You or someone else will be back on this one.If you repair or replace, check the ground connecting the unit back to building circuit.I have seen old systems that used the conduit for ground and you could read voltage from body of unit to ductwork,this is the reason we hear of techs. getting shocked or killed when climbing over equipment.I have seen 115 volt control transformers grounded to gas lines because units had no ground - just waiting for us to complete the circuit.Be careful.

 

I still dont know how this thing has been running a week

 

I still dont know how this thing has been running a week

Was the resistance at the compressor terminals with all the wires off the compressor? If not, it may be something beside the compressor that was causing the resistance to ground.

You didn't say what the resistance to ground was. A standard breaker doesn't sense current to ground - only high current on the phase legs. If the resistance was high enough, the unit can continue to run with some current going to ground. If there were a GFCI on the circuit, it would trip the GFCI.

 

Please pardon this seemingly stupid question ... but I just gotta ask it. And by the way, I would expect you to ask it of me if the shoe was on the other hoof.

Did you strip the terminals down to their bare minimum brass before you made this measurement?

In other words, there were no OTHER conductors attached to the compressor terminals while you made your continuity measurements!?!?!?!?

Remember ... I wrote the book on dumb moves. I dont mind you getting in a chapter of your own now and then, but please DO NOT attempt to hord in on MY territory


We all have been there/done that!

 

I agree I think there was some other connection there, OR just maybe there was a small grounding path that blew out when it started again, a small chance, and also I would agree with these other answers. Sometimes when you are checking things you may miss something obvious or there is something really wierd going on. I was checking a lighting circuit the other day and read open on my meter but when I hooked it up, the electronic ballast worked and let there be light. The other lights showed resistance. Still working on that, but figure has something to do with my meters ability to read some level of resistance.

 

Freaky, eh?..I've had that happen on a couple of cond fan motors. Run winding is grounded about half way through and the motor runs..Kinda like running your vac pump on a cheater cord off a 230 volt unit.

I had a 230 volt cond. fan motor(psc) that was grounded and ran half speed when the relay de-energized. Relay broke 1 leg only.

The load is still there, it's not a direct short to ground, but comp should be changed for anyways for safety reasons mentioned.

If the building had ground fault protection, you'd be changing that comp right now hehe

[Edited by sonc on 07-26-2004 at 06:40 AM]

 

you can also read through the oil in the crankcase. also what ohms scale are you on, that will show a ground if your on the wrong scale.

 

Sorry guys, I've never seen a grounded compressor run?? Were you using a digitial meter on a low scale?

 

Sorry guys, I've never seen a grounded compressor run?? Were you using a digitial meter on a low scale?

My Fluke 16 on a humid morning on auto range will read everything to ground. Meg-ohm meter will also be affected by extremely humid conditions. I definately agree that you must remove all the wires from the compressor and check it at the terminals with the wires off. I have been on at least a dozen calls over the years to give competitive quotes on compressor replacements and found a compressor lead wire shorted, bad external overload, defective molded case circuit breaker, etc. so use everything in your bag of tricks to make sure. You don't want to keep throwing fuses at a bad compressor however as you can add to a burnout condition.

 

Sorry guys, I've never seen a grounded compressor run?? Were you using a digitial meter on a low scale?

+

 

I had to have been on the high scale, i have an amprobe ACD-12 which i think goes up to 4 megs so now i look like an ass. Unit is still running.

No I did not isolate the comp, when i got there the comp was cool to the touch and I had 208 volts to run and start. Shut down and measured resistance between winding and found incorrect readings then checked for grounds, measured a ground with wires still connected(rushing) and just condemmed it.my mistake i guess!

So i must have measured maybe 2 million ohms to ground thats why it is still running for a while anyways!

 

The breaker never tripped at all, i guess the internal must have been open .i duno just floundering!

 

Shut the power off to the unit and measure the resistance of the compressor windings at the compressor terminals with the wires disconnected. Megar the contactor and all components to varify for grounds. Bare in mind as you do this that EVERYTHING must be isolated and checked individually...motors, controls and all wiring. Your digital meter can give false ground readings on high resistance scales especially if the batteries are weak.

 

went on a call once and found a grounded comp comp 7.5 ton semi-hermetic 3 phase, thing ran for 1/2 hour then tripped breaker, there was a punget odor in the gas, we had our own rebuilding shop so when the comp was brought in i asked the guy to let me know what was up with it and sure enough it was grounded

 

This gas also had a pungent odor thats when i condemmed this running compressor.lol!

 

Relax, I have seen that as well, weird but in time it will go bonkers and take out the breaker, just a matter of time, it is toast.
Once we had a new Payne, 3 ph 5 ton condenser unit installed, it was tripping on it's cute little 3ph protector board. It did that till we shut off another, older 5 ton condenser sitting next to it, then it ran fine.
We checked out the older unit and the comp read grounded, and the freeeezone smelled burnt too, but it was running??
We advised bldg owner and changed comp on old unit, all OK after that. Talked to local Carrier/Payne service guy about it, very sharp dude. He never heard of it either.
That's what makes this field so crazy sometimes, ya never know.

 

yep they turned it back on called me a liar and a couple of days later.....

Have had more than one call about a self contained fridge zapping people that turned out to be a grounded compressor but the only reason they weren't tripping breakers was a bad ground to the unit.

 

bet the ground was burnt.........

 

I have had several motors run with low rresistance to ground.
The first was a mg set the megger read 0, so I used the fluke and got a reading in the thousand ohm range, both on the 3 phase 480 m0tor and the 200+ DC generator. I figured it was carbon dust. Tried to get my employeer to clean motor and redip it, but no cost too much. I think it is still running today.
The second was as a volunteer at Spirit West Coast, We were putting disconects and cam locks on a 480 to 208 transformer to power up mail stage. some one walked past and asked if that was the transformer that had been underwater in the flood. We megged it 0 megg ohms. Used a fluke and got a low ohm rreading. With the festable the next day we had to try, it did not blow. We use that transformer each year.
I do not explain it I just saw it.
Len

 

I've seen problems with the neutral buss itself not being connected.

 

Ive never ever ever seen a grounded compressor not tripping breaker or blowing fuses..wow thats a wild one. I know one thing..i wouldnt lay my bare hands on that unit without cutting power first.

 

Thanks Dice...now i feel a little better and i will now always try to restart a grounded compressor so some maintence guy does not show me up! If this customer would have just said go ahead we would have never learned this .

Brian

Glad to see others share their experience on here.