Megger

[jmele]

I need some information on using an insulation tester (megger). I have used one a couple of times to test motors. Now I have to test a set of 3 phase wires that are approximately 1200 feet long. Most of it is on poles above the ground. About 300 feet of the run is underground. It's a 480 volt service. My main question is, will a fluke 1507 handle this chore? If not what should I do? Thanks in advance.

 

[JRW 70]

Type of Wire

Type of Wire

On the overhead, if the three phases are separate lines,
and not a single cable, the tester will not really tell you anything.
If it is a cable, the conductors will probably be close enough to check
against each other and if the underground section is in a metal
conduit, check all phases against that. (All loads that could be damaged,
or skew the results should be removed.)

On the overhead, if on insulators and separate lines, and the spacing is far
enough apart, the reading will probably be infinity or the max GOhm of
the meter. In my area we check our high value insulators by x-raying them
to look for cracks, but these are on 230 & 500 kV insulators. The little ones
are considered disposable and are replaced upon failure or corona (likely
eminent failure)

So cable or lines on insulators will tell if what you want to do will render
any meaningful results.

JR

 

[jmele]

The overhead wires are a quad cable and the underground is in aluminum pipe. Thanks for the information. What about the fluke 1507? Is that a sufficient tester for the length of wire? I haven't tested anything this long before.

 

[JRW 70]

Since this is a cable, then I think the meter will
be fine. Look for at least 75 to 100 MOhm as a
minimum ( personal opinion only ). The meter
needs to stay on the line until the resistance
remains constant and that will be the best results.
If the meter does di-electric absorption ratio (DAR)
take that as well if you intend on doing trending or
just the Meg. test for a one shot test.

And sorry about the formatting above, it looked good
when submitted, and I hope your testing goes well.

JR

 

[jmele]

Great, so that distance is not a issue? I appreciate all the help.

 

[big john]

...What about the fluke 1507? Is that a sufficient tester for the length of wire...?

Specs on that say it will put out 0.2A of short circuit current, that's a whole lot more than many other meggers. That would be more than enough to charge a whole lot of capacitance: I think you'll be fine.

I'd do a PI: Record your 1 minute value and your 10 minute value. By that point you will have nulled out all the capacitance and will be measuring mostly leakage and polarization. While I agree that the underground is your primary concern, it's not unheard of to find bad insulators: We just found a mess of bad strain bells by doing a simple megger test.

 

[jmele]

Thanks for all the help.