Unusual 3-Phase Delta Voltage Measurements

[Runway24Right]

Hello,

I've recently been asked to install an new panel fed from an existing 240V Delta service. While metering the service, I encountered some unusual results:

A-N = 121V
B-N = 218V
C-N = 122V

A-B = 121V
A-C = 243V
B-C = 119V

The phase-neutral readings appear normal for a Delta service, however the ~120V measurement between phase A-B and B-C seem unusual... my understanding with 240V Delta services whether "open" or "closed" is that voltage between any two phases should be 240V.

Does this sound like it could be an issue with the utility, or perhaps some unusual transformer configuration on the pole which is actually intended to yield 120V between A-B and B-C that I've not encountered before?

Thanks!

 

[hillbilly1]

Is this a fused service? It would appear “B” phase fuse is blown or open, but if that was the case, b-n would be low. Do a fop test on the fuse or main breaker.

 

[ptonsparky]

Were there some 3 phase motors still attempting to run? Almost sounds like an accidental rotophase in operation. Yes ,a SEWAG on my part.
I agree, do a FOP.

 

[Elect117]

I think they wired it incorrectly at the pole. Do you have a picture of the transformers on the pole? Are the secondary bushings / wiring visible?

And then the wiring at the weatherhead?

 

[LarryFine]

What determines voltages to ground is which conductor is (intentionally, hopefully) grounded.

 

[wwhitney]

The readings are incompatible with A,B,C, and N having fixed voltages. So presumably one of the conductors is floating and its apparent voltage depends on which conductor you are measuring it against.

That is, start with observing A-C is 243V. Since A-N + C-N = 243V = A-C, we know N must be on the line segment connecting A and C, and since A-N is almost equal to C-N, close to the midpoint of A and C.. Likewise, the A-B and B-C measurements imply that B must be close to the midpoint of B-C. [A-B + C-B is only 240V, which is a contradiction with the measured A-C of only 243V, but the difference is small and presumably attributable to measurement imprecision or a slight change in the system voltages between non-simultaneous measurements.]

So that means that if all the voltages points are stable, B-N must be very close to zero. Yet you are measuring 218V. Therefore one of the conductors is floating, likely B as per post #2.

Cheers, Wayne

 

[Runway24Right]

Thanks for the responses!

I was able to return to the location yesterday to evaluate more thoroughly.

Starting at the pole - it is does a appear to be a closed Delta system based on the transformer connections. Then metered at the service entrance - all phases measured normally.

A-B = 245V
A-C = 245V
B-C = 247V
A-N = 122V
B-N = 213V
C-N = 122V

Returned to the existing panel intended to source the new panel and discovered that the main lugs on the panel also metered normally, only after the breaker intended to service the new panel did voltages on the B-Leg start to drift wildly. Removed the breaker and metered the busbar directly, saw the same issue - it appears that there's something bizarre going on at that particular position on the busbar, not present in other locations.

Will plan to return later this week once I can take the (very few) loads on the B-Leg offline to determine if one of them is influencing the panel or if it's some sort of a defect in the panel/busbar.

Thanks Again!

 

[hillbilly1]

Thanks for the responses!

I was able to return to the location yesterday to evaluate more thoroughly.

Starting at the pole - it is does a appear to be a closed Delta system based on the transformer connections. Then metered at the service entrance - all phases measured normally.

A-B = 245V
A-C = 245V
B-C = 247V
A-N = 122V
B-N = 213V
C-N = 122V

Returned to the existing panel intended to source the new panel and discovered that the main lugs on the panel also metered normally, only after the breaker intended to service the new panel did voltages on the B-Leg start to drift wildly. Removed the breaker and metered the busbar directly, saw the same issue - it appears that there's something bizarre going on at that particular position on the busbar, not present in other locations.

Will plan to return later this week once I can take the (very few) loads on the B-Leg offline to determine if one of them is influencing the panel or if it's some sort of a defect in the panel/busbar.

Thanks Again!

That’s where the Fall Of Potential test comes in. Leave the loads on, test voltage across each phase of the breaker. Line side A to load side A, same for B and C. You should get 0 volts on each test. If you get anything more than zero, you have a problem with that connection.