what the heck is this?


Senior Member
Boston, MA
I've uploaded a picture of the subject matter.

It's a Westinghouse electronic breaker (something like 150A frame) with what would appear to be a zero sequence CT above it wired into the shunt trip of the breaker. It appears to be serving as ground fault protection. Are any of you familiar with a such a product? Any idea who the manufacturer might be. Don't think it's Westinghouse. I suspect this came along after the fact but I could be wrong. All I know is that my Eaton guy isn't too sure either.

Appreciate the help,




Senior Member
Yes, That's a Zero sequence CT.
Specifically it's a Westinghouse or Cutler-Hammer Type GFM Class 1 ground fault protection devide. This one is has a 4" window so it's a GFM4 but it has an SCR output designeed to operate a remote, low energy, flux transfer shunt trip mount4ed in the breaker. As such it soes not need any supplementary control power. This model would be a GFM-4A.
It has a place for an optional "rating plug." Without installing a rating plug the GFM is rated 5a. There are (4) rating plugs available the GFMP-10, 25, 50, and 100 for 10, 25, 50, and 100a.
Normally a visible indicator and reset button is provided but the SCR version isn't provided with this feature since the SCR is used in the output circuitry which automatically resets when the MCCB opens to clears the GF.