Breaking Capacity

Every protective device — whether circuit breaker, fuse, or MCCB — has a maximum current it can safely interrupt. Beyond this limit, the device may fail to extinguish the arc, resulting in sustained arcing, device destruction, enclosure damage, or fire. IEC 60947-2 defines two breaking capacity ratings: Icu (ultimate breaking capacity) is the maximum current the device can interrupt and remain intact but may not be fully operational afterward, while Ics (service breaking capacity) is the maximum current after which the device remains fully operational. Ics is typically expressed as a percentage of Icu (25%, 50%, 75%, or 100%). For most installations, the device's Icu must equal or exceed the prospective fault current at its location. In situations where the prospective fault current exceeds the device's breaking capacity, back-up protection (cascade coordination) with an upstream current-limiting device can be used — the upstream device limits the let-through energy to within the downstream device's capability. Miniature circuit breakers (MCBs) typically have breaking capacities from 6 kA to 10 kA, while moulded-case circuit breakers (MCCBs) range from 25 kA to over 150 kA for industrial applications.