Yesterday I was connecting a large toroidal transformer to an unused 240V 15A circuit in my apartment (US split phase) for testing. I plugged it in and I heard the breaker trip after a few seconds.

I wasn't expecting that much magnetizing current so I probed the outlet with my DMM and got a very strange reading—80V line to line and 40V between both lines and ground.

I was worried the 30+ year old breaker might have "partially" tripped (presenting a fire hazard) so I didn't think to test it with my low-impedance meter. I immediately shut off the breaker by hand and re-tested the outlet, which now read 0V (so I don't think it was ghost voltage anyway).

Does this sound like the breaker is faulty and partially tripped? I doubt it's ever tripped in its life as it's normally used for the in-wall air conditioner. Are there any other potential scenarios that could cause the breaker to act in this way?

I plan to redo the test later today and if it fails in the same manner I'll contact the landlord to have it replaced, but I want to make sure it really is faulty first.

If it is indeed faulty and I'm only using it under supervision with proper fusing on the DUT and current monitoring, are there any potential risks to using it while we await replacement? There are no other devices on this circuit; the only other 240V device is the stove and that has its own 40A circuit...so unless the breaker has some internal resistance I can't see any problems.

  • Well, this is an unusual question! Sorry to be blunt, but what the heck are you trying to accomplish with a "large toroidal transformer" Is it new? Used? Did you have anything connected to the secondary? Was it shorted? Breakers are pretty darn reliable, they do fail, but very infrequently. I'd suspect a defect in the transformer or miss-wiring it. – George Anderson Jan 20 at 17:38
  • It's brand new. Open secondary; just verifying the connections. It has two 115V primaries which I have wired in series—the polarities are marked and everything. I'll double check the wiring on it but I don't feel like that's the issue...who knows! I've blown up transformers wiring them incorrectly before, but I learned my lesson there ;) – flashbang Jan 20 at 17:40
  • As for what it's for...it's going to be used for a high current power supply. Output ~46VDC 16.5A. My concern is less that it caused the breaker to trip (I have a soft start circuit designed for it) and more that my outlet was still live at 80V after it tripped. – flashbang Jan 20 at 17:42
  • Does your DMM have an "amp clamp" on it and do you have access to an individual wire to measure amp draw? If so, you could measure the current draw and see if it's a breaker issue or a transformer issue. Highly unlikely to be a breaker issue. – George Anderson Jan 20 at 17:43
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    It's not a FP although that is the manufacturer of the main panel in our building's basement (sigh). It's a Wadsworth, which I understand to be good quality if not very, very old. For all intents and purposes let's just say I overloaded the circuit somehow and the breaker tripped. Why would I still be reading 80V L-L and 40V L1/L2-G? That's what I'm most worried about. I can fix the current draw issue and my final design is even fused and has a soft-start. – flashbang Jan 20 at 17:56

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