ECN Forum Forums General Topics of the Trade General Discussion Area Ungrounded Delta Service

In several of our local government buildings we have a 230V, 3Ph, 3W. electrical service. Measuring voltage to ground; A-G=230 volts, B-G=230 volts, C-G=0 volts. Why isn't there any voltage to ground on one phase?

[This message has been edited by Frank Cinker (edited 09-13-2005).]

Phase C is grounded. If this is supposed to be an ungrounded service, then there is a fault somewhere. But this is very likely 'corner grounded' delta rather than 'ungrounded' delta.

-Jon

Ungrounded systems are common in powerplants or substations. That way, IF one phase does become grounded, life goes on. Control of operations isn't effected.
Now a ground on a 2nd phase is another thing.

But I agree with Winnie. You most likely have a corner grounded system.If that is the case, your phase to ground reading is irrelevant.

HP

That conductor should be white.

You're so prejudiced!

The jokes just write themselves from there don't they.

I occasionally run into corner grounded delta in this area. It is rare and I have never gotten a good explanation for why that configuration appears in just one building in a neighborhood.
It seems to be old installations.

Anybody got a definitve answer?

It is a very clean way to install 3p delta for motor loads if you don't need to serve 120v loads. You can use 2 pole equipment, one fewer service conductor and have 240v to run your motor on vs the 208 you get with wye.
Do you like "red leg" better? That comes with it's share of confusion too.

240 with a high leg is very common in this area.

Two of the 240 corner grounded delta have been on an elementary school and a private school that was formerly an orphanage. Both had small a\c loads - split systems and small rooftops -no chillers. Both systems served lights and receptacles.
3 pole main disconnect with a solid plug in the grounded phase. 3 wires in and 3 wires out.

I think if this were an ungrounded system your measurement from phase "A" to ground and from phase "B" to ground would not be that close to each other. With an ungrounded system, the phase voltage to ground is erratic.