Lost phase?

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dstjohn

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Hello all, I lost the 'c' phase off the load side of a 120/208 transformer. Come to find out, I had a direct short (ungrounded conductor to equipment) that didn't trip the breaker. I located the problem (neutral not bonded at xfrmr) but am wondering why the xfrmr shut down the 'c' phase (which was the phase that the short was on) and my a & b phases were reading 208 to ground instead of 120. any ideas?
 
dstjohn said:
Hello all, I lost the 'c' phase off the load side of a 120/208 transformer. Come to find out, I had a direct short (ungrounded conductor to equipment) that didn't trip the breaker. I located the problem (neutral not bonded at xfrmr) but am wondering why the xfrmr shut down the 'c' phase (which was the phase that the short was on) and my a & b phases were reading 208 to ground instead of 120. any ideas?
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The transformer did not shut the c phase down. It just moved voltage to the equipment. That is why you were reading 208 from A & B to ground.
You are lucky you or no one else was killed.
 
Just to expand upon the above comments:

With no neutral to ground bond in place, the fault from the C leg to ground _created_ the only ground bond. This essentially made your system into a 'corner grounded' system.

The C phase coil was not shut down; it still had 120V from one end of the coil to the other. If this coil were normally bonded to ground, then the 'neutral' end of the coil would be at 0V relative to ground and the 'C' end of the coil would be at 120V.

But in your fault case, the 'C' end of the coil was forced to 0V relative to ground by the unintentional bond, and the _neutral_ voltage was raised to 120V relative to ground, and the A and B voltages were raised to 208V.

-Jon
 
Thanks Jon for the explanation, I guess all the other people who responded w/ smart comments are way to important to help someone.
 
dstjohn said:
Thanks Jon for the explanation, I guess all the other people who responded w/ smart comments are way to important to help someone.
Click to expand...

John, I asked you a real question about checking the primary.

I don't know you and I have no idea what your level of troubleshooting ability is. My thought was perhaps this transformer was fed from a fused switch with an open fuse on C and that perhaps you had not noticed that.
 
You come asking for free advice. My answer best fits your understanding...but might also be dead wrong. Don't go insulting the others here who made real efforts to give useful responses. We gave you our time, for free.

-Jon
 
No solutions just talking.

I had the same problem about 16 years ago on a 460 volt three phase system. I had 480 to ground on two phases and 0 on the third phase, and the lumber mill was running with no problems. We shut down the mill and the voltage leveled out to 240 to ground on all three phases. We started up one motor at a time until the voltage drifted again. We found a conveyer belt motor that had a short to ground, and like you said it did not tripped the breakers. All I know is that we had three old transformers (probable original from the 50s) wired in Delta. I believe in time that the mill replaced them and changed to a Y system.

Frank
 
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