High Current Testing

Circuit breakers are controlled in various ways. Nearly all small, molded case circuit breakers are controlled locally and tripped on overload by internal, often thermal, devices. The only way to check these breakers is to inject them with full rated current. Often, three times the rated current is used for long-time delay, and the trip time is compared to factory-supplied specifications. The breaker is also tested on all other functions (e.g., instantaneous trip).

The same procedure can be followed for larger power circuit breakers. Some systems use current transformers (CT) and separate trip units that respond to secondary CT currents. These can be tested by injecting secondary currents through the overcurrent relays. While secondary current injection methods are usually satisfactory, occasionally the current transformer secondary circuit is faulty, and the apparently successful test has bypassed the faulty circuit.

DYMAX accordingly recommends testing with full rated currents injected through the primary circuits of the breaker. Though this involves the use of larger, heavier equipment and more testing time, the results are more reliable.