Selective coordination is the application of circuit protective devices in series such that under overload or fault conditions, only the upstream device nearest the fault will open. The rest of the devices remain closed, leaving other circuits unaffected. In the following example a short circuit has occurred in the circuit fed by branch circuit breaker “C”. Power is interrupted to equipment supplied by circuit breaker “C” only. All other circuits remain unaffected.
Circuit Breaker Coordination
Time current curves are useful for coordinating circuit breakers. If the trip curves of main breaker “A”, feeder breaker “B”, and branch breaker “C” are placed on the same graph, there should be no overlapping, indicating the breakers are coordinated. The three circuit breakers in the following example have been coordinated so that for any given fault value, the tripping time of each breaker is greater than tripping time for the downstream breaker(s). In the following illustration, circuit breaker “C” is set to trip if a 400 amp fault current remains for .04 seconds. Circuit breaker “B” will trip if the fault remains for .15 seconds, and circuit breaker “A” if the fault remains for .8 seconds. If a 400 amp fault occurs downstream from circuit breaker “C”, it will trip first and clear the fault. Circuit breakers “A” and “B” will not trip.
When selecting circuit breakers, it is extremely important to know both the maximum continuous amperes and the available fault current. NEC® article 110.9 states:
Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.
There are two ways to meet this requirement. The first method is to select circuit breakers with individual ratings equal to or greater than the available fault current. This means that, in the case of a building with 65,000 amperes of fault current available at the service entrance, every circuit breaker must have an interrupting rating of at least 65,000 amperes.
The second method is to select circuit breakers with a series combination rating equal to or greater than the available fault current at the service entrance. The series-rated concept requires the main upstream breaker to have an interrupting rating equal to or greater than the available fault current of the system, but subsequent downstream breakers connected in series can be rated at lower values. For example, a building with 65,000 amperes of available fault current might only need the breaker at the service entrance to have an interrupting rating of 65,000 amperes. Additional downstream breakers can be rated at lower values. The series combination must be tested and listed by UL.