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?