ECN Forum Forums General Topics of the Trade General Discussion Area Another brain teaser....480 on a 240 volt delta

I work at a utility. We got called out on a customer complaint to a grocer who had burned up his 3 phase motors.

His service was a 120/240 volt open delta secondary fed from two phases of a 4 wire wye, 12.5/7.2 Kv primary.

When we got there, we read across the "lighting" legs and got 120 volts from A to neutral and 120 volts from B to neutral. A to B read 240 volts, as did B to C.

So far so good. So.....

A to C read 480 volts (instead of 240) and C to neutral read 360 volts (instead of 208).
Neither transformer fuse was blown.

Hint: There had been many outages due to storms a few days before and lots of backfeeding had been going on.

Did the transformers end up on the same input phase?

Dammit! Dammit! Dammit!
Didn't even get past the first guy.

I don’t know what back feeding has to do with this but you can’t feed a three phase transformer bank with only 2 phases. By feeding both transformers from the same two phases your secondaries are connected in series giving the voltages you see.

Curt

I’ve got to learn to type faster.

Quote:
"I don’t know what back feeding has to do with this but you can’t feed a three phase transformer bank with only 2 phases. By feeding both transformers from the same two phases your secondaries are connected in series giving the voltages you see."

Right on the second half.
You CAN feed a 3 phase open delta bank with only two phases and neutral. I didn't mean to give the impression that they were going phase to phase on the primary with the common floating. The common is bonded to the neutral.

Quote:
"I’ve got to learn to type faster."

Indeed. The second response was going to win a brand new Porsche.

...oh, wait. I was the second response.

Oh, man lookit what I started....