240 To Ground = 0

Bubbles

Were on a commercial job doing lighting and just for fun we took some numbers on one of the subpanels. We get 240 to ground on 2 and the center phase we get 0. On a 2 pole 20 amp breaker we get 240 to ground and 240 to ground on both screws. I've seen all kinds of other numbers on 3 phase weve been on just havent seen this one yet. Going to have to get books out but is there an old timer out there that has seen this and has some insight on this one. It doesnt affect our job there at all just curious.

Thanks in advance

JohnJ0906

Sounds like a corner grounded delta. 240v 3 ph.

The leg you are getting 0v to ground is the leg that is grounded. You should have 240v between any 2 legs.

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John from Baltimore
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All replies based on the 2008 NEC

Magnettica

How is it that you get "240 to ground and 240 to ground on both screws?"

You should be reading 240 to ground, and 480 between the two phases.

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mahlere

ok...someones got bad numbers....

is it 240? 120? or 277? to ground?

RobertWilber

Absolutely agree.
Just 3 wires coming in?
No neutral, right? No 120 off this service? There is another 120/240 service here for receptacles, right?
Like he says, you will read 240 between any two "hot" conductors, but 2 read 240 to ground and one reads 0 to ground.
Straight 240 delta, corner ground.
Old, rare dog.

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mahlere

just like this EC from philadelphia I heard about...

mdshunk

I don't think I've ever worked on a corner grounded 240 delta, but the measurements taken pretty much square with what you'd expect from such a system. I wonder if the OP could say what sort of place this was where these measurements were taken? I expect that it's a very old industrial building of some sort.

Bubbles

Yeah. Good eye everyone. I think this is a corner delta. Its definate a 3 wire delta config coming out of CT to main dissconnect then all kinds of transformers and subs all over to get 120v where needed. Its a big 20,000 sq ft warehouse. All the high bays are running on 240 now some 120. I think it was built in 50's or 60's. Appreciate the tips.

JohnJ0906

The reason I recognize this is that a funeral home we do service for has a dual service - 120/240 1ph and 240, 3ph delta for heat loads and the elevator. POCO still provides it in a few areas.

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John from Baltimore
"One Day at a Time"
All replies based on the 2008 NEC

Sparky Joe

The power company will still feed a building with 3 wire 240 3 phase, but grounding one of the phases to everything metal is something that's done on site.

mdshunk

Yeah, I have a feeling that these are overhead services, derived from a bank of pole top kettles. If it comes from a padmount, I'd bet you're getting the neutral whether you need it or not.

HVAC_NW

Can I assume that transformer is dedicated for that customer? If not, how is it determined which phase is the one to be grounded?

If there are more than one customers on that transformer and customer one decides to make phase one a ground, customer two decides phase to a ground, well bad things happen.

goose134

I'm not sure if it it's actually stipulated anywhere in the code but I believe traditionally the B phase is the one that is grounded (at least around here). Same goes for high leg systems. A and C will have 120 each to ground, and B will be 240. Only seen 2 of these but if you start seeing circuits 1,5,9,11.. you'll know.

Sparky Joe

The corner ground delta utilizes a high impedance ground. which still allows high fault tolerence that an ungrounded system does, but without all the personnel safety issues of a direct corner ground. And an added benefit of troubleshooting a fault because you still have current flow in the faulted conductor and the system is still up and running.

I highly doubt the power company would feed two customers a delta system from the same transformer, but if they did it would not be a problem.