Protective Device Coordination

Protective Device Coordination


Protective device coordination is the process of selecting and arranging protective devices to limit the effects of an overcurrent (short-circuit) situation to the smallest area. This study can be used to determine fuse sizes, settings for breakers and relays, or as an aid to selection of new equipment.

Protective devices should be coordinated by a qualified engineer when an electrical system is first designed, and updated when major modifications or renovation takes place.

Protective devices are typically designed to interrupt the current more quickly for higher current values and slower for lower current values. Each type of protective device has a unique curve or set of curves that manufacturers and engineers use to represent its operating characteristics. From these curves, you can tell how long it will take for the protective device to interrupt at any value of current. Most relays and electronically controlled breakers have a wide range of field-adjustable trip settings and operating curves. Protective device coordination involves analyzing the characteristics of each protective device to confirm the sequence in which they’ll operate and adequate margin between the operating times of each.

Often maintenance technicians do not understand the implications of changing protective device settings on the overall coordination scheme. Any time a change is planned, an engineer needs to verify that it doesn’t affect the coordination.

We perform these studies in accordance with IEEE Std. 242-2001 (Buff Book)

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