Breaking capacity

The breaking capacity of a circuit-breaker is themaximum level of fault current (kA) it can
safely interrupt.

Each circuit-breaker has two breaking capacities:

• Ultimate BreakingCapacity (Icu) –It is required that the circuit-breaker should be selected so that its Ultimate breaking capacity is equal to,or greater than, the prospective fault current (Ipf) at the point at which it is installed.
• Service BreakingCapacity (Ics) –It is recommended that the Service breaking capacity of a circuit-breaker should be equal to,or greater than, the prospective fault current (Ipf) at the load end of the circuit it is protecting.

Ultimate BreakingCapacity (Icu)

Icu is defined as the ‘Ultimate Breaking Capacity’ of a circuit-breaker – Icn is the ‘Normal Breaking Capacity of an MCB, with the same meaning as Icu.

This is the maximum level of fault current that the circuit-breaker can safely interrupt. Therefore, Icu (or Icn – as applicable) has to be equal to or greater than the maximum prospective fault current at the point of installation.

Service Breaking Capacity (Ics)

Ics is defined as the ‘Service Breaking Capacity’ of a circuit-breaker and is the maximum level of fault current that the circuit-breaker can interrupt, subject to more stringent requirements than those applicable to Icu. The requirement for Icu is simply to safely interrupt the fault current and it is acceptable that, having done this, the circuit-breaker may well be no longer serviceable. For Ics, the circuit- breaker must be able to perform a further close/open operation under fault conditions.

For some circuit-breakers, the Ics rating may be lower than the Icu rating (as low as 50%). For larger circuit-breakers the two ratings are often the same.

The standard, BS EN60947-2, recommends that any circuit-breaker should be selected and installed such that its Ics rating is equal to or greater than the maximum prospective fault current at the load end of the circuit in which it is installed. This is a recommendation not a requirement. Ics and its application, is not mentioned in BS 7671:2018 Amd 2. Where this recommended condition is not met, a warning message will be displayed.