Insulation Resistance test with PV mods in circuit

[SunNinja]

I'm trying to figure out whether it is safe & appropriate to megger - or insulation resistance test- positive to negative on a single string connected to PV modules. I do troubleshooting exclusively with a new job, a lot of arc fault and ground fault problems. I recently used a Seaward meter at a commercial site for IRT tests with modules that are in the sun, producing voltage. We disconnect all positive wires at the fuses in combiners, and run the test between each + string leg and the negative bus, and I was assuming to ground from + strings as well.

I have a Fluke DMM with IRT capability, and I went to do a test between + and - at a single string disconnected from the inverter, but there was a little sparking going on when I went to connect the meter lead to the + side string. Double/ triple checked all connections for polarity and plugging into the appropriate spot on the DMM for IRT.

Made me second guess whether this is something that is commonly done, or whether a Seaward meter has some fancier internal protections or??
The company has some conflicting guidance that gets painfully specific and elementary about how to test BUT never says test + to - on a string, or + to ground. But there's one line that says if panels are in the circuit, IR must be above a certain amount.

Any experience/ information appreciated.

 

[GoldDigger]

Unless the panels are totally covered or it is a dark moonless and starless night there will be close to normal voltage on the panels with an open circuit. This will invalidate simple ohmmeter measurements or break the meter. Even in complete darkness the panels will act as diode stacks.

With a megger you are likely to damage the panels with either a (+) to (-) or (+) to ground test.

Just like any megger test on building wiring with load devices or surge protection still in circuit. Don't do it.

If you, say, disconnect (-) end and keep the megger voltage below the system voltage rating of the panels you might get away with it.

 

[yesterlectric]

There are companies that make testers that they claim can be used to test through modules but in many cases you’ll find that the module manufacturer recommends against it. So regardless of whether it works or not you could run the risk of voiding the warranty on the modules.

 

[ggunn]

There are companies that make testers that they claim can be used to test through modules but in many cases you’ll find that the module manufacturer recommends against it. So regardless of whether it works or not you could run the risk of voiding the warranty on the modules.

You could megger the home run conductors while they are disconnected at the array. It seems to me that that would be the thing to do anyway so that there is no conductive path between them.

 

[SunNinja]

Alrighty, thank you all for the feedback. I’ll have to dig in to what & how we were working with the Seaward.
My Fluke megger was smart enough to refuse to test when there was voltage in the circuit.