[RE-wrenches] To Megger or not to Megger

John Raynes john at raynes.com
Tue Apr 28 20:00:43 PDT 2009


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: re-wrenches-bounces at lists.re-wrenches.org
>[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Richard L
>Ratico
>Sent: Tuesday, April 28, 2009 12:18 PM
>To: re-wrenches at lists.re-wrenches.org
>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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