# A,B,C 277v to ground missing 480 A to B



## cycotik (Jul 12, 2011)

Ok here it is. 277/480 Nema Motor Starter. I was trouble shooting loss of power the other day and this plow lost power. So of course I begin checking voltage. A,B,C all have 277v to ground. A to C, B to C have 480. A to B has 0 volts. So I knew the problem was before the starter because it was losing voltage on the line side of the starter. The power cable plugs into a 480 3 phase plug in the ceiling. The way the plugs are you can't stick your probes in to test voltage. They have a push in interlock and regular meter leads don't go far enough. Anyways I took the plug out and all the inserts after I checked power and did a continuity test through the inserts. I found two were bad. I took the inserts apart and the contacts were burned from arching or just too much usage. They inserts are seperated from each other through mutilple layers. I found the problem but what I don't understand is how I had 277 to ground at all phases and lost 480 between two. It is a grounded system. I'm not sure if the MCC is feed with a Delta or Wye, but I don't think it would matter anyways because a Delta or Wye that's tapped different depends on the Neutral and as everyone knows 480v motors don't use a neutral. The meter I'm testing with is a fluke 747. 

Finding the problem wasn't too hard I'm just confused on the theory of the lost 480, when I still had 277v on each phase. Please help me understand this.


----------



## HARRY304E (Sep 15, 2010)

cycotik said:


> Ok here it is. 277/480 Nema Motor Starter. I was trouble shooting loss of power the other day and this plow lost power. So of course I begin checking voltage. A,B,C all have 277v to ground. A to C, B to C have 480. A to B has 0 volts. So I knew the problem was before the starter because it was losing voltage on the line side of the starter. The power cable plugs into a 480 3 phase plug in the ceiling. The way the plugs are you can't stick your probes in to test voltage. They have a push in interlock and regular meter leads don't go far enough. Anyways I took the plug out and all the inserts after I checked power and did a continuity test through the inserts. I found two were bad. I took the inserts apart and the contacts were burned from arching or just too much usage. They inserts are seperated from each other through mutilple layers. I found the problem but what I don't understand is how I had 277 to ground at all phases and lost 480 between two. It is a grounded system. I'm not sure if the MCC is feed with a Delta or Wye, but I don't think it would matter anyways because a Delta or Wye that's tapped different depends on the Neutral and as everyone knows 480v motors don't use a neutral. The meter I'm testing with is a fluke 747.
> 
> Finding the problem wasn't too hard I'm just confused on the theory of the lost 480, when I still had 277v on each phase. Please help me understand this.


What it sounds like to me is phase A or B is shorted to ground and that is why you are getting 277 instead of 480 Volts.


----------



## MDShunk (Jan 7, 2007)

With only reading the title, I'm going with blown fuse/dropped phase. Motor or similar load on load side.


----------



## nitro71 (Sep 17, 2009)

You were reading voltage through something. My eyes sorta glazed over reading your paragraph. IE: One leg dropped, reading voltage through a winding.


----------



## cycotik (Jul 12, 2011)

*Thanks so far*

It wasn't shorted to ground. Otherwise I wouldn't have gotten 277 on each leg. This is a grounded system and I don't think I would read the 277 to ground in this one spot where the ground is continuous through out the system. There was no blown fuses, just a bad 480 plug and all the phases were isolated from each other. I wasn't reading through a winding either. The problem came from a fused disconnect to the plug. I was reading the voltage at the line side of the motor starter, so the motor was not on and the coil on the motor starter was not made, so the starter wasn't on. I did drop a phase I just don't understand why 277 would be there and no 480.


----------



## freeagnt54 (Aug 6, 2008)

I think you lost either the A or B phase up the line and the A and B conductor were shorted together. That would explain why you were measuring 277 to ground on each wire.


----------



## cycotik (Jul 12, 2011)

*Thats it*

Yep that's it freeagnt54. I knew I lost 277v, and I originally thought this same senerio where one phase was lost but not shorted and A phase at 277v bleed to B phase that only showed 277. This could happen and a fuse won't blow depending on where you lost it. The trouble I was having is that if all the phases are completely isolated I don't know how it would pick up 277. Inductance wouldn't produce the same voltage, and the inserts in this plug are isolated then three layers of plastic cover them also. Other than that the spacing between the conductors are about 2". So I just can't understand where the bleed would come through. With the systems it doesn't seem like it would be given the opportunity. The plug was undamaged as well insulators and inserts. Just the brushes in the inserts were damaged.

I shouldn't be able to get feedback on the line side of the starter with if off to read through B phase should. It doesn't make sense to me.


----------



## Service Call (Jul 9, 2011)

You will read 277 to gnd thru the contacts that work off the heaters and the coil itself. But you won't get 480 due to its the same phase. The contactor does not need to be pulled in, in order to read it because the wiring for the overloads are off the line side of the starter. For instance, lets say (without a switch installed) L1 feeds the control ckt. It then goes to the overloads, then thru the coil, then to L2 for a 480 coil.


----------



## Service Call (Jul 9, 2011)

cycotik said:


> It wasn't shorted to ground. Otherwise I wouldn't have gotten 277 on each leg. This is a grounded system and I don't think I would read the 277 to ground in this one spot where the ground is continuous through out the system. There was no blown fuses, just a bad 480 plug and all the phases were isolated from each other. I wasn't reading through a winding either. The problem came from a fused disconnect to the plug. I was reading the voltage at the line side of the motor starter, so the motor was not on and the coil on the motor starter was not made, so the starter wasn't on. I did drop a phase I just don't understand why 277 would be there and no 480.


You will read voltage thru a winding. Its just a long piece of wire wound up.


----------



## ralpha494 (Oct 29, 2008)

It seems like you lost B phase someplace and that A & B phase are tied together somewhere, hence 277V to ground and 0V across A to B. Like the control tranformer. I'll bet they used A & B phases. 

If you ever find out what the problem was, please let us know.


----------



## cycotik (Jul 12, 2011)

*Got it*

It wasn't the heaters or that coil, Instead I found a hidden 480v coil that interrupted between the starter and the plug. So it was reading through a sperate coil thanks for everything.


----------

