480v 3phase

[john.swanger.5]

Have 480vac system......check voltage line to line and loosing 1 line but check line to earth ground and get 272 line to ground........ Confused......if I loose line shouldn't I loose voltage when checked to ground?

 

[JD6400]

Is it back feed ?

 

[john.swanger.5]

It's possible. I guess it boggles my mind cause I know I'm single phasing cause all my motor cares about is line to line voltage being 480 across but when checking to ground normal know what line I'm loosing cause it will read 0v.......and checking my cable to ground I get no ohm reading so I know I'm not grounded, unless it's in 2 not touching ground.

 

[JD6400]

How far back have you checked for the loss , is it as far back as the service head ?

 

[john.swanger.5]

It is fed from my disconnect(which reads good line to line) to my control panel 200+ feet way. So checking end to end.

 

[JD6400]

I am sorry mate , I am not trying to be a smart arse . But I am having a hard time to understand what you are trying to describe ?

 

[john.swanger.5]

Let me try again. This is an oilfield installation about 30+ years due for an upgrade. Got call and was told motor 1 hums and motor 2 doesn't nothing. I check my voltage in top of control panel and have 480, 480, 0 line to line, so I goto my main disconnect 200+ feet away and I have 480, 480, 480. So I tell myself I have a bad underground on one of my lines, so I check phase to ground and I have 272, 272, 272 to ground. So I then kill out the power and check my cable for a ground/bad area and find nothing so I'm clueless as to which phase is bad. It's hard to check for fault with fault finder if you can't find which line of the 3 phases is bad.

 

[JD6400]

Where are you based , 480v 3ph ?

 

[john.swanger.5]

Yes sir

 

[JD6400]

Yes sir

I do not understand ?

 

[Knobhead]

He's a septic.

480V 3Ph = USA

Why are you asking on a UK site?

You don’t say if it’s star or delta supply. On some delta systems one supply leg is grounded, others use a zig-zag reference to ground.
This sounds like a delta systems one leg grounded

You need to give more details for me to give any help.

 

[Knobhead]

What’s the betting he’s realised he’s dropped a monumental on the supply.

 

[Richard Burns]

If you had linked a phase then you would get those results.
E.g. at supply L1, L2, L3 and at load L1, L2, L2
at the supply full line to line voltage; at load, because L3 was dropped and two L2 brought up you woudl get full voltage on L1, L2 and the second L1, L2 but zero on L2, L2. However all Uo would be 272V.

 

[Engineer54]

What i can't understand is if you have determined that a phase is dropped out, why are you continuing to test on a live cable, when dead testing will quickly and more importantly safely locate the problem.

Have you checked the disconnect switch, ...is it a fused disconnect or just a switch disconnect?? If fused have you checked the fuses?? On the switch side, have you checked that the all the contact blades are all fully engaged and making contact in the ON position?? Can't count the number of times i've found that happen on American disconnects, both fused and unfused. Not the best designed switch/contact arrangement out there!! lol!!

 

[Marvo]

.......I check my voltage in top of control panel and have 480, 480, 0 line to line, so I goto my main disconnect 200+ feet away and I have 480, 480, 480.........

If you're getting 480-480-0v line to line then the third line has assumed the same potential (voltage) as one of the other phases due to a load being connected somewhere probably. The load isn't necessarily a motor, it could be something very small such as indicator lamps etc. This also explains why you're getting 272, 272, 272 with respect to earth or ground as you call it.

As already suggested you know you've dropped a phase somewhere so start dead testing to localise the problem.

 

[chcnorris29]

Reminds me of a post I seen before where there was voltage on all lines but total was showing a line down. Was caused my dno fault where fuse had blown cutting one line but the heat had welded 2 phases together causing the voltage at each phase.