Minimum Megger Readings

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sfav8r

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I realize that different wire will have different specification, but I have sort of a general question.

I was on a job yesterday that had a bathtub overflow for several hours. Our job was to determine if it would be safe to turn the power back on to the areas that got wet. After opening up fixtures, looking in the drywall where the plumber opened etc., I meggered the wiring. The home run to the bathroom was great +2,000 Megs. Some of the jumpers from plug to plug were what I would normally find acceptable at over 100 Megs. Two of the jumpers were only 2 megs. The interesting thing is that if we disconnect everything and check each jumper individually, we get over 100 megs on each jumper. Once we connect them together, it drops down to 2 megs or so. I would normally say this indicates a leakage at the wire nuts but the wires are sticking out of the box and are a good 6 inches from the box. (FYI, I'm talking hot to ground measurement)

OK, so even with 2 megs of resistance, it's not going to trip a breaker,and I don't think it is likely to pose a safety issue, but I need to understand what's is going on here before I power it up. This particular job is 12/2 MC cable with steel boxes.

Thanks
 
Can we assume that walls and circuits were opened outside of the bathroom, would be my first question?

No GFCI was mentioned in your OP as some protection with-in the bathroom! (?)

Are these baseboard RECP. or other, that are outside of the bathroom?

Did you see or could one see where the overflown ran to from the floor above?
 
Can we assume that walls and circuits were opened outside of the bathroom, would be my first question?

Yes, everything beyond the bath is disconnected.

No GFCI was mentioned in your OP as some protection with-in the bathroom! (?)

All devices were removed before megger testing

Are these baseboard RECP. or other, that are outside of the bathroom?

Everything after the bath is disconnected.

Did you see or could one see where the overflown ran to from the floor above?

I'm not exactly sure what you mean. The water ran through the light fixtures (which we removed) down the walls, etc. Sheetrock ws opened in the ceiling, but not the walls.
 
Well look at romex meaning the make up of the install and the conductors and jacket lets say its age or the type of install meaning a staple hammered into jacket many conditions can give you a lower reading one can see if water entered the internal jacket of the romex it just might give a bad reading you think .

But i say your fine you dont have to have the same high megg reading on all conductors it can change and 2 megs is a good reading for a water bath .

Dont do it if you feel its wrong but turn it on !

Romex has conductors in a jacket fill it with water its capacitance has changed now between the plates of its structure the plates are the conductors at high voltage yes it will read low at 120 volts no.


Hook up a load on the circuit let her warm up under load in a few days it will come up to a higher megg reading as the wire drys out megg readings change from day to day i know
this because i megg hundreds of conductors each year and ive turned on many conductors with lower readings than 2 meggs
wire resistance doesnt have to be 999 meggs the damp and wet will effect the megg reading even on the outside of that conductor or jacket take a roll of wire megg it then drop it in a tub of water megg it you will see what iam trying to tell you turn it on your fine !
 
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ohmhead said:
Well look at romex meaning the make up of the install and the conductors and jacket lets say its age or the type of install meaning a staple hammered into jacket many conditions can give you a lower reading one can see if water entered the internal jacket of the romex it just might give a bad reading you think .

But i say your fine you dont have to have the same high megg reading on all conductors it can change and 2 megs is a good reading for a water bath .

Dont do it if you feel its wrong but turn it on !

Romex has conductors in a jacket fill it with water its capacitance has changed now between the plates of its structure the plates are the conductors at high voltage yes it will read low at 120 volts no.


Hook up a load on the circuit let her warm up under load in a few days it will come up to a higher megg reading as the wire drys out megg readings change from day to day i know
this because i megg hundreds of conductors each year and ive turned on many conductors with lower readings than 2 meggs
wire resistance doesnt have to be 999 meggs the damp and wet will effect the megg reading even on the outside of that conductor or jacket take a roll of wire megg it then drop it in a tub of water megg it you will see what iam trying to tell you turn it on your fine !
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I think you're probably right about the readings improving after heating it up. We were also thinking of just letting it air out another day or 2 to see how much it improves. I was mostly puzzled by the fact that each individual reading was good but when made-up they dropped.

thanks for your input
 
sfav8r said:
I think you're probably right about the readings improving after heating it up. We were also thinking of just letting it air out another day or 2 to see how much it improves. I was mostly puzzled by the fact that each individual reading was good but when made-up they dropped.

thanks for your input
Click to expand...

Well I see 2 meggs as good 1 megg is passing theres so many things that could or could not be or was or maybe or its just not a bigg deal if you ask me the areas water damage is done so what do you have to lose wait it out dry it out and youll save yourself lots of work for nothing .

Example gear room we megg in a damp morning conductors at 7 am we megg afternoon at 2 pm higher by hundreds of meggs why florida has humid weather big time from morn to noon is a major change its were you are and whats going on in that condition thats important no one gets the same reading every time they test .

I like the guys who pull out feeders because it doesnt read the same megg reading on all conductors water/dampness burns off with heat it drys out try bring in a dehumidifier in that space for one week tell me what megg readings you get?

COOK IT WITH A LOAD!
 
Someone gave this out on ET. It's pretty good for finding a base line to start with.
 
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