What are the implications of a failed Neutral - Earth insulation resistance test?

[markusjames]

Greetings!

Newbie here! We have 3 phase installation in our Lighting dimmer rack. During Insulation Resistance testing on Earth-Nuetral, readings was 0.33Mohm on 500v test. What causede the failed test and how can I troubleshoot? Thank you!

Markus

 

[Pete999]

Greetings!

Newbie here! We have 3 phase installation in our Lighting dimmer rack. During Insulation Resistance testing on Earth-Nuetral, readings was 0.33Mohm on 500v test. What causede the failed test and how can I troubleshoot? Thank you!

Markus

What earthing system do you have?

 

[Strima]

Damaged insulation on a neural conductor somewhere, you need to break the circuit down to trace the fault.

 

[Wilko]

Hi - Perhaps try testing at 250V to see if it’s some inbuilt surge protection that’s started to wake up.

 

[radiohead]

0.33 megs is a typical reading for surge protection, as above, check that out. Have you disconnected final circuits from the rack and tested them separately?

 

[Lucien Nunes]

Does the rack have any indicator lights for polarity or earth fault? If so, the reading you are getting might be caused by the indicator's series resistor. What make and model of dimmer is it? Have you consulted the manufacturer's instructions about what types of insulation test are permitted?

 

[markusjames]

What earthing system do you have?

Damaged insulation on a neural conductor somewhere, you need to break the circuit down to trace the fault.

I should start tracing from the Neutral insulation first? How about the Earth insulation? Thank you!

 

[HappyHippyDad]

I should start tracing from the Neutral insulation first? How about the Earth insulation? Thank you!

I have deleted my post (albeit too late) as I don't give DIY advice (sorry Markus) as I think it is dangerous.

However, you have beat me too it and copied my post. Just be safe and get an electrician.

 

[markusjames]

It doesn't really work like that Markus. The problem could be many, many different things. It's OK to do some detective work and from your own experience think about the most likely causes (in fact it's a very good idea), this can help save time but it's unlikely to find the fault by it's self.

You just need to break the circuit in half somewhere then IR test either side (at the broken part). One side will have the fault, one side wont. Liven up the good side (leaving the circuit broken at the testing point) and everything that works can be eliminated....continue the process. It's a little more complicated than this if the fault is not actually accessible but it generally is.

Thank you for info. I will start the breakdown to find the faulty cable/circuit somewhere.

 

[Lucien Nunes]

It's not clear what the OP is testing. Is it the 3-phase supply cable from the distribution board to the dimmer rack? Is the rack still connected to the cable? Are the outgoing lighting circuits still connected to the dimmers?

Again, what make and model of rack is it? Please give us more information. A few of us here have decades of experience specifically testing entertainment lighting dimmer installations, but we can't help you without details.

 

[markusjames]

It's not clear what the OP is testing. Is it the 3-phase supply cable from the distribution board to the dimmer rack? Is the rack still connected to the cable? Are the outgoing lighting circuits still connected to the dimmers?

Again, what make and model of rack is it?

Is it the 3-phase supply cable from the distribution board to the dimmer rack?
Yes!
Is the rack still connected to the cable?
Yes!
Are the outgoing lighting circuits still connected to the dimmers?
Yes!

Everything is still connected during Insulation testing in this particular dimmer rack. And only this rack failed the insulation test on 500v test with reading of 0.3Mohms.
I will start removing individual connection and run the test until I find the issue.

 

[Pete999]

I should start tracing from the Neutral insulation first? How about the Earth insulation? Thank you!

Are the loads disconnected? sorry already been asked.

 

[markusjames]

Are the loads disconnected?

loads not disconnected.

 

[Pete999]

loads not disconnected.

Disconnect and retest

 

[markusjames]

Disconnect and retest

Will do. Thank you so much!

 

[Lucien Nunes]

At the moment, when you test N-E you are testing the supply cable, all the electronics in the rack, all the outgoing circuits, all the lighting fixtures powered by those circuits, literally the whole system at once. This is not a helpful test (unless it passes!)

Also, the manufacturers may not permit testing just N-E at 500V as this can put the test voltage across the output semiconductors. You must test in accordance with the manufacturers' instructions or you might damage the dimmers.

I can't add any more until you tell the model of the dimmer rack.

 

[markusjames]

At the moment, when you test N-E you are testing the supply cable, all the electronics in the rack, all the outgoing circuits, all the lighting fixtures powered by those circuits, literally the whole system at once. This is not a helpful test (unless it passes!)

Also, the manufacturers may not permit testing just N-E at 500V as this can put the test voltage across the output semiconductors. You must test in accordance with the manufacturers' instructions or you might damage the dimmers.

I can't add any more until you tell the model of the dimmer rack.

We are using
C21 DIMMING SYSTEM
please refer to the photo.
This system supplied by 3 phase power.

 

[goasis]

Why are you testing it??

 

[markusjames]

We are using
C21 DIMMING SYSTEM
please refer to the photo.
This system supplied by 3 phase power.

Why are you testing it??

Annual inspection to check the integrity of the system. We're checking the electrical installation.

 

[dmxtothemax]

it may be that there is an internal neutral to earth jumper in one of the dimer racks ?