chris.doyle.391
Member
- Location
- Ukiah ca
Hey thanks guys for all the help
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Sent from my iPhone using Tapatalk
How am I getting 480v from each leg
- Thread starter chris.doyle.391
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Michael15956
Senior Member
- Location
- NE Ohio
Chris, sounds like there is a grounding conductor mixed wired with a phase wire after the disconnect.
If there was a grounded wire he would only get 480V on two of the legs
big john
Senior Member
- Location
- Portland, ME
Something is ungrounded. There's just no way in the world a solidly grounded wye system is gonna give you those voltages.
The description is confusing, so I'm not at all clear what you're up against, but I can say with certainty if you've actually got a shifting phase-to-ground voltage it's because of a ground-fault on an otherwise ungrounded system.
The description is confusing, so I'm not at all clear what you're up against, but I can say with certainty if you've actually got a shifting phase-to-ground voltage it's because of a ground-fault on an otherwise ungrounded system.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Or a compromised neutral and asymmetric leakage?
Any chance the disconnect opens a grounded conductor?
Any chance the disconnect opens a grounded conductor?
Definitely a puzzler.
If it's phantom voltage as it appears most are suspecting, can anyone explain how it's higher than actual line-to-ground voltage?
Are there any other loads that are energized when the motor disconnect open that has circuit conductors in close proximity with the motor conductors?
When disconnect is open, did you check line-to-line voltage on the load side terminals? If so, what did you get? If it's phantom voltage, I'm thinking line-to-line voltage should be relatively close to zero because the motor windings are still connected with one contactor is stuck closed.
If it's phantom voltage as it appears most are suspecting, can anyone explain how it's higher than actual line-to-ground voltage?
Are there any other loads that are energized when the motor disconnect open that has circuit conductors in close proximity with the motor conductors?
When disconnect is open, did you check line-to-line voltage on the load side terminals? If so, what did you get? If it's phantom voltage, I'm thinking line-to-line voltage should be relatively close to zero because the motor windings are still connected with one contactor is stuck closed.
That's inline with what I'm thinking.... but you'd need two faults, with at least one having fairly high resistance to ground, to read 480V to ground on the load side of an open disconnect and also not trip any OCPD when the disconnect is closed... and that's assuming it's an ungrounded system.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I have to think about this more and maybe draw it out, but here's my initial raw train of thought.
Don't forget it's a crane, so maybe the power feed to it is through duct-o-bar. It's likely only 3 bars, no ground bar for reference because on old crane systems I have found they sometimes were grounding through the trolley wheels to the rail. Now 60 years later there is a layer of dust built up that is, in essence, insulating the trolley from the building ground or at least creating a high resistance to ground. If that's the case, AND a line is shorted to the frame, you have an energized ground plane on that part of the crane. Because of the bad connection through the wheels, it's like a localized Delta or HRG system. Fault tolerant, doesn't blow the OCPD, but gives funny readings on sensitive DMMs. And of course, now VERY dangerous if anyone makes a better ground connection from the trolley to the building iron!
What doesn't fit is the part about ALL 3 reading 480V to ground. The one that is shorted should read zero because it would be at the same potential. Was there perhaps some ASSuming going on here? Did you perhaps forget and leave the "Peak Hold" button on?
Note: No Peak Hold on a Wiggy!
K8MHZ
Senior Member
- Occupation
- Electrician
My Ideal Vol-Con II is one of my favorite tools.
The only down side is it uses button cell batteries, like 6 of them, and they aren't cheap. But still very much worth it.
Not getting any "vibe" on your "dust" supposition at present.
But all three measuring the same voltage to ground isn't hard to conceptualize with a stuck contactor. Any one fault to a motor conductor would put all three at the same potential.
But to read 480V to ground from those conductors requires "ground" to be at the same potential as one of the other phases... which deduces to another line-to-ground fault on the other side of the open disconnect. Possible on an ungrounded, perhaps even an HRG system, but one, the other, or both faults would have to be quite resistive to not trip any OCPD.
meternerd
Senior Member
- Location
- Athol, ID
- Occupation
- retired water & electric utility electrician, meter/relay tech
Pull the motor leads out of the load side of the starter, then measure everything again. If all reads as expected until the motor is connected, you've got a motor wiring problem somewhere. Sounds to me like you have a three wire motor load but you are measuring line to ground (neutral). If the neutral is not solidly grounded at the source, a line to ground reading will be meaningless. Make sure you really have a 277/480 grounded Wye and that the neutral is grounded AT THE SOURCE. Downstream grounds are a no-no. Bet you'll find a wiring error somewhere. "Pantom voltages" are usually on floating conductors, not those tied into a circuit.
ptonsparky
Tom
- Occupation
- EC - retired
Last old cranes I was around used wound rotors and banks of resistors for speed control. Grounding was not high on the list. Thinking none, as Jraef described. This was a 40 ton overhead. We had some obvious potential problems...don't remember how I fixed it. Back in the mid to late 70s. Dang CRS.
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