Protective device coordination refers to the clearing of faults by the protective device closest to the fault, rather than by devices further upstream. Device coordination is important because clearing of the fault by the device closest to the fault avoids a wider than necessary power outage. Device coordination becomes very important when high value processes have the potential to be interrupted.
Consider a simple example of a sawmill with a main circuit breaker for the facility, and a small circuit breaker supplying power to a waste conveyor. In the event of a ground fault in the conveyor motor (a not so infrequent occurence), it is very important the fault be cleared by the small circuit breaker, avoiding a trip of the main breaker, so that the processing of logs can continue while the waste conveyor motor is repaired. When this correct fault clearing sequence occurs for all faults that are possible at the conveyor motor, the conveyor circuit breaker and the main breaker are said to be properly coordinated. If the main breaker were to trip in the event of a fault at the conveyor motor, there is miscoordination between the devices, and the breaker settings should be adjusted to correct the condition.
The tripping characteristic of a protective device determines at what time and current level the device will operate. In the case of electronic trip units and protective relays, the tripping characteristic is adjusted fairly easily through front panel switches or programming software. In the case of fuses, a fuse link with the desired characteristic is placed into the fuse holder.
Bench are experts in the selection of devices, coordination analysis, and recommendation of optimal settings for overcurrent and other protective devices. Please contact us for coordination analysis on your new or existing facility.
In many cases, protective device settings can be chosen to provide proper coordination while also reducing arc-flash incident energy levels. For this reason, coordination is often evaluated during an arc-flash hazard analysis.