120 v from neutral to ground, and 0 volts from b phase to ground.

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Could use some ideas on how and why.

I got a call that some office propane heaters were not working and got there and fan was working but no fire from igniter. Circuit board was giving code for reverse polarity or poor ground. Everything looked normal but 120 volts from neutral to ground. Tracing it back I found a feed through panel and I measured 120volts from isolated nuetral bar to ground. Tracing back again I find a switch gear fed from a transformer feeding four 120/208 panels going to main fuse in switch gear I find O volts from B phase to ground and 208 from other two phases to ground. Should mention that other test points read the same 208 to ground on A and C but zero to ground on B. All three read 120 to nuetral. I figured to find a blown fuse but all was good. I shut down all loads and it went back to normal. Then energizing one panel at a time I found that one panel created the problem. Then borrowed it down to one 20 amp single pole breaker when energized create the problem. It is in position 9 so it would be the B phase. It was labeled recepticles but could not find where power was lost on the floor. Not sure where the best place is to go from here. The ground is apparently energized when you turn that breaker on but no fuse blows it just assumes potential of b phase.



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Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Could use some ideas on how and why.

I got a call that some office propane heaters were not working and got there and fan was working but no fire from igniter. Circuit board was giving code for reverse polarity or poor ground. Everything looked normal but 120 volts from neutral to ground. Tracing it back I found a feed through panel and I measured 120volts from isolated nuetral bar to ground. Tracing back again I find a switch gear fed from a transformer feeding four 120/208 panels going to main fuse in switch gear I find O volts from B phase to ground and 208 from other two phases to ground. Should mention that other test points read the same 208 to ground on A and C but zero to ground on B. All three read 120 to nuetral. I figured to find a blown fuse but all was good. I shut down all loads and it went back to normal. Then energizing one panel at a time I found that one panel created the problem. Then borrowed it down to one 20 amp single pole breaker when energized create the problem. It is in position 9 so it would be the B phase. It was labeled recepticles but could not find where power was lost on the floor. Not sure where the best place is to go from here. The ground is apparently energized when you turn that breaker on but no fuse blows it just assumes potential of b phase.



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It sounds like you have a 3 phase wye transformer with the center point or the HO not bonded to ground. Circuit 9 has a ground fault which has consequently made the B phase grounded and gives you the high A-B and C-B readings.

Find the the fault in circuit 9, correct it, then bond the HO properly. In your case I believe bonding at the transformer may be best.

Do you have someone with more experience to help you?

Might be a good time to watch and learn.
 
You say HO and I'm thinking primary side. I am thinking XO (secondary side) should be bonded to building steel.
Just want to make sure I am on the same page. primary side should have equipment ground if I am thinking right


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kwired

Electron manager
Location
NE Nebraska
Sounds to me like missing system bonding jumper, and a ground fault on phase B - on the mentioned circuit #9.

ETA: I guess that is what was said in post 2:ashamed1:
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
You say HO and I'm thinking primary side. I am thinking XO (secondary side) should be bonded to building steel.
Just want to make sure I am on the same page. primary side should have equipment ground if I am thinking right


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Yes, XO. I Stand Corrected again. Glad someone is paying attention.

HO shouldn't have anything attached to it.
 

rian0201

Senior Member
Location
N/A
Ground fault on an ungrounded system.. Phase a to g should measure 208.. As well as c to g..


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