Ground Fault Relay Setting for Breaker Protecting Motor

[PE (always learning)]

I've got an electronic trip 250 amp breaker at the switchboard that has both phase and ground fault settings. This breaker is protecting a 75 HP motor at 480V with a FLA of 96A. My question is: What should I set the ground fault pickup and ground fault delay time at? The ground fault pickup ranges from 50 to 250 amps and the ground fault delay ranges from .05 to .8 seconds


Typically on the phase side my settings are high to avoid the motor inrush, but what is best practice on the ground fault setting?

Any advice appreciated

 

[David Castor]

If the only thing downstream is the motor, I'd set it at minimum pickup and delay If it trips during starting, increase the delay. There is nothing this ground fault device needs to coordinate with downstream, so it can be set quite low. Any ground current indicates a problem in the motor. 50 A of ground fault current in a 75 hp motor indicates a definite problem.

 

[PE (always learning)]

If the only thing downstream is the motor, I'd set it at minimum pickup and delay If it trips during starting, increase the delay. There is nothing this ground fault device needs to coordinate with downstream, so it can be set quite low. Any ground current indicates a problem in the motor. 50 A of ground fault current in a 75 hp motor indicates a definite problem.

There is an elevator control panel downstream from this breaker. It is most likely a bussman power module with a fusible disconnect which then feeds over to the elevator motor.

Would you still set the ground fault settings low given this information? I understand the point of setting it low, but I'm just curious if I need to be concerned about nuisance tripping as well? ( I guess I could always set it higher as you suggested if this did become an issue)

 

[tom baker]

Can you ask the elevator supplier about the setting when they do the elevator startup? They may be familiar with the equipment

 

[mayanees]

The selection of the ground-fault pickup and delay is subjective I think because there isn't a requirement for ground-fault protection for the elevator assembly, only for 120V receptacles.
So in order to provide some level of protection, I'd probably set the gf pickup at some value less than the 430.52 requirement, or in this case around 80% of 250 or 200 amps with a delay that occurs where the 250-amp phase breaker comes off instantaneous pickup. For a 250-amp Eaton JG breaker that occurs around 0.5 seconds. This is all to say that I'd look at a Time Current Characteristic curve to make the determination.
I think the true test for your settings is that the protection does not nuisance trip, but that some level of gf detection is enabled. If I had the time to play with it, I'd probably set per post # 2 at some minimum level and adjust it up if there's a nuisance trip, but those types of opportunities are rare.

 

[JoeStillman]

The title of Table 430.52 is "Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices" (Emphasis mine).

 

[mayanees]

The title of Table 430.52 is "Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices" (Emphasis mine).

Yes, so I'm recommending the gf protection be set below the phase protection, which in this case is 250 amps. But I don't see anything in the NEC that says an elevator controller must have gf protection.