Insulation Resistance Test One or Two?

[Colonel Hathi]

Hi
I am aware that test 2 at 250v with L+N joined is recommended where there is a risk of damaging sensitive equipment.

What I admit I don't really understand is why test 2 is not used in all cases

Can anyone please explain why/when test 1 is necessary?

Before I get my head blown off - this is a genuine question - I simply wonder why.

I know that some people start off with a 250v test 2 - and if this is ok, go on to perform 500v test1.

Again, I just dont really know why

Thanks in advance chaps

CH

 

[marksparky]

generally test 2 is applied wen sensitive equipment cannot be disconnected transformers etc the reason test 1 is preferred is simply because it wud detect any faults between phase an neutral

 

[Colonel Hathi]

Thanks - that was quick

Ok - but if test 2 would not detect a phase neutral fault, wouldn't you think that a test between phase and neutral at 250v would be used at some stage - rather than going straight to 500v?

Patience now!!!

Cheers
CH

 

[oldtimer]

Unless you can be sure that you will not test electronic equipment then its a nono Let me explain when I did alarm systems I lost count of the customers who phoned me to say that they had electrical work done and "the electrician mentioned that the alarm panel went off for no reason" Well I said me thinks your spark has meggered the DB and blew the alarm panel PC. The problem is not only neon lamps fitted to switches that can give you a duff reading but there are now things like microprocessor time clocks and boiler control PC boards, smoke and Carbon manoxide detectors along with ariel booster units(hidden in the attic) not to mention new age fridge/freezers cookers dishwashers etc so if you inject 500v dc between live and neutral then you could be talking many hundreds of pounds compensation to the customer.

PS forgot to mention electric showers that have microprocessor controls fitted

 

[Jason Smith]

Er um, if you join neutral and phase together, there is no potential difference between them so why cant you test at 500 vts, no current will flow in the sensitive devices. Go for it you lot - I need an answer. (obviously sticking to the book because if you step out of known procedure etc. )

 

[roukel01]

you use 500V as a kind of "stress Test" to make sure the insulation doesn;t break down under higher voltages.
(I am guessing by the way)

Plus you'll get a more accurate result using a higher voltage.

Going back to an earlier post on here, provided you test L-N to E you shouldn't damage anything, thats how a PAT tester tests an appliance.

 

[pevvers]

You would use the full test when testing new installation works, test 2 when conducting PIR's, as you will have no idea normally whether there are any hidden things here there and everywhere that you could quite easily kill with a zap of 500VDC. Not too bad in domestic, but when you're working in commercial it's a lot easier to destroy things that you don't even know are there!!

Of course, this has to be an agreed limitiation with the client, although I've yet to come across a client who has the foggiest idea what I'm going on about,

 

[acat]

Correct test for sensitive equipment is live and neutral on one clip and earth on the other at 500v in the comments part quote 612.3.1 thats what I do anyway may be wrong

Chris

 

[andyb]

Er um, if you join neutral and phase together, there is no potential difference between them so why cant you test at 500 vts, no current will flow in the sensitive devices. Go for it you lot - I need an answer. (obviously sticking to the book because if you step out of known procedure etc. )

Provided there was no fault.

If there was an earth fault 500v could flow through the equipment to earth.

 

[Jason Smith]

Thanks mate, thats why it says after a satisfactory test with l+n joined and tested between this and CPC at 250 vlts, you can test at 500. A satisfactory test would give a good reading, and therfore no faults. Then test at 500 and a better result would be obtained.

 

[pevvers]

Provided there was no fault.

If there was an earth fault 500v could flow through the equipment to earth.

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.

There is no need to test at 250V when line and conductor are connected together for test method 2. Trust me, I've meggered a lot of installations at 500V, small and huge, and I have yet to kill anything.

Although I did nearly destroy a load of freezers when I forgot to reconnect the neutral after an IR test, the smell of burning electronics is not something you forget for some time....

 

[acat]

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

 

[Jason Smith]

probably the best thread I have ever read. Thanks Pevvers. Your quite right, the smell of burning electrics is horrendous. However, Imagine the scene. The whole place was rewired, it was my first one. All tested fine. A quick call to old sweat and he said throw the switch. He had purosely kept away to build my confidence. I switched it all on. At that precise moment in time, someone put a match to a gallon of parrafin over a bonfire next door. The windows were open and all I could hear was raoring flames and a smell of plastic and paraafin burning. I hit the switch to off and pulled the main fuse as I bundled myself out the door. Something wasnt right, or was right, as it were. As I got out the front door, there was a man stood in next doors garden holding a rake. The instance I noticed the bonfire, I calmed down. I stood there taking a few deep breaths before I carried on.

 

[Graeme Harrold]

The reason I was given for the reduced voltage is for sensitive chokes and filters that are connected to a functional earth. I blew a digital thermostat on a fridge using the 500V test

 

[pevvers]

The reason I was given for the reduced voltage is for sensitive chokes and filters that are connected to a functional earth. I blew a digital thermostat on a fridge using the 500V test

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

 

[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

 

[andyb]

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).

That's not the point.
If you know the resistance was 1M there would be no need to test anyway.
You are testing for lower readings than 1M, plus the fact that a lot of IT equipment uses the earth path and you do not want to be putting 500v across it.

 

[pevvers]

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

It's a very good course, I was invited on the pilot back in November. It's nice to see things from their eyes, and to see how they want you to test and certificate, and you will come away knowing that 99% of certificates are filled out incorrectly....

It's good fun too, like Doom but a bit slower!

 

[andyb]

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.

Colonel, this post is about PIRs,
Any work you do must be tested accordingly and insulation resistance should be tested @ 500v.

Also, if you get a poor reading with 250v then you should investigate further before putting 500v to the circuit.

 

[pevvers]

That's not the point.

Well there is a point isn't there, if he says there's no current when there is?

You would not be putting 500V across the equipment if line and neutral are connected together!! I did 175 domestic PIR's a few years ago, used test 2 at 500V every time, with EVERYTHING plugged in. If I had broken something I'm pretty damn sure I would have known before I left the house. This is why we use the 2nd test in periodic situations.... I would happily stand there in front of you and megger my own house at 500V that way with computers plugged in all over the place just to prove a point.

 

[andyb]

Well there is a point isn't there, if he says there's no current when there is?

You would not be putting 500V across the equipment if line and neutral are connected together!! I did 175 domestic PIR's a few years ago, used test 2 at 500V every time, with EVERYTHING plugged in. If I had broken something I'm pretty damn sure I would have known before I left the house. This is why we use the 2nd test in periodic situations.... I would happily stand there in front of you and megger my own house at 500V that way with computers plugged in all over the place just to prove a point.

612.3.2 of brb

Where surge protection devices (SPD) or other equipment are are likely to influence the verification test, or be damaged, such equipment shall be disconnected before carrying out the insulation resistance test. Where it is not reasonably practicable to disconnect such equipment (e.g fixed socket-outlets incorporating an SPD), the test voltage for the particular circuit may be reduced to 250v dc, but the insulation resistance shall have a value of at least 1M.

Testing your house @ 500v proves no point at all, neither does the fact that you tested 175 installations @ 500v (so have I)

The op was asking why you can use 250v in some instances, that is the answer.

 

[pevvers]

612.3.2 of brb

Yes mate, I am aware of the reg. It's just that I have found that at 250V it's not always possible to identify problems, and so therefore devalues doing it a bit. The same can be said for connecting line to neutral and testing to protective earth as well. But if you do both in same test then this maybe isn't going to show up a fault.

I know everything has gone a bit off topic, so I apologise for that!

If there was an earth fault 500v could flow through the equipment to earth.

BTW, volts don't flow....

 

[andyb]

Yes mate, I am aware of the reg. It's just that I have found that at 250V it's not always possible to identify problems, and so therefore devalues doing it a bit. The same can be said for connecting line to neutral and testing to protective earth as well. But if you do both in same test then this maybe isn't going to show up a fault.

I agree and if anything at all shows up then further investigation is needed
Also testing with 250v is very rare, I can't remember the last time I needed to do it (you can usually isolate things for a shoet period)

BTW, volts don't flow....

Damm!!!

 

[pevvers]

Damm!!!

I'm glad you've got a good sense of humour!

I'm gonna leave this topic now as I think it's done, see you on the next one,