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.

I don't think you can measure voltage from phase to ground on a straight delta system, only phase to phase.

Lear with a typical DVM you can measure voltages to ground from an ungrounded system.

However I agree with Ryan that the readings you get will be erratic and meaningless.

If you have a delta system feeding line-to-neutral loads, it's not corner grounded, but center-tap grounded. The "main" winding has the tap, just like your 240/120v. home, and the other two windings meet at the "high leg".

Nope, seperate neutral. Three phase service with one leg at "0" potential to ground and 240 volt between all phases.

Twice in 20 years and I'm not sure those faciities are still wired as such.

If you had a delta system with a neutral tap on one of the secondaries, and a corner ground, then you would get a pretty dangerous situation that would _work_ with most loads. For 3 phase loads you would have 240-240-0, just like a normal corner grounded system. But for single phase loads you would have 240-120-0 instead of the normal 120-0-120 (all voltages relative to ground). Standard 120V lamps would function in sockets, but the socket would _always_ be energized. Add anything with a ground-neutral bond and _bang_.

-Jon