800 Amp Bucket Tripping

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Drake5265

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It hasn't rained in like 2 or 3 days now I didn't think that those reading would be that low maybe I need to find where we attach to the ground ring at lol:)
 

Drake5265

Member
I have located the problem with a branch circuit in the panel fed by the 800 amp circuit breaker it is caused by the branch circuit powering a rooftop air handler it rained this morning the breaker tripped I megged it out and my result was the B phase wire had a reading of .096 MOhms
 

Electric-Light

Senior Member
I have located the problem with a branch circuit in the panel fed by the 800 amp circuit breaker it is caused by the branch circuit powering a rooftop air handler it rained this morning the breaker tripped I megged it out and my result was the B phase wire had a reading of .096 MOhms

What test voltage?

.096Mohm is 96kOhm, which even at 277v, it would only let through 2.9mA (3/1000th of an A). Allow fluctuation for environmental variations and drop it to 1kOhm and you're still only pulling 300mA (which is just about the right current of a properly working 3 lamp fixture), it's not even remotely close to activating overload or shot circuit.
 
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What test voltage?

.096Mohm is 96kOhm, which even at 277v, it would only let through 2.9mA (3/1000th of an A). Allow fluctuation for environmental variations and drop it to 1kOhm and you're still only pulling 300mA (which is just about the right current of a properly working 3 lamp fixture), it's not even remotely close to activating overload or shot circuit.


Mmmm. I sure have failed a lot of underground and motors with better readings. Guess I have been wrong for several years now. Strange that things 'worked' after correcting such a small problem.

My semieducated guess from here is he is on the right track.
 

hurk27

Senior Member
What test voltage?

.096Mohm is 96kOhm, which even at 277v, it would only let through 2.9mA (3/1000th of an A). Allow fluctuation for environmental variations and drop it to 1kOhm and you're still only pulling 300mA (which is just about the right current of a properly working 3 lamp fixture), it's not even remotely close to activating overload or shot circuit.

What your missing is carbon flashover, you don't apply the resistance like you would a load.

96kohm is a very good indicator of bad insulation that with the right conditions will flash over into a bolted fault.
 
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