Overcurrent coordination and interrupting rating
CERT 3 · Commercial & Industrial Systems · ~11 min
Commercial protection asks two questions a residential breaker never faces. First: can this device safely interrupt the largest fault current available at its location? That is the interrupting rating (AIC). A standard 10 kAIC breaker installed where 22 000 A of fault current is available will not merely fail — it can rupture. Available fault current is highest close to the transformer and falls with distance as conductor impedance grows.
Second: when a fault happens, does the NEAREST upstream device open, or does the main? That is selective coordination. Properly coordinated, a fault on one branch opens only that branch's device and the rest of the building keeps running. Poorly coordinated, the main trips and the whole facility goes dark for one bad receptacle. Coordination is achieved by comparing time-current curves so downstream devices always clear faster than upstream ones.
Fuses and breakers behave differently here. Current-limiting fuses clear in less than a half cycle, cutting off a fault before it reaches peak — valuable where available fault current is very high. Breakers are resettable and adjustable but generally slower.
Exam focus: define AIC and know it must equal or exceed available fault current; define selective coordination and know that the downstream device should open first; know that available fault current decreases with distance from the source.
Quiz — 3 questions
1. A breaker's interrupting rating must be:
2. In a selectively coordinated system, a branch-circuit fault opens:
3. Available fault current at a panel located far from the transformer is: