Insulation Resistance Test One or Two?

[Graeme Harrold]

Was that with line and neutral connected together, and only tested to cpc?

Yes, thats how most machines work, they link L and N internally and buzz between that and earth.

 

[sparkypants]

Remember that the tests you are conducting are DC - the RMS equivalent - and 230VAC is the RMS equivalent of single phase voltage - so you need to test 500V DC as with AC the voltage rises and falls upto about 346VAC. I was not aware that you need to do a 250 V DC test first if you have removed vulnerable equipment.... But you live and learn

 

[pevvers]

I was not aware that you need to do a 250 V DC test first if you have removed vulnerable equipment....

That would be because you don't have to!

 

[andyb]

I completely disagree, as nothing can flow through the equipment because the voltage cannot see (i.e. no potential difference) the equipment as the line and neutral are connected together. All that will be picked up is any insulation faults between the cpc and either of the live conductors, which is what you want to see.

To pick up an insulation fault current has to flow, and the pd will be 500v.

Sorry folks quoted the BRB of the top of my head correct one is 612.3.3 and I agree with the above no need to use 250v
Chris

If you read 612.3.2 it says that if it is not reasonably practicable to disconnect such equipment the test voltage may be reduced to 250v.

That would be because you don't have to!

Indeed you don't have to, this is just something I do.

If there is sensetive equipment that may be damaged and I cant disconnect it during a pir, I test @ 250v.

 

[devlin maguire]

Sticking 500V down the conductors just because the book says so is a perilous path, if you know what connected fine if you dont then dont, even 250v applied to the wrong circuit can cause damage I once wrecked a radio alarm because a cleaner plugged it back in after it had been removed, old dear came in the back door never even knew she was in the house
Mac

 

[chris gray]

i use 250v just to prove that there are no loads, and for no other reason. things can me missed.

 

[Forde]

Have to agree with those who err on the side of caution and use 250v. Big difference in my book between a periodic and a new installation. That's why the limitations gets a regular workout on Pir's.

 

[pevvers]

Have to agree with those who err on the side of caution and use 250v. Big difference in my book between a periodic and a new installation. That's why the limitations gets a regular workout on Pir's.

Most definitely limititations should be used for PIR's, but I have yet to cause any problems using the 500V with line and neutral connected tested to protective earth. 250V doesn't always show up an insulation fault, due to the reason as previously stated with 250VDC not being the same as 250VAC.

But I suppose it's better than doing no test at all, which I often find when I see periodics with >299M across the board, as from experience testing a whole board on a periodic you will not often find such a perfect reading...

 

[Forde]

Most definitely limititations should be used for PIR's, but I have yet to cause any problems using the 500V with line and neutral connected tested to protective earth. 250V doesn't always show up an insulation fault, due to the reason as previously stated with 250VDC not being the same as 250VAC.

But I suppose it's better than doing no test at all, which I often find when I see periodics with >299M across the board, as from experience testing a whole board on a periodic you will not often find such a perfect reading...

Agreed it's best to do 500V and I do as often as I see fit, however sometimes I just feel it's best not to. A periodic IMO is a tricky procedure and experience is vital, ultimately in some specific scenarios doing nothing ie no test is the best option. Anybody who has ripped apart a 30 year old installation to make sure they do a comprehensive test report may one day bite off more than they can chew, been there and it wasn't pretty!!!

 

[pevvers]

Anybody who has ripped apart a 30 year old installation to make sure they do a comprehensive test report may one day bite off more than they can chew, been there and it wasn't pretty!!!

Maybe, maybe not. I managed to test the Museum of Wales in Cardiff, and the Museum of Welsh Life just outside of it correctly, and some parts of that installation were way over 30 years old. Some things I saw in there I'd never seen before in my life!!

Never had any comebacks at all, all tested at 500VDC with test 2 used.

I agree that you have to be careful though and get it set up correctly, cos the moment you see that instrument reading 0.00M you heart does sink a bit...

 

[Jason Smith]

Right, trying to get this sorted. My NICEIC inspector (this very day), described the test , as described by a previous post on this thread, as a stress test on the two conductors concerned. No current can flow. All the test measures, is millions of paralell resistors in the cables, trying to leak out of the insulation. The lowest acceptable limit is as we know is 1MO. Thats a resistance of 1000 O. Even with a poor reading like this, No current will be flowing out of the wire , IMO. If current could flow out of the wire, to my mind, we would have a short circuit, which is what the test is designed to predict. Hence the recomendation for when the next test should be carried out. Yearly with poor results like 1MO.

 

[Risteard]

The lowest acceptable limit is as we know is 1MO. Thats a resistance of 1000 O.

1 Megohm is 1,000,000 (one million) ohms.

 

[Jason Smith]

Ta mate, I did of course know that, got my o's in a mucking fuddle.

 

[pevvers]

No current will be flowing out of the wire , IMO.

Well there will be some current, but for a resitance of 1M ohm @ 230V it would be around 0.23mA @ 230V.

Even with a resistance of 7667 ohms @ 230V you would still only have just 30mA flowing (just enough to trip your RCD).

 

[Colonel Hathi]

Just a quick thanks for all the postings.
Really interesting.
I certainly note the point about using LIMS on periodics.
Not having had too much experience, there is a danger that in wanting to be thorough, I could go too far in investigating/testing an old installation and actually make matters worse!!!

I have the 2392 - which being focussed on Initial Verification does lead you to want to undertake every test in the book - but I am learning!

I had an issue recently with some very minor work in a kitchen.
The pro forma minor works certificate shows IR as an essential test - which I didn't want to do because of neons/no acccess behind appliances etc etc.

I think I will use the LIMS more in future - and only do a 500v test if I get very poor readings on the 250v test 2.

By the way I am booked on the NICEIC Virtual Reality PIR course later this month.

I will post to let you all know how I find it.

Thanks again

CH