Field measured total clearing time of protective devices and its effect on electrical hazards

Properly analyzing power system clearing times is vital in determining what the actual hazards are. These clearing times are measured in cycles or milliseconds. Many arc flash hazard analysis are done using the relay curve of a protective device when the actual clearing time of the circuit involves many more devices such as CT's, interposing relays and circuit breaker action. This actual clearing time of the new and service aged equipment in the field is very important in this overall analysis.