ECN Forum Forums General Topics of the Trade General Discussion Area 3 phase voltage readings

I'm doing a job at a auto repair garage that has a 400 amp 3ph panel. Reading the voltages I got 120v to ground on all 3 phases and between phase A & B it read 215v, B & C was 215v, but A & C was 0 volts. I have never come across this before. Also not sure why I got 215v between phases instead of 208v or 230v. Transformer problems? Any ideas? Thanks..
Al D

That's a hot set-up! Sounds like supply is a an open "Y" with one phase repeated, ie. in parallel with another phase. Is the neutral cooking?

Is the 3-phase being supplied from the PoCo pole in a wye or delta configuration? has there been any recent work by the PoCo on the pole with the transformers? There is a possibility that the transformer phases on the secondary side of the pole have been hooked up in correctly. This gives a 200v+ reading between the phases at the transformers, while still maintaining the 120v to ground. This does not explain the 0v volt reading between the A and C phases. Some more information is needed as to the type of 3-phase configuration and the setup.
(3 or 4 wire). In my opinion though, this is a PoCo problem. If you can get me some some more info, I will look into it more.

I don't know of any work done by the power co, but I called them out to check it tomorrow. I'll try to find out what type of transformer is being used.
Thanks,

Al,

It sure sounds like you have 1 phase 3 wire coming from a 4 wire Wye system. The readings of voltage to ground on all 3 lines removes some doubt that the system is of another configuration [described later].

Line B is common to either A or C [there's a voltage being read between these lines] and each line has voltage to ground, so this is really looking to me like someone, somehow and somewhere has connected the service feeders for line A and line C to only the "X1" or "X3" termination point from the Wye bank.

Also, as someone has mentioned, the system may be an Open Wye, which explains the 215 VAC from L-L [that should be somewhat normal on a lightly loaded or unloaded Wye setup, especially when the Transformer Impedance is a high percentage - like 5.0% or more], and for some reason, someone installed feeders to "Appear" like a 3 phase setup - why this would be done is anyone's guess.

Don't really see the benifits of installing an Open Wye system in a commercial enviroment, but who the heck knows?

If there's some way you could trace things back - along with verify if the Transformer [or bank of Transformers] is setup as a 4 wire Wye system, you would stumble across the above mentioned mistake in connections.

If you were using a DVM for testing, the 215 VAC L-L would be very common. You would maybe get higher L-G readings, but not necessarily.

Question: Are you measuring Line To Ground voltage between one Ungrounded Conductor and the Grounded Conductor, or to the Grounded Equipment [EGC]?
This doesn't change anything, I am just curious to the L-G voltage readings.

Anyhow, if the situation had L-L voltage equally between A-B, B-C and A-C, but L-G voltage was not "Typical", that would be a clue towards some type of Delta system.
Higher than expected L-G voltage readings would be a result of an ungrounded Delta.
One Line [such as "B"] having zero volts to ground, and the other two having the same voltage to ground as is read L-L would be the results of a corner grounded Delta.
All three Lines having voltage to ground, but one Line is quite higher than the other two read out would be the results of a Delta 3 wire with the center tap of one coil grounded at the Transformer.
Once again, this stuff is only "FYI", not crucial to your situation.

Hope you get the problem taken care of without too many headaches!

Scott SET

Question: Are you measuring Line To Ground voltage between one Ungrounded Conductor and the Grounded Conductor, or to the Grounded Equipment [EGC]?
This doesn't change anything, I am just curious to the L-G voltage readings.
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Thanks Scott, and yes I did measure from both Line to neutral and L-G and all 3 were still 120v, pretty strange ??
I have to wire in a 40a single phase compressor a hope it works ok on phase A&B or B&C, until I can find out the problem.
I'll let you know what I find.
Thanks again,

I found the problem today. The customer had some so-called electricians out to do some work previously and they shorted something and blew one of the 3 legs (Phase A) at the meter where the panel was fed from, and they could'nt figure it out so they tied the circuits that were off to breakers on the other working phases just to get it working.
The reason I was getting reading on all 3 phases is it was being backfed through some load.