checking for ground shorts on 480 circuit before power turne

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doneright

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I just finished hooking up a 480 volt aircompressor and before i turned on any power i used my ohm meter to check for any possible ground faults. I tested from all lines to each other and then all lines to ground, no faults. until i turned on the power. bang smoke and a tripped circuit breaker. :shock: whats the best way to check for ground faults before power, that is correct and doesn't provide a 4th of july show for the plant manager who was standing nearby at the time.

ps i checked both the line and load side of both the safety switch and the motor starter. the short occured when purple wire was nicked by MC connector, and yes an anti short bushing was used, however thats supposed to protect the wire from the cable assemblie.

any info would be great
thanks DR will
 
If the short was inside, or load side, of normally open starter, then checking compressor terminals may have been the only way to see it.

If the short was before the starter, and your meter missed it, maybe resistance was too high. Checking equipment installs with a Megger is not STD Operating Procedure, neither are wire tracers, even if they could pick up a signal on a high-Z ground. Anyone??
 
You need to use a megger and not an ohm meter for this type of testing. The voltage is too low in an ohm meter to detect many faults of this type. The megger will use 500 or 1000 volts, the ohm meter will use 9 or less.
Don
 
If a ohmmeter is all you have then you did all you could. I agree with Don that a meggar was a better choice and the only choice when fault currents are going to be high. Not that anyone wants to "smoke" test any electrical system. But you need to be trained on how to use a meggar safely. They can damage equipment and kill you.
 
stormywyo said:
If a ohmmeter is all you have then you did all you could. I agree with Don that a meggar was a better choice and the only choice when fault currents are going to be high. Not that anyone wants to "smoke" test any electrical system. But you need to be trained on how to use a meggar safely. They can damage equipment and kill you.
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This is going to sound awfully stupid but can my portable megger really take my life? I have always been very careful with them because I assumed the shock from the test circuit would be painful. But until I read your post it had not occurred to me that the battery powered megger might produce enough current to kill me.
 
It can sure give you a shock. Navy electricians used to shock mechanics wit them ( I was a mechanic)
Fluke has a great new combination megger-voltmeter, it sells for about $250.
Well worth having
 
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