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Neutral pickup setting for three phase motor protection

Neutral pickup setting for three phase motor protection

Neutral pickup setting for three phase motor protection

(OP)
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?

RE: Neutral pickup setting for three phase motor protection

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

RE: Neutral pickup setting for three phase motor protection

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

RE: Neutral pickup setting for three phase motor protection

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

RE: Neutral pickup setting for three phase motor protection

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

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

RE: Neutral pickup setting for three phase motor protection

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

RE: Neutral pickup setting for three phase motor protection

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.

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