Odd readings at meter socket

[Skubie]

Here's a puzzler. We rehabbed a legacy power supply that was feeding yard loads - 120/208V. This was fed from a 277/480V (Delta) supply, and stepped down (120/208V Wye) before being distributed to the branch circuits. When we did the rehab, we put a meter upstream of the stepdown transformer in the 480V and here's what measures at the meter panel:

Meter line in:
A,B,C to Neutral - all 277V
A-B,B-C,C-A - all 480V

Cool so far, but then when we set the meter, load out:
A-Neutral - 480V
B-Neutral - 480V
C-Neutral - 15V (that is not a typo)
Ph-Ph were all 480V
And the meter would not energize

We tried two different meters and got the same results. I know there is something "obvious" here, I'm just missing it. Thanks for any insight.

 

[Jraef]

Here's a puzzler. We rehabbed a legacy power supply that was feeding yard loads - 120/208V. This was fed from a 277/480V (Delta) supply,

No such thing as 277/480 Delta. If it is Delta, it is just 480V, and since there is no neutral, the voltage readings to a ground point are going to be irrelevant. You might get 277, you might not.

and stepped down (120/208V Wye) before being distributed to the branch circuits. When we did the rehab, we put a meter upstream of the stepdown transformer in the 480V and here's what measures at the meter panel:

Meter line in:
A,B,C to Neutral - all 277V
A-B,B-C,C-A - all 480V

Cool so far, but then when we set the meter, load out:
A-Neutral - 480V
B-Neutral - 480V
C-Neutral - 15V (that is not a typo)
Ph-Ph were all 480V
And the meter would not energize

We tried two different meters and got the same results. I know there is something "obvious" here, I'm just missing it. Thanks for any insight.

If you are using a new "smart" meter that is supposed to be connected to a wye service, and you have a delta service, it will detect the mismatch and will not energize the built-in contactor. If the meter is for a wye and you have a delta, the meter CTs would become your neutral point for the entire srevice and they are of course not capable of that.

Most likely you have a 480/277V wye service, not a 480/277V wye but your meter is for a 480V corner grounded delta, or vice versa. The PoCo is supposed to know what your service is and put in the correct meter. If you thought you had a delta service and bought the wrong socket, I would have thought the meter jaws would not allow the wrong meter to plug in, but maybe with the smart meters now, the sockets are all the same and the meter just doesn't energize. I have not had to buy a meter socket since the smart meters came along, maybe someone else knows for sure.

 

[jim dungar]

...fed from a 277/480V (Delta) supply, and stepped down (120/208V Wye).

277/480V(delta) is not a valid system. You either have a 277/480V wye or you have an ungrounded 480V delta.
What is your "neutral" to ground voltage?
What tye of meter arrangment are you using (how many elements)?

 

[Skubie]

You are correct, sir! The 277/480V Delta reference was wrong - should be 480V Delta.

 

[don_resqcapt19]

This appears to be an ungrounded delta system. With no loads the voltages will often look like a 480/277 wye system. With loads the voltages from phase to earth will be all over the place, but based on your readings, I would suspect a ground on C phase.

 

[jim dungar]

You are correct, sir! The 277/480V Delta reference was wrong - should be 480V Delta.

It sounds like you are supplying an ungrounded 480V delta service to the meter socket. As Don said, if the customer's service is installed as grounded-phase, then your load readings may be appropriate.

 

[don_resqcapt19]

It sounds like you are supplying an ungrounded 480V delta service to the meter socket. As Don said, if the customer's service is installed as grounded-phase, then your load readings may be appropriate.

Jim,
I was suggesting an accidental ground fault on the load side of the meter. I would expect zero volts to ground on the grounded phase of an intentionally grounded system.

 

[jim dungar]

Jim,
I was suggesting an accidental ground fault on the load side of the meter. I would expect zero volts to ground on the grounded phase of an intentionally grounded system.

My choice of 'installed' was poor.
The OP provided L-N voltages then said it was a delta system, I assumed the supplied values were 'abouts' and not 'exacts'.