Breaking Capacity
Breaking capacity is the largest short-circuit current that a fuse or circuit breaker can interrupt without damaging itself and without sustaining an arc. If the prospective short-circuit current is above this value, the protective device may burst while opening or its contacts may weld; the circuit is not interrupted.
The only things limiting the current that flows during a short circuit are the internal resistance of the source and the resistance of the cable. When a battery is shorted directly, thousands of amperes can flow; interrupting this current is a far harder job than interrupting the rated current, because an arc forms between the contacts as they separate.
- In fuses: sand-filled bodies are designed to quench the arc; empty glass bodies have a lower capacity.
- Near the battery: the prospective short-circuit current is highest here, so the breaking capacity of the main protection matters especially.
- Selection: the breaking capacity on the device label must be above the prospective short-circuit current at that point.
This value is independent of the rated amperage: of two 30 A fuses, one may interrupt 1 kA and the other a much higher short-circuit current. In caravan and machinery systems with high-capacity battery banks, do not select the main protective device by looking at the amperage alone.
Context in which this term is used: Installation, redundancy and field continuity