Neutral pickup setting for three phase motor protection

[andieh]

I am wandering what to set the neutral pickup setting to for a medium voltage motor relay. I am taking the amp readings from a zero sequence CT. I realize that there should not be any current flowing if every thing is balanced. Should this setting be 1% of FLA or 10% of FLA?

 

[davidbeach]

In the 5A secondary world I'd set it in the 0.25 to 0.50A (secondary) range.

 

[waross]

Not usually a neutral connection on three phase motors. Any current coming back on the neutral is probably ground fault current. I would treat it as a ground fault indication and set the protection accordingly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[andieh]

That is correct what would you recommend the pickup value be set to?

 

[waross]

David Beach is more familiar with protection settings than am I. I defer to his suggestion.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[living2learn]

(assume solidly grounded) I would also coordinate it with the upstream GFI protection. Obvoulsy wire to breaker not contactor.

 

[LionelHutz]

You usually need to know something about the system, such as it being resistively grounded. It'd be useless setting the current to 20A if your system has a 15A NGR.

If the system is solidly grounded then the exact setting doesn't matter too much. The setting will not limit the ground fault current anyway.