Coordinating and setting protective devices is an art in itself. It’s mostly at the engineers discretion and the clients priority when finding the middle ground for device settings. In any case, the following items should be considered when coordinating protective devices:
Ground (or Zero Sequence):
- In solidly grounded system set the ground relay pickup element to coordinate with the upstream (utility) relay. Make sure the settings coordinate with fuses as well.
- In resistance grounded system or ungrounded systems, the relay pickup can be set more sensitive i.e. in the 25-50Amp range. Coordination with the fuses is not necessary since the ground fault current can be so low that fuses take a lot of time to clear relative to the ground relays.
- Coordination between two ground relays located on either side of a delta-transformer is not required. The delta winding, be it in primary, secondary, or tertiary location, does not allow the ground fault current to permeate into the primary system.
- Provide at least 0.2 seconds of delay between two electronic relays, 0.3 seconds for electromechanical.
- The pickup settings should match either the drawings which indicate it (for example, the trip ratings for the breaker, trip MVA for the lines etc.) or follow the recommendations provided by NEC depending on what the relay is protecting.
- The pickup settings should also consider the load and that it is set well above it.
- The instantaneous and short time pickups should be set by giving due consideration to the downstream PDs and motor starting current. Typically, relays should not trip for 6 to 10x the motor FLA for 10 seconds.
Coordination of instantaneous trip settings between two series connected PD’s is possible only when there is significant impedance between them. A transformer or a long cable typically provides this impedance. Another important consideration is the available fault current. Although certain portions of one PD curve can overlap the other PD’s curve (like instantaneous region or pick up region) they should, however, coordinate in the region where the fault current occurs.
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