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.
The diagram of a simple electrical system resembles a tree configuration. The main power source is the trunk, and the primary feeder circuits and branch circuits are the large and small branches. Protective devices are located at strategic points in the system — usually at the main power entrance and the start of each primary and branch circuit.
If a fault occurs near the end of a branch circuit, the fuse or breaker immediately upstream should open before any other protective devices, limiting the resulting power outage to only the portion of the circuit downstream of the protective device. Similarly, if the fault occurs on a primary feeder, the fuse or breaker for that feeder should open before any other upstream protective devices.
Circuit breaker, relay, and fuse operating characteristics are graphically represented by time-current curves. These curves are a plot of operating time vs. current level. 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.
A correctly performed and enforced protective device coordination can prevent unplanned outages and workplace accidents, and extend the life of 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 — but no less frequently than every five years.