (RT) and 60 °C);
— operating mode of the load shall be co
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