Properly performed, megohm resistance testing is not hard on
insulation, although its close cousin, hi-pot testing, can be.
Insulation resistance testing is usually performed at 100-500 volts
on low voltage (<600V) circuits, 1000 volts on higher voltage
circuits. Just enough voltage to get enough leakage current to
resolve a meaningful resistance value.
The actual dielectric breakdown (failure) voltage for any component
or insulated cable rated for a 600V circuit, will normally be on the
order of 2500 to 5000 volts dc, much higher than insulation
resistance test voltages. The old rule of thumb for hi-pot testing
(which is looking for breakdowns) used to be "2x rated voltage + 1000
volts", which will give you a hi-pot test voltage of about 2500 volts
for a 600V circuit when you consider the peak voltage seen on an AC
circuit. So components that are designed to see test potentials of
2500 volts or so during hi-pot shouldn't have any problem at the
lower megohm resistance potentials.
Insulation resistance measurements will vary widely on the same exact
piece of equipment depending upon relative humidity. Way back when,
I used to do electrical testing in the Middle Atlantic area, which
can see humidity anywhere from 40 to 100%. Insulation resistance
readings that would read >10,000 Megohms in dry conditions could drop
as low as 10 Meghoms in high humidity. If that's the case, more
often than not the leakage is happening across dirty contaminated surfaces.
The general rule that I remember for protecting active circuits from
high test potentials during insulation resistance tests was to short
all of the terminals together on the active side. That could present
practical problems with PV arrays of any size, though, so I'm not
sure what the best practice should be. Megohm testers can't deliver
enough fault current to cause overcurrent-induced damage. But,
depending upon how the test is conducted, it is possible that under
insulation fault conditions, particular cell P-N junctions could see
close to full test voltage and suffer reverse breakdown damage. The
best preventive measure in that case is to use a tester that allows
you to bring the voltage up gradually (and back off if you're seeing a short).
To be picky, only Biddle makes "Meggers". Everybody else makes
megohmeters. Just thought I'd add that in, no extra charge!
John Raynes
RE Solar
Torrey, UT
At 01:56 PM 4/28/2009, you wrote:
25 years is hard on everything. If it is marginal out of the gate, it will
be on fire soon.
Bill.
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Richard L
Ratico
Sent: Tuesday, April 28, 2009 12:18 PM
To: [email protected]
Subject: Re: [RE-wrenches] To Megger or not to Megger
Bill and Matt,
Meggering is hard on insulation. It may, or may not, be hard or harder on PV
cells .... that is the question. Could you guys please elaborate on that?
Dick
Solarwind Electric
P.S. Bill, your recent class here in Vermont was great. Thanks.
---Bill Brooks wrote:
Just to add punctuation to this thread, I always recommend that contractors
megger their arrays, because it has saved my butt several times.
--- end of quote ---
--- Matt Lafferty wrote:
I only meggered the actual arrays on projects where it was a requirement and
I only did that following specific procedures provided by the module
manufacturer. No procedure from the mfr = No array megger.
--- end of quote ---
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