ISO 10924-2:2025
(Main)Road vehicles — Circuit breakers — Part 2: Guidance for users
Road vehicles — Circuit breakers — Part 2: Guidance for users
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.
Véhicules routiers — Coupe-circuits — Partie 2: Recommandations pour les utilisateurs
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Standards Content (Sample)
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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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
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
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
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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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
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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
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 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.
ISO 1
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