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Joined: Oct 2005
Posts: 141
L
Member
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.

Joined: Jul 2004
Posts: 625
S
Member
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.)

Joined: Aug 2006
Posts: 356
Member
[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


Be kind to your neighbor, he knows where you live

Joined: May 2003
Posts: 2,876
E
e57 Offline
Member
Originally Posted by Tripp
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?

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.


Mark Heller
"Well - I oughta....." -Jackie Gleason
Joined: Aug 2001
Posts: 7,520
P
Member
Originally Posted by Trumpy
I can't say I understand this system at all, it's totally foreign to what I've been trained in.


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. wink

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.

Joined: Jun 2007
Posts: 301
J
Member
Sometimes we like to make things as complicated as we can. I am not one of them.

Joined: Oct 2005
Posts: 141
L
Member
Originally Posted by JValdes
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

Last edited by LoneGunman; 06/24/07 11:40 PM.
Joined: Jun 2007
Posts: 301
J
Member
Originally Posted by LoneGunman
Originally Posted by JValdes
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.

Joined: Jul 2006
Posts: 272
L
Member
Originally Posted by Niko
[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


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) wink


Luke Clarke
Electrical Planner for TVA.

Joined: Feb 2007
Posts: 20
J
Junior Member
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.

Quote
Checking voltage to ground is what has you confused. I don't use the practice as I have found that it complicates simple problems.


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.

Last edited by joncon; 06/25/07 06:04 PM.
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