Ground fault protection and its relation to phase device coordination

[Magic Gorge]

In regards to displaying time-current coordination curves, is there anything meaningful to learn from showing a main breaker ground fault against a downstream 3-phase breaker? How would the downstream breaker "see" the ground fault and operate? If the phase and ground overlap in this instance, should that be of concern? Also, would the same logic apply to medium voltage relays?

 

[topgone]

In regards to displaying time-current coordination curves, is there anything meaningful to learn from showing a main breaker ground fault against a downstream 3-phase breaker? How would the downstream breaker "see" the ground fault and operate? If the phase and ground overlap in this instance, should that be of concern? Also, would the same logic apply to medium voltage relays?

A ground fault will be displayed in the TCC as an instantaneous trip curve (vertical). If the breaker has intentional delay, the characteristic line will look like a definite time trip curve.

 

[JoeStillman]

Ground fault protection exists because the regular "phase" trip points are too high to clear a damaging ground fault. Phase current sensors "see" the ground fault, they just don't trip because it's too low.

Let's say you had a 2000A main breaker with GF upstream of a 1000A breaker that didn't. If the setpoint of the main GF is 1000A, then you don't have coordination. That's why it's meaningful to look at downstream phase setpoints vs. upstream GF.

The same applies with medium voltage relays unless there is a delta-wye transformer between the MV device and the downstream breaker of interest. A ground fault downstream of the transformer will not appear as a ground fault on the primary. In that case, you might compare the downstream GF setpoint to the upstream phase. Mis-coordination is a lot less likely here.