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landmark
04-10-2011, 09:45 AM
Hi, I had a compressor today that I was told by an electrician kept tripping the circuit breaker. He kicked it and got it running. When I go there all appeared well. It was a single phase compressor and when I checked for resistance between a couple of the compressor pins and earth I was getting only a couple of ohms. I can't understand how it was working and not tripping the circuit breaker. Has anyone heard of this or got any explanation?

mikeref
04-10-2011, 11:29 AM
Tripping circuit breaker when sparkie was there and now it runs with short to earth after kicking it in the guts? Yeh... sounds fishy to me. First of all, (if you are not pulling my leg), than the sparkie should be sacked as he would know better than to let faulty item pass his sight without tagging and locking it out of service.

landmark
04-10-2011, 12:18 PM
I am definitely not pulling anyones leg. Yes the sparky should have tagged it and locked it out of service. I used two separate metres to get these readings so it wasn't a dodgy meter. Do you think if he disconnected the earth it would be possible for the compressor to still run. Seems to me though if I'd touched it with it turned on I would have been zapped, in which case this sparky should be in jail.

mikeref
04-10-2011, 12:45 PM
Not for me to decide on legal judgement. Disconnecting the earth wire on any electrical item is beyond belief, let alone on something that is proven faulty. No registered electrician would ever allow anyone's life to be put in danger. Incident report should be submitted.

install monkey
04-10-2011, 01:47 PM
Did u test the insulation with an insulation tester or a multimeter? Anything under 1 meg ohm will trip a rcd,if its a mcb it will still run but trip occasionally until the insulation fails. Did u disconnect the compressor when u tested it,are u sure its not siezing and tripping on overcurrent!!

texas64
04-10-2011, 02:41 PM
Are the resistances between each pin to the others correct per compressor manufacturer? If you have a motor winding that is making contact to earth, then your ohm reading would be off.

texas64
04-10-2011, 02:42 PM
Another thing, when you checked the pins originally, you did remove all the terminals off the compressor, right?

paul_h
04-10-2011, 02:43 PM
You did disconnect the wiring to the compressor when testing it didn't you? So you were testing bare terminals at the compressor and nothing connected to it?


^ edit: Ha Ha, beaten by a minute! ^

texas64
04-10-2011, 02:49 PM
Haha, just a minute...

install monkey
04-10-2011, 03:47 PM
Paul- post 5 beaten by nearly an hour haha

paul_h
04-10-2011, 05:51 PM
I missed that.

:o

Now I'm worried if TX was being sarcastic or not.

Help me!

Yuri B.
04-10-2011, 07:32 PM
All is possible. If earth shorted is the to neutral connected winding, and by that in the point where potential is close to zero, the motor will work, however with higher amperage (and will soon burn). MCB may hold the heightened current. The moment you turn places the neutral and phase, the mcb should momentarily switch power off.

install monkey
04-10-2011, 07:33 PM
paul could have been worse- it could be a 3yr old thread haha !!!

I missed that.

:o

Now I'm worried if TX was being sarcastic or not.

Help me!

Quality
04-10-2011, 07:47 PM
All is possible. If earth shorted is the to neutral connected winding, and by that in the point where potential is close to zero, the motor will work, however with higher amperage (and will soon burn). MCB may hold the heightened current. The moment you turn places the neutral and phase, the mcb should momentarily switch power off.

I was thinking very similar with regards to the neutral short. But still would not remove the earth connection and leave it running

chillerman2006
04-10-2011, 08:18 PM
I am definitely not pulling anyones leg. Yes the sparky should have tagged it and locked it out of service. I used two separate metres to get these readings so it wasn't a dodgy meter. Do you think if he disconnected the earth it would be possible for the compressor to still run. Seems to me though if I'd touched it with it turned on I would have been zapped, in which case this sparky should be in jail.

Hi landmark

No as you would need to remove more than just the compressor terminal earth on most units

Question: when you checked resistance, was this with a multi meter ?

and what ohms ? mega ohms ?

best to check with meggar not multi meter as they dont have the juice to check properly

R's chillerman

install monkey
04-10-2011, 08:23 PM
chillerman post 5 thank you!!!

Hi landmark

No as you would need to remove more than just the compressor terminal earth on most units

Question: when you checked resistance, was this with a multi meter ?

and what ohms ? mega ohms ?

best to check with meggar not multi meter as they dont have the juice to check properly

R's chillerman

monkey spanners
04-10-2011, 08:27 PM
If the short is near the neutral end of the windings it would run (assuming the rest of the windings are ok)

Remember finding a single phase fan on a three phase unit once, no neutral so the fitter had just connected the neutral from the fan to earth, had been running 10+ years like it.

Jon :)

landmark
04-10-2011, 10:02 PM
Thanks so much for your help. My question has been answered.

landmark
05-10-2011, 02:04 AM
Yuri B gave the correct answer and yes I did remove all the wiring when checking the pins.