measuring insulation resistance on multimeter

[skyman]

Hello!

I have difficulty interpreting the result of the insulation’s resistance measurement I have done ( without short circuit ) :

Testing the insulation resistance between the negative electrode and earth gave 2 Megaohm

But testing between the positive and earth
Showed OL on the multimeter.

What does that mean ?

 

[wpns]

Usually means infinite ohms (no conductivity). Make sure when you are measuring HIGH impedances that you are not touching either of the probes, as the conductivity of a human is a megohm or two.
Also note that this says nothing about the breakdown voltage of the wire, so you may have a short at high applied voltages. Ask me how I know this. 8*)

 

[hwy17]

Interesting backstory:

Interpretation of multimeter displaying "OL"

Multimeters normally displaying "OL" when measuring resistance in ohms and the cables are not connected. My friend told me that apprentices in Germany learn it is "Overload", wh...

electronics.stackexchange.com

But, a multimeter is not capable of doing real insulation testing. It can test if there's a short or not but you need a megger to actually judge the insulation.

 

[timselectric]

You can't test insulation resistance with a multimeter.

 

[740GLE]

It’s a good test to verify you don’t have a dead short, but nothing more than that.

Megaohmeter applies dc voltage to a disconnected line and then reads the crazy small current flow. Typically they are well outside the DIY budget (unless one falls off your work truck/bag and gets lost).

 

[skyman]

Usually means infinite ohms (no conductivity). Make sure when you are measuring nigh impedances that you are not touching either of the probes, as the conductivity of a human is a megohm or two.
Also note that this says nothing about the breakdown voltage of the wire, so you may have a short at high applied voltages. Ask me how I know this. 8*)

Thanks, well I measured in the night( low voltage )

I have two strings: first string’s VOC 310 volt

Second string’s VOC 620 volt

I measured voltage to earth and was ~ 0 volts ( for all strings )

while measuring the insulation resistance of the first string gave :
Positive to earth 17 megaohm
Negative to earth 16 megaohm

Measuring insulation resistance for the second string gave

Negative to earth 2 Megaohm
Positive to earth OL!

 

[skyman]

You can't test insulation resistance with a multimeter.

What ! Are you really sure about that ?

Well there are two ways to measure insulation resistance :

• 1.To short the positive and negative electrodes of the PV string, and measure the insulation resistance between the shorting point and earth.
• 2.Measuring the insulation resistance between the positive electrode and earth and between the negative and earth separately without shorting.

I have a digital clamp multimeter, the only thing is that my meter can’t do a shortcircuit insulation resistance measurement but measuring resistance between + or - to earth is possible

 

[skyman]

It’s a good test to verify you don’t have a dead short, but nothing more than that.

Megaohmeter applies dc voltage to a disconnected line and then reads the crazy small current flow. Typically they are well outside the DIY budget (unless one falls off your work truck/bag and gets lost).

Yes I know however they should still be able to find if there is an earth fault, according to the inverter‘s manufacturer one should measure voltage between + and - to earth ( must be 0 volt )

And measure insulation resistance between + and - to earth ( without shortcircuit ) and result should be > 1 megaohm

 

[skyman]

Also note that this says nothing about the breakdown voltage of the wire, so you may have a short at high applied voltages. Ask me how I know this. 8*)

well this is actually a very important tip.

My inverter shows an isolation fault after I reboot it!

My inverter is a transformer-less ( No Galvanic isolation ) so every morning before it connects to the grid it measures the voltage to earth and if it’s fine it starts producing power.

as I said I noticed that if I reboot it, it shows an isolation fault immediately ! Despite that it has been working fine before the reboot ( on the same day ) I guess it has something to do with the highe VOC voltage !!!!

 

[timselectric]

What ! Are you really sure about that ?

Absolutely
You are not measuring insulation resistance, with a multimeter.
You can measure if there is isolation. Or if insulation is missing.

 

[tinyt]

Some insulation testers:

This is affordable for me, but I am not sure about performance.

This is expensive for me, but familiar brand.

Or you can get the really expensive Fluke brand.

Be careful when selecting the higher test voltages.

 

[Zwy]

You need a megger to measure wire insulation resistance.

 

[tinyt]

You need a megger to measure wire insulation resistance.

Or a 'hipot' tester. But if the device being tested fails, it gets damaged. Sometimes you can clean where it arc'ed and rate it at a voltage lower than the value it failed.
The meggers I know are the old analog vacuum tube devices. Even the hipots I am familiar with use large knobs and analog meters.
Shows how old I am.

 

[skyman]

You need a megger to measure wire insulation resistance.

Please don’t get me wrong :

I understand that a megger is needed if you want to measure the insulation resistance by shortconnecting the positive and negative electrodes of the PV string, and measure the insulation resistance between the shorting point and earth ( the megger applies testing voltage for this kind of measurement )
Read this

Insulation Resistance Measurement for the Safety of Solar PV Systems | Hioki

Insulation damage can cause power loss, overheating, and fires. Electrical devices, parts, and equipment in industrial buildings and facilities, including PV systems, must undergo insulation assessments. This aids in reducing electrical shocks and short circuits.

www.hioki.com

 

[skyman]

@wpns

You mentioned that sometimes there may be
a short at high applied voltages.

Could you please explain a little ?


Thx

 

[Warpspeed]

Skyman, you can have something like a very thin layer of paint or maybe rust on something, measure the "resistance" and your low voltage ohm meter says its a complete open circuit.
But if you test it with a much higher voltage, the paint will break down, and then you measure a dead short.
So what is it, an open circuit, or a dead short ?

Testing insulation resistance, is just about always safety related. You want to know if its a good safe insulator against dangerous voltages.
You not only want to know the resistance, but will the insulation break down at say 220v and electrocute you.

You can use a special high voltage insulation tester that measures the resistance with say 500 or 600 volts applied.
This is a pretty standard insulation test, and there are special instruments especially designed for doing it.
EVERY electrician has such an instrument for testing the safety of electrical wiring and appliances.
He will also have in his toolbox, a conventional low voltage multimeter for measuring ordinary resistors at a low voltage.

 

[skyman]

Skyman, you can have something like a very thin layer of paint or maybe rust on something, measure the "resistance" and your low voltage ohm meter says its a complete open circuit.
But if you test it with a much higher voltage, the paint will break down, and then you measure a dead short.
So what is it, an open circuit, or a dead short ?

Testing insulation resistance, is just about always safety related. You want to know if its a good safe insulator against dangerous voltages.
You not only want to know the resistance, but will the insulation break down at say 220v and electrocute you.

You can use a special high voltage insulation tester that measures the resistance with say 500 or 600 volts applied.
This is a pretty standard insulation test, and there are special instruments especially designed for doing it.
EVERY electrician has such an instrument for testing the safety of electrical wiring and appliances.
He will also have in his toolbox, a conventional low voltage multimeter for measuring ordinary resistors at a low voltage.

Thank you for your comment and information.
I am aware of the safety precautions, but let’s forget the insulation resistance for now and focus on measuring the voltage to earth.when a ground fault is present, measurement will show Voc between positive and negative conductors ( dc+ to earth should be 0 voc ) and ( dc- to earth should be 0 voc )

So I expect that my low voltage ohm meter can handle this measurement with low risk of damage.

What I still don’t fully understand is what if the voc to earth is 0 but the insulation resistance of the string is lower than 1 megaohm?

 

[wpns]

@wpns

You mentioned that sometimes there may be
a short at high applied voltages.

Could you please explain a little ?


Thx

The AC connection of my 10KW grid-tie 120/240 SMA inverter kept tripping the 60A breaker, though my Fluke 87 DVM (which only applies a volt or two) showed infinite resistance. On closer inspection the white wire had turned black (!) and became a short at high applied voltage. Swapped out that wire and made a note to myself to tighten the screws yearly, and I was back in business.

 

[skyman]

@wpns

Great find ! I am happy for you
My inverter is a 3 phase inverter but not hybrid.

I didn’t have yet issues with tripping the AC breaker and the AC wires ( on the right side ) look fine, my issue is that my string inverter shows an isolation error immediately after reboot.
let’s suppose i would reboot it now, it will shows prompt after startup an isolation fault despite it has been working fine before the reboot

 

[knexbuilder]

You can't test insulation resistance with a multimeter.

Agreed. Megger testing checks insulation integrity