480v Question

Status
Not open for further replies.
360Youth

360Youth

Senior Member
Location
Newport, NC
We just finished a job upgrading an ATS and generator for a water co. It is a 480/277v system. When we got everything installed and truned back on, all lights and motors were working great. (Lights and receptacles are powered from step down transforner.) The problem is when everything is up and running voltages readings to ground are 0, 480, 480. :-? :-? When we turn the main off voltages are normal. Phase to phase readings are fine. We double and triple checked bonding and connections. Any ideas?? There is no neutral in this system. There is only 3 wires feeding the 400a disconnect. Bonding is done through the equipment and bushings (all that is pre-existing.) I had no thoughts or reason to check voltage before hand, so I don't know if the same readings were there before we started.
 
The source is wired as a corner grounded Delta, I am assuming the readings were on the primary side of the "step down transformer".

Roger
 
480

480

I see a problem in the statement "it is a 480/277 system" and "there is no neutral"...it should be one or the other.... from what you say it appears to be a 480 system..and one phase has gone to ground downstream of the main (since it clears with the main off). By isolating feeders, then branch circuits, you should be able to locate the "fault" (possibly a bad term). If the supply is actually 480/277 then the neutral should have been brought to the service and the condition you show would result in a overcurrent device opening. If its a 480 delta system, I would suggest addding phase monitoring to alert you when the fault condistion occurs.

(sorry, I was typing slow while others were answereing)
 
augie47 said:
I see a problem in the statement "it is a 480/277 system" and "there is no neutral"...it should be one or the other.... from what you say it appears to be a 480 system..and one phase has gone to ground downstream of the main (since it clears with the main off). By isolating feeders, then branch circuits, you should be able to locate the "fault" (possibly a bad term). If the supply is actually 480/277 then the neutral should have been brought to the service and the condition you show would result in a overcurrent device opening. If its a 480 delta system, I would suggest addding phase monitoring to alert you when the fault condistion occurs.

(sorry, I was typing slow while others were answereing)
Click to expand...

The original feed is 3 wire with grounding through conduit and bushings. No grounded conductor pulled. 480 phase to phase, 277 to ground.
 
360Youth said:
The original feed is 3 wire with grounding through conduit and bushings. No grounded conductor pulled. 480 phase to phase, 277 to ground.
Click to expand...

The generator may be a wye source with 480V line to line and 277V line to neutral, but if you don't bring out the neutral, then you only have 480V available.

If the generator neutral is not bonded, then you have a 480V ungrounded system.

If you have a 480V system and one of the phases has a ground fault, then you will get voltages of 480V, 480V, and 0V.

-Jon
 
winnie said:
The generator may be a wye source with 480V line to line and 277V line to neutral, but if you don't bring out the neutral, then you only have 480V available.

If the generator neutral is not bonded, then you have a 480V ungrounded system.

If you have a 480V system and one of the phases has a ground fault, then you will get voltages of 480V, 480V, and 0V.

-Jon
Click to expand...

All readings are normal under genereator power. It is under utility load that we have the readings of 0, 480, 480 to ground. Turn the main off and voltage to ground is 277 on all phases at the line side. Phase to phase readings are good under all conditions.
 
don_resqcapt19 said:
You have an ungrounded supply and a ground fault on the load side of the main.
Don
Click to expand...

This may sound like an unschooled question, but any suggestions on finding the fault? Everything we tested checks good. Step down transformers handle lighting and controls. It was a very long 2 days doing the changeover, so we only did preliminary checks before it was time for a long drive home. I wish I knew if the same faults were there before we started.
 
480V corner grounded delta

480V corner grounded delta

I have only worked on a system of this type once in 30+ yrs and it was in a poco plant where one of the employees told me that one of the windings in a generator grounded and they made it this way to still be able to use the old generator. It was on a 5000 amp distribution old copper bussbar system in the poco plant. The company I was working for was upset because we didnt know the concrete decks were 3ft thick and we had to run conduit between decks. OUCH! I naturally tested all voltages myself and it was eerie to say the least. Not a place for beginners.
 
Don is correct

Don is correct

don_resqcapt19 said:
You have an ungrounded supply and a ground fault on the load side of the main.
Don
Click to expand...
Try turning off all of the main distribution loads and turn them back on one at a time until you see your voltages change. Then you can further investigate the ground fault without disturbing the majority of the building. May I suggest the amprobe 2005 series for finding the ground fault just follow the directions in the manual it will seem a little strange however you will loose tone when you pass the ground fault It is well worth the money unless you have a lot of time to wait fo one on ebay. Word of warning I suspect that the padmount xformer has a grounded Xo in the xformer enclosure grounding stirrup probably 2 groundrods one primary side one secondary side of the vault enclosure. IF there is a ground fault on a phase AND it turns out that the service ground is inadequate to clear the ocp at the mdp you probably have fault current running on the ground path back to the xformer. That being said be extremely careful pulling off ground wires as you may become a victim of the current never goes to ground group as you are screaming in a panel doing the funky chicken waiting for the ground fault to trip. The main is probably set at 200 amps so try to make that wire back to the busbar or metalic pipes in the room. That being said I think the ground fault is probably on a 480/277 volt lighting ckt as this is the most logical place for a ground fault with all the fixture splices and colorblind people trying to figure out high voltage colors to the fixture wires.
 
Last edited:
but any suggestions on finding the fault?
Click to expand...
1) turn off all feeder breakers
2) check the voltages
3) turn the feeder breakers on one at a time checking the voltages each time
4) you now know what feeder has the ground fault so you turn the branck breakers off one at a time checking the voltages each time....when you turn off the branch breaker that has the ground fault, the voltages will return to normal.

Note that the ground fault could be on a load and will not show up unless the load is on. Also assuming that the generator has its grounded conductor bonded to the grounding system, the ground fault is not a solid fault...it is was, a breaker would trip.
Don
 
ditto?

ditto?

don_resqcapt19 said:
1) turn off all feeder breakers
2) check the voltages
3) turn the feeder breakers on one at a time checking the voltages each time
4) you now know what feeder has the ground fault so you turn the branck breakers off one at a time checking the voltages each time....when you turn off the branch breaker that has the ground fault, the voltages will return to normal.

Note that the ground fault could be on a load and will not show up unless the load is on. Also assuming that the generator has its grounded conductor bonded to the grounding system, the ground fault is not a solid fault...it is was, a breaker would trip.
Don
Click to expand...
We agree totaly scary huh??
 
As far as I can tell auggie and winnie have give you the best insite. You gave some voltage readings that pretty well rule out solidly grounded 480/277 for the utility or it's not wired per code.

When you say 480/277 then either its solidly grounded and the neutral has to go the the first disconnect or it's impedance grounded 480/277 - go look for a neutral grounding resistor - uncommon if the xfm belongs to the utility.

However, more important, it doesn't sound like you have any prints. I'd start by figuring out exactly what the incoming service is. If the customer can't give you a one-line, I'd recommend making one. Nothing says the original installation was anywhere near code.

Hope there is enough money for you to figure this one out. It would bother me to walk away from this one.

carl
 
Augie said the same thing

Augie said the same thing

coulter said:
As far as I can tell auggie and winnie have give you the best insite. You gave some voltage readings that pretty well rule out solidly grounded 480/277 for the utility or it's not wired per code.

When you say 480/277 then either its solidly grounded and the neutral has to go the the first disconnect or it's impedance grounded 480/277 - go look for a neutral grounding resistor - uncommon if the xfm belongs to the utility.

However, more important, it doesn't sound like you have any prints. I'd start by figuring out exactly what the incoming service is. If the customer can't give you a one-line, I'd recommend making one. Nothing says the original installation was anywhere near code.

Hope there is enough money for you to figure this one out. It would bother me to walk away from this one.
carl
Click to expand...
But probably scared the op away when he mentioned phase monitoring equipment
 
quogueelectric said:
But probably scared the op away when he mentioned phase monitoring equipment
Click to expand...
Maybe, but shouldn't have. Even at $1/W this is a $.5M job. Three transformered 480V pilots are not much to buy and install.

carl
 
quogueelectric said:
But probably scared the op away when he mentioned phase monitoring equipment
Click to expand...

No, I just had to go to bed. Thanks to everyone for sticking with me so far. We are looking at getting some mor monitoring equipment anyway because many customers with generators are upgrading their loads and having problems with generator keeping up. We gotta start checking peak loads, blah, blah...


quogueelectric said:
Try turning off all of the main distribution loads and turn them back on one at a time until you see your voltages change. Then you can further investigate the ground fault without disturbing the majority of the building.
Yes, thank you. That is our next step. As I mentioned earlier, we ran out of day to dig farther.



quogueelectric said:
That being said I think the ground fault is probably on a 480/277 volt lighting ckt as this is the most logical place for a ground fault with all the fixture splices and colorblind people trying to figure out high voltage colors to the fixture wires.
Click to expand...

There is no such lighting. 480v motors with step down transformers handling 120v controls, lighting, and basic receptacles. Proper grounding is present at receptacles.
Click to expand...
Click to expand...
 
Status
Not open for further replies.
Top