Got a call about a rooftop A/C unit not working. Open the panel and find it is three-phase, with those tell-tale blanks every few spaces. The A/C is on a 3-phase breaker. With my meter I get the following readings, on both the main lugs and the A/C breaker: For all three phases I get 120v to ground. Phase A to Phase B I get 240v. Phase A to Phase C I get 240v. But Phase B to Phase C I get NOTHING. I expected to get 208v. Is this a problem at the transformer? How can B have voltage to ground, but not to another phase?

Did you try measuring voltages with the main open?

If you lost the incoming phase B from the xfmr, you could be reading voltages on that phase via some 240V device which is connected between B and C (maybe the A/C unit itself).

Checking voltage to ground is what has you confused. I don't use the practice as I have found that it complicates simple problems.
Ignore the ground readings for a moment. If you do not have voltage between two legs of the 3 phase, you can count that the voltage is not present.
If you are missing a leg you have to look for it. Start somewhere closest to the supply power and work your way back to the equipment. Possible areas of concern are XFMR's, breakers, contactors and possibly the wiring which is doubtful. Just work your way through this thing and keep the meter on the wires, NOT to ground.

As what JValdes saying but let me add one more thing is check the poco supply sometime they will supply the open delta format or open wye so check out the can[s] if you see just two cans both not same size that is open delta but if you see 3 cans if two are same and third one is diffrent that genrally it is delta but some case they used same size all the way thru with cans so watch for secondary taps that is surefire hint the first and third can will have no netural hook up at all but if this is on underground service the bet is off due the transformer set up.

What ya have to do is go back to the main panel and check the voltage on main breaker to make sure you have straght 240 v delta some case you will run into corner grouned delta [ not the case if ya have 4 wire service ]

Merci, Marc

What you describe sounds like 120/240 single phase, connected to a three-phase panel, with one leg of the 240 connected to both the B and C phases of the panel.

Also, why are you expecting 208 between the B and C phases? I know of no system that would have 240 between two pairs of phases, and 208 between the remaining pair.

No voltage between phases means voltage is not present? EXCUSE ME?????
Maybe ... just maybe ... this is the wrong panel for the service. It's time to start over.

CHECK YOUR ASSUMPTIONS!

How many wires are coming in from the PoCo? How many transformers do you see on the pole?

B and C leg will show no voltage between them if they are both powered by the same leg. Yet, they'll still read 120 to ground.
I see this when someone puts a 3-phase panel in a 1-phase location. (As Solar Power described)

Guys,
I'd like to get more info on this type of system, there is a bit of talk about "high legs" and grounded delta's in the chat room from time to time.
I can't say I understand this system at all, it's totally foreign to what I've been trained in.
Can anyone point me to a thread or site that explains it all in detail?.
Thanks,
Mike.

as pauluk said you would be reading voltage via some other 208 volt breaker feeding through a load.working back to the source for that panel until you get voltage between phases narrows down the problem area.question though,could a amprobe be put on the phases and look for the low or no existant amperage on the feeder to narrow down the offending bad phase. this reminds me of a tip to offer.some of you have been to a building with partial lights and sproatic power outages throughout building and know that there is a lost phase.besides checking the neighbors and opening panel, go out to the meter and look for a missing or flashing a,b or c.then you know its on the poco side.

Mike, although I don't believe this has any bearing on the OP's problem, (it sounds like a phase has been lost for whatever reason)see the illustration below of a "Center Grounded Delta"

Roger



Note that the term"Center Grounded" is only refering to the center of one winding when speaking of a Delta configuration of windings.

One thing you're all missing, from a service man's point of view is that a short time ago this AC worked, just as it's connected. No one jumped up on that roof and did some wierdo wiring just for fun. I can't quite work it out from the info given, but one of the phases is out upstream from the disconnect. Find the point where you DO have correct voltages and work downstream.

Ill keep my mouth shut on the not having voltage between two phases means there is no voltage present, Reno has already addressed that.

Stupid question, are you using a meter or a Wiggy (or other solenoid type tester) Maybe you are getting a false reading from the meter, its reading voltage but theres nothing behind it. Cant say Ive ever seen a ghost voltage above 50 or 60 volts though.

It could be 240/120 open delta, with the high-leg transformer between the B and C phases, but no power to the primary of the high-leg transformer. (Which, with a minor modification, would explain why Tripp is expecting 208--not across B-to-C, but across B-to-ground.)

[quote=Tripp] with those tell-tale blanks every few spaces.

It is probably what SOLARPOWER said, it is 120-240 and high leg 208 to ground.

Check supply to the panel if all voltage is present then the breaker is either bad or is not making a good contact or burned contact on the bus bar.

If not above, then as suggested by others work yourself up the stream to find the problem.

Niko

The OP describes, and expects some wierd voltages here. The 120's to ground make it sound like a Wye, but phase to phase sounds like Delta. Either way some of the measurements may not be what are quoted. If say there were an open, or even shorted winding he may be reading across or through a load which could throw all the numbers out the window. I would suggest checking voltage with all loads open, and at least determin what the system and voltage that they should be is.

Mike,

It's certainly a peculiarly North American configuration. I was trying to explain it to a nearby neighbor a few weeks ago and was getting that "You gotta be kidding" look.

I have to admit that when I first learned about the high-leg delta it seemed a little odd. But when you look at how and why it originated, it makes perfect sense.

Sometimes we like to make things as complicated as we can. I am not one of them.

Assuming there is no voltage because your meter registers no voltage is scary. What happens when you take a voltage reading when each conductor is on the same phase, your meter will not register voltage but there can be voltage present.

Ive ran across a three phase panel being fed from a single phase service with the extra phase tapped off of one of the others.

Maybe I misunderstood you post

The electrician should know what he/she is measuring. If you do not know what you are doing, you should not have a meter. I do not mean you "personally"
Of course no one is perfect. But to troubleshoot an open leg of a three phase system does not require a rocket scientist, it requires a knowledgeable electrician.

I agree with Niko on this. It sounds allot like a breaker that has a worn out/loose connection with the bus bar. One of those 120V to ground readings is coming back through a load in the unit. (As e57 also described)

Two words.

Backfeed.

I guess that's one word.

The voltage you were reading on the hi leg was backfeed thru the compressor from another phase.



Exactly.


Also, it is also confusing when people refer to the Delta leg being "208" . "208" should be reserved for the 120/208 systems. The Delta leg is usually (around here) 196-200 to ground.

I had the same service call and I found the overhead utility line (hi leg) laying down in the alley.

I have been tricked a few times working on 3 phase with a bad connection. I figured out long ago it is always easier for me to draw a picture of how I think things are wired and transfer the meter readings to the drawing as I do them. For me, seeing it on paper makes more sense that trying to remember what I was looking at and what I got several measurements ago.
If I actually have a wiring diagram and there is a copier nearby I will copy the page I am looking at so I can have one I can write all over.