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. This presentation will look at recent field test data collected that measures the actual clearing time of devices. The timing tests will include the inception of short circuit current until the final clearing time of the power contacts. From the time the initial current wave hits the current transformer through the relaying protection and lockout relay (or tripping relay) to the breaker trip coil circuitry, mechanism and power contacts. The presentation will review the test protocol and the results of these tests will be discussed along with the impact on electrical safety. A complete analysis of the ldquofirst triprdquo tripping time will be presented. It is typical for circuit breakers that are in service and have not operated for some time to operate much slower on their first trip. Once the breaker is exercised even one time, the speed of the clearing time is much faster, but in many cases it is that first trip time that will determine the actual hazard clearing time. The presentation will look at test procedures to determine first trip timing and results of these tests will be presented. A variety of scenarios will be covered utilizing typical in service equipment of different vintages, designs and configurations. The results and their impact on the hazards will be discussed.