megging wires

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bayy99threeway

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hi eveyone .i'm starting a new job and it is requiring me to do a lot of megging of wires #2-4/0 wire i think good wires should meg 30 megaohms or more. if anyone has any information on megging send it to me at ............Moderator Edit; send a PM
 
cadpoint

cadpoint

Senior Member
Location
Durham, NC
ptonsparky said:
Don't forget to discharge the cables after megging!!
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Great point!

If I might add a different thought! Just a reminder it is a coil of wire!
I've always grabbed the wire of new big reels and touched them to a grounded conductor, before anything happens!!!

Even more so if the wire is for medium and high voltage application, which are always tested at the plant. Only lot samples are taken for 600V or less wire, unless of course it's a special condition that requires it.
 
Rockyd

Rockyd

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Location
Nevada
Occupation
Retired after 40 years as an electrician.
That roll of wire you just megged is now a giant capacitor. If you don't drain it, it will ring your bell!
 
ultramegabob

ultramegabob

Senior Member
Location
Indiana
thats a new one on me, I thought capacitors were made of two conductors seperated by an insulator. how does a single coil of wire make a capacitor?

*edit, you are talking about megging bewteen multi conductor cables arnt you?
 
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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
When you test the cable to ground, you charged that cable to 1000v, if the cable is good, you have charged a capacitor and it has no place to blead off unless you remember to allow that new tester time enough to discharge it. Makes a heck of a spark even several minutes after.
 
cadpoint

cadpoint

Senior Member
Location
Durham, NC
ultramegabob said:
thats a new one on me, I thought capacitors were made of two conductors seperated by an insulator. how does a single coil of wire make a capacitor?

*edit, you are talking about megging bewteen multi conductor cables arnt you?
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Yes I would assume they'd measure everything!
The top five that are dangerous to those that go there. JMO
In no preticular order:
Batteries, Transformers, Capacitors, coils of wire, megging a wire, & a live circuit, add others if you feel the need too, please ...
I always have to take a moment when any of these are in my line of work!

Most pole transformers are looped together across the terminals when in storage.

It's not so much that its a Cap, other than it can store a charge due to coiling and can release high voltage under a no load condition, don't be that path...
 
Rockyd

Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
It's not so much that its a Cap, other than it can store a charge due to coiling and can release high voltage under a no load condition, don't be that path...
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600'+ feet of wire in a pipe is the same as cable, the longer the run, the greater the storage area (plate equivalent of a cap)....I have met ZAP and support the big drain off!

Anyone else remember back in apprenticeship class leaving a charged cap laying around for the unsuspecting? Nice introduction to stored energy and a good teacher of things don't always "appear to be what they are" in regard to what danger may look like.
 
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Bentworker

Bentworker

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Location
State of Jefferson
Make sure you don't pull a bonehead move and Megger the wire at a voltage over what the insulation is rated for. With a 2500 volt Megger you can destroy things in a short manner.

On a multi-conductor cable I usually get some small soft drawn or a wire nut and tie all of the wires together except one, then measure from the single wire to the rest of the conductors. I continue rotating until I have checked all conductors, then I tie them all together and measure from them to ground (and shield if it has one). Make sure the other end is isolated, and that you are not going to ruin someone's day by lighting them up with the Megger.

The newer Meggers are a little safer, but be careful. You might want to make up a spreadsheet where you can record your readings in megohms for each cable.
 
Bentworker

Bentworker

Member
Location
State of Jefferson
brian john said:
Oh you gotta love the new millenium, what use to be a test report is now a...
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Some guys idea of a test report is the back of a Carl's Jr receipt. Excel is nice because you can save it, and in the future you can add onto it. The company I worked for before I went back to the dark age keeps all test reports on a shared hard drive. That way years later you can pull up all the information you want, and add to it if an upgrade or expansion is in progress. Just an idea, but it is probably overkill unless you have a cradle to grave responsibility for whatever you are building (like a utility company).
 
zog

zog

Senior Member
Location
Charlotte, NC
brian john said:
Oh you gotta love the new millenium, what use to be a test report is now a
Click to expand...

Spreadsheets are so 90's. power DB forms are what we use now, powerful tool and communicates with most of the newer (Megger) test equipment for diret downloads to the reports from the test equipment. Drop down menus and built in NETA specs are also helpful.
 
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