Last edit: 04/07/2025
The doubt
What is the breaking capacity of a Circuit Breaker of of a Fuse (Branch Circuit Protective Device or BCPD)?
In North America, that is called the Interrupting Rating.
Consideration 1: the overload and the short-circuit
One of the problems related to the dangerousness of electrical current is the fact that cables and electrical equipment can catch on fire if crossed by a current bigger than the one foreseen in its normal use.
The two phenomena which can cause the occurrence of these cases are the following:
Overload: An overload is a condition in which a cable or an electrical equipment is subject to a current much bigger than is nominal current, but not at a level that can be caused by a short-circuit; generally we are in a situation where the circuit is healty, meaning fault free. This increase in the current, although not immediatly dangerous, like in the case of a short-circuit, can lead to an overheating of the cables and, in the long run, at the deterioration of the isolation and, potentially, a fire. The reference standard is IEC 60364-4-43, which defines the overload as a condition in which the current through a conductor exceeds its nominal value.
IEC 60364-4-43: 2023] 430.3 Terms and definitions
430.3.5 Overload: operating conditions in an electrically undamaged circuit, which cause an overcurrent.
430.3.2 Overload current: overcurrent occurring in an electric circuit, which is not caused by a short-circuit or an earth fault.
This can happen when too many devices are connnected on the same line or when a motor faces mechanical friction.
[IEC 60364-4-43: 2023] 430.3 Terms and definitions
430.3.1 Overcurrent: electric current the value of which exceeds a specified limiting value.
Note 2 to entry: An overcurrent is an overload current or a short-circuit current.
Short-circuit: a short-circuit is defined as a fault in an electrical circuit, in wich two points with different potential (for example, phase and neutral conductor, or two phases) are connected with very low or no resistance, causing a sudden and noticeable increase in the electric current.
[IEC 60364-4-43: 2023] 430.3 Terms and definitions
430.3.4 short-circuit: accidental or intentional conductive path between two or more conductive parts forcing the electric potential differences between these conductive parts to be equal to or close to zero.
This increase of current, much bigger than the nominal current of the circuit, can lead to damages, overheating, fires o malfunctioning of the connected devices.
Consideration 2: The Breaking Capacity
The Breaking capacity is the maximum current that a BCPD (Circuit Breaker or Fuse) is capable of interrupting in a safe way, without damaging itself, in the occurence of a Short Circuit.
[IEC 60947-2: 2024] 4.3.6.2 Rated short-circuit breaking capacities
4.3.6.2.1 General. The rated short-circuit breaking capacities of a circuit-breaker are the values of short-circuit breaking capacity assigned to that circuit-breaker by the manufacturer for the rated operational voltage, under specified conditions.
A rated short-circuit breaking capacity requires that the circuit-breaker shall be able to break any value of short-circuit current up to and including the value corresponding to the ratedcapacity at a power-frequency recovery voltage corresponding to the prescribed test voltage values […]
Unfortunatelly , for an appassionate reader of this topic, a BCPD can have more than one Breaking Capacity. The Product standards that defines all those values are two:
- EN 60947-2: for Circuit Breakers used in industrial applications
- EN 60898-1: for CB used in domestic applications
EN 60947-2: 2016. Low-voltage switchgear and controlgear – Part 2: Circuit-breakers
It defines two breaking capacities:
The Icu (extreme breaking capacity) defined as the maximun current that the CB is capable to open when the following sequence is applied: O-t-CO:
[IEC 60947-2: 2016] 8.3.2.6.4.4 Test operations. The following symbols are used for defining the sequence of operations:
- O represents a breaking operation.
- CO represents a making operation followed, after the appropriate opening time, by a breaking operation:
- t represents the time interval between two successive short-circuit operations which shall be as short as possible, allowing for the resetting time of the circuit-breaker (see 2.19), but not less than 3 min. The actual value of t shall be stated in the test report.
The maximum resetting time shall be 15 min or such longer time as may be stated by the manufacturer, but not exceeding 1 h, during which time the circuit-breaker shall not be displaced. Attempts to reclose the circuit-breaker during the resetting time shall be spaced by at least 1 min.
The maximum value of I2t (see 2.5.18 of IEC 60947-1:2007) during these tests may be recorded in the test report (see 7.2.1.2.4, item a)).,
After the test, the CB needs to maintain its nominal current In.
[IEC 60947-2: 2016] 4.3.6.2.2 Rated ultimate short-circuit breaking capacity (Icu) The rated ultimate short-circuit breaking capacity of a circuit-breaker is the value of ultimate short-circuit breaking capacity (see 2.15.1) assigned to that circuit-breaker by the manufacturer for the corresponding rated operational voltage, under the conditions specified in 8.3.5. It is expressed as the value of the prospective breaking current, in kA (r.m.s. value of the a.c. component in the case of a.c.)
The Ics (service breaking capacity) defined as the maximun current that the CB is capable to open when the following sequence is applied: O-t-CO-t-CO.
[IEC 60947-2: 2016] 8.3.2.6.4.4 Test operations. The following symbols are used for defining the sequence of operations:
- O represents a breaking operation.
- CO represents a making operation followed, after the appropriate opening time, by a breaking operation:
- t represents the time interval between two successive short-circuit operations which shall be as short as possible, allowing for the resetting time of the circuit-breaker (see 2.19), but not less than 3 min. The actual value of t shall be stated in the test report.
The maximum resetting time shall be 15 min or such longer time as may be stated by the manufacturer, but not exceeding 1 h, during which time the circuit-breaker shall not be displaced. Attempts to reclose the circuit-breaker during the resetting time shall be spaced by at least 1 min.
The maximum value of I2t (see 2.5.18 of IEC 60947-1:2007) during these tests may be recorded in the test report (see 7.2.1.2.4, item a)).,
After the test, the CB needs to maintain its nominal current In.
[IEC 60947-2: 2016] 4.3.6.2.3 Rated service short-circuit breaking capacity (Ics) The rated service short-circuit breaking capacity of a circuit-breaker is the value of service short-circuit breaking capacity (see 2.15.2) assigned to that circuit-breaker by the manufacturer for the corresponding rated operational voltage, under the conditions specified in 8.3.4. It is expressed as a value of prospective breaking current, in kA, or as a % of Icu (for example Ics = 25 % Icu). Ics shall be at least equal to 25 % of Icu.
Normally, the Ics is smaller than the Icn.
EN 60898-1: 2020 – Circuit-breakers for overcurrent protection for household and similar installations
It defines the Icn as the nominal beaking capacity
[IEC 60898-1:2020]: 5.2.4 Rated short-circuit capacity (Icn)
The rated short-circuit capacity of a circuit-breaker is the value of the ultimate short-circuit breaking capacity (see 3.5.5.1) assigned to that circuit-breaker by the manufacturer.
NOTE: A circuit-breaker having a given rated short-circuit capacity has a corresponding service short-circuit capacity (Ics).
The Icn, as the IEC 60898 states, needs to have some requirements of sequence similar to that of the Icu described in the IEC 60947-2 but, despite this, it will have some differences.
[IEC 60898-2: 2021]9.12.11.4.3 Test at rated short-circuit capacity (Icn)
Replacement of the first paragraph:
a) The test circuit is calibrated according to 9.12.7.1 and 9.12.7.2, on alternating current with a power factor according to Table 17, or on direct current with a time constant according to 9.12.5.
Addition:
c) In the case of direct current, the test sequence for single- and two-pole circuit-breakers is:
O – t – CO
Two operations are made, the circuit being closed once by the auxiliary switch A and once by the circuit-breaker.
Example
It exists a value k, being the ratio between the Ics and the Icn, which varies depending upon the Icn considered.
It is important to know that the number indicated in a rectangule in the front of the component, is the Icn according to EN 60898-1.
Here in the picture we can see a Circuit Breaker made by bticino (G23AC32 model). On it, if we observe in the low left corner, we will see it’s breaking capacity reported, wich indicates, like we see from the referential schedule, the value of the Icn according to IEC 60898-1.
Conclusion
The Breaking Capacity or Interrupting Rating of a Circuit Breaker is the maximum current that this device can interrupt.
There are two product standards, one is for CBs used in domestic applications, and the second is for those used in industrial applications.
The breaking capacity for the industrial standard is in general bigger than the one according with the “domestic” standard. Moreover, if a Circuit Breaker is compliant with the IEC 60898-1, it can also be used in industrial installations. But not the other way around.
For what regards the machinery, the manufacturer of the electrical equipment, for the calculation of the SCCR, can refer to the Icu according to the IEC 60947-2