A phase to ground 0 Volts - B & C phase to ground 208 Volt

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Bryanhard1982

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I have a weird one for you all

we have a 75 KVA transformer 277/480 to 120/208

we are getting the following readings

PRIM.

A/B-277 Volt A/B -480 A/N-277 Volt
B/G-277 Volt B/C -480 B/N-277 Volt
C/G -277 Volt C/A -480 C/N -277 Volt

N/G - 0 Volt

Secnd.

A/B -0 Volt A/B -208 A/N-208 Volt
B/G -203 Volt B/C -208 B/N-208 Volt
C/G -205 Volt C/A -208 C/N -208 Volt


N/G - 120 Volt
 
If A/B is really 0, then the rest of the numbers can't be what you have (look at a diagram to see that). Did you measure directly on the transformer terminals?

Also, is there really a neutral connected to the primary? Usually the pri is configured as a delta feed without one.
 
the prim reading were taken in the panel that feeds the XFMR the Secondary readings were taken in the 120/208 panel on the line side of the main
 
Are you sure that you meant A/B and not A/G in the first column and first row on both the primary and secondary voltages?
I think the first thing to note is that N/G is 120V on the secondary, which indicates that the neutral is not bonded to equipment ground. And so I'm not sure how informative the other measurements are going to be if the neutral is not correctly bonded. The voltages to ground may change vs. loading, etc. if the neutral is allowed to "float".

One could speculate that A of the secondary is connected to ground if you meant that A/G is 0 volts, because that would explain the 120V on N/G, and also B/G, C/G both being close to 208V.
However, A/N, B/N, and C/N all being 208V is puzzling. Are you sure those are the correct measurements?
I think the bottom line is that the N/G bonding on the secondary should be checked first, and also the rest of the connections on the secondary to make sure they are correct.
 
zbang said:
If you can, safely, check the voltages directly at the transformer and check the connections as well. (That is, don't trust that they're correct.)
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I've seen where the installer accidentally swapped the phase conductor coming off the generator and bonded it at the transfer switch. Installer did not identify the conductors before install. Oops. Fortunately the owner called me before they started any loads up.

That transformer secondary won't care either.
 
Reading the values, I am fairly certain that the first 'A/B - 0' should really read 'A/G - 0'. The first column of both primary and secondary measurements says 'A/B - .... B/G - .... C/G - ....', suggesting that the first column is phase to ground measurement. The first row of both primary and secondary measurements says 'A/B - ... A/B - .... A/N -....', two measurements with _different_ values identified as A/B.

The secondary readings look perfectly correct for phase-phase and phase-neutral, but the phase-ground and neutral-ground numbers are wrong. It is pretty clear that the neutral is _not_ bonded to ground but phase A is bonded or faulted to ground.

This isn't the swap described in post #7 measured at the disconnect. If what you thought was neutral was really phase A, then the line-line and line-neutral voltages would be wrong. However this could be the swap described in post #7 if the measurements were taken at the transformer.

Or this could be a system where the neutral bond was never connected but a phase faulted to ground.

-Jon
 
winnie said:
The secondary readings look perfectly correct for phase-phase and phase-neutral, but the phase-ground and neutral-ground numbers are wrong. It is pretty clear that the neutral is _not_ bonded to ground but phase A is bonded or faulted to ground.
Click to expand...

Shouldn't the phase-neutral voltages be closer to 120V instead of the 208V that's listed?
 
synchro said:
Shouldn't the phase-neutral voltages be closer to 120V instead of the 208V that's listed?
Click to expand...
winnie said:
*self-administered dope slap*

I read those as 120V, not 20xV.

Well there goes that assessment :)

-Jon
Click to expand...
That's OK. I agree with every word of your post except for the phase-neutral voltages.
I'm thinking that the 208V A-N, B-N, C-N voltages shown were not correct.
Hopefully you'll be merciful next time I mess something up. :)
 
winnie said:
Reading the values, I am fairly certain that the first 'A/B - 0' should really read 'A/G - 0'. The first column of both primary and secondary measurements says 'A/B - .... B/G - .... C/G - ....', suggesting that the first column is phase to ground measurement. The first row of both primary and secondary measurements says 'A/B - ... A/B - .... A/N -....', two measurements with _different_ values identified as A/B.

The secondary readings look perfectly correct for phase-phase and phase-neutral, but the phase-ground and neutral-ground numbers are wrong. It is pretty clear that the neutral is _not_ bonded to ground but phase A is bonded or faulted to ground.

This isn't the swap described in post #7 measured at the disconnect. If what you thought was neutral was really phase A, then the line-line and line-neutral voltages would be wrong. However this could be the swap described in post #7 if the measurements were taken at the transformer.

Or this could be a system where the neutral bond was never connected but a phase faulted to ground.

-Jon
Click to expand...
Jon,
my mistake you are correct see the revised readings below these reading were taken in the panels on either side we are going back the XFMR is hanging 20' up on a column

this is not a new install its been in service for 4 years

PRIM.

A/G-277 Volt A/B -480 A/N-277 Volt
B/G-277 Volt B/C -480 B/N-277 Volt
C/G -277 Volt C/A -480 C/N -277 Volt

N/G - 0 Volt

Secnd.

A/G -0 Volt A/B -208 A/N-208 Volt
B/G -203 Volt B/C -208 B/N-208 Volt
C/G -205 Volt C/A -208 C/N -208 Volt


N/G - 120 Volt
 
Bryanhard1982 said:
I have a weird one for you all

we have a 75 KVA transformer 277/480 to 120/208
Click to expand...
Did you mean to say 208Y/120? 120/208 would be single phase.
Bryanhard1982 said:
Secnd.

A/B -0 Volt A/B -208 A/N-208 Volt
B/G -203 Volt B/C -208 B/N-208 Volt
C/G -205 Volt C/A -208 C/N -208 Volt


N/G - 120 Volt
Click to expand...
If A/B was supposed to be A/G then I'd guess your looking at a 480Y/277 to 208 delta transformer. As others have requested posting a photo of the namplate would help.
 
tortuga said:
Did you mean to say 208Y/120? 120/208 would be single phase.

If A/B was supposed to be A/G then I'd guess your looking at a 480Y/277 to 208 delta transformer. As others have requested posting a photo of the namplate would help.
Click to expand...
Question if it is a 208 delta secondary is what is the "neutral connected to?
 
kwired said:
Question if it is a 208 delta secondary is what is the "neutral connected to?
Click to expand...
if i had to wager a guess, i'd say what the OP is calling the neutral is a grounded buss in the secondary panel, what we typically call a neutral buss. And the ground measurements are to the panel can.
 
tortuga said:
if i had to wager a guess, i'd say what the OP is calling the neutral is a grounded buss in the secondary panel, what we typically call a neutral buss. And the ground measurements are to the panel can.
Click to expand...
If it were a 208 delta secondary the readings do all make sense other than the 120 volts N-G and A-B being 0 volts.

But I believe he said these are readings at the secondary panel, so we don't know exactly where each of those conductors lands at the transformer and are presuming they are continuous to the transformer.

Having that 120 and a 0 volt reading seems to indicate it maybe is 208/120 secondary but something is not connected correctly. If there is loads connected vs unloaded secondary could make some differences in readings as well.

open neutral to the source could make you see 120 N to G easily if there is active load connected to the N, A or B could be open, but have a load between them causing them to read same reading yet no voltage between them.

ETA: I guess I am right now leaning toward there being open neutral and either A or B is also open, but there is some connected load(s) causing backfeed and confusion. Maybe disconnect all loads and then check readings again?
 
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