Insulation Resistance, can anyone explain?

[CaptainKidd]

Can someone please explain to me in lamans terns insulation resistance.

So the meter reads >200 on 500v which means everything is ok but at what point is it a fail is it at 100 or 50 etc?

And how does 1M ohm realtes to all this.

Ive always had a bit of a hard time taking this in

Cheers

 

[EFLImpudence]

The limit is actually 1MΩ (@230V) but anything under 2MΩ should be investigated.

Ideally it should be much higher than this. The >200MΩ you state is the maximum your meter will measure. Some are 500 or 1000.

If you divide 230 by the number of Ohms you will get the leakage current in amps - multiply by 1000 for milliamps

Edit - I should have said it is the measurement of the resistance through the insulation between conductors.

 

[CaptainKidd]

Thanks for the reply, so on the meter if i get a reading of 143M Ohms then this is ok its even 3M Ohms is ok its only when it reaches 2 MOhms that things should be looked at? Is it that simple or have i got it all wrong?

Thaks

 

[RF Lighting]

It's kind of like a sliding scale. There isn't a magical reading where you suddenly have to investigate.

If you are testing a new installation then I would not accept a reading of less than say 100MΩ, where as testing an old installation it would probably be no cause for concern at all.

 

[securespark]

Pretty much.

In an ideal world you should get a good resistance, as has been said, much higher than the minimum. But this depends very much on what is on the circuit.

With a RF circuit, if appliances are plugged in, you'll get a lower reading than the max. With some appliances, you will get a near-zero reading.

 

[EFLImpudence]

Thanks for the reply, so on the meter if i get a reading of 143M Ohms then this is ok

Yes.

its even 3M Ohms is ok,

Yes, as far as the regs. are concerned, but you have to bear in mind the circumstances.
On a very old installation it may be expected but if it were a brand new cable something must be wrong.
Or if it were >200MΩ between L & E but 3MΩ between N & E then something is not right.

its only when it reaches 2 MOhms that things should be looked at? Is it that simple or have i got it all wrong?

Yes.

 

[CaptainKidd]

Thanks EFLImpudence, RF Lighting and securespark for the replies i didnt realise it was that simple.

So basically the older the install the lower the reading and even althought 1MOhm is the minimum your not really wanting anywhere near this as problems could start in the very near future.

 

[EFLImpudence]

Yes, that's right.

 

[Adam_151]

Bear in mind that 1meg is the minimum across a DB, not per circuit. So if you had an installation with 12 circuits each measuring 10 meg then technically that would be a fail.

Its not unusual to find that measuring across a whole existing DB that in many cases the needle doesn't even move at all. (often the fault thats eventually found has been present since original installation!) And anything above 5meg is considered good, anything above 20, excellent, and anything above 50meg, rarer than hens teeth!

What you might also get is a reasonably low value but the figure slowly increasing as the test button is held, this usually a sign of good insulation but long lengths of cable present (it takes time for all the cable capcatance to charge to the test value), if there was a fault present it would inhibt this and act as a bleed resister

 

[bernardgreen]

(it takes time for all the cable capcatance to charge to the test value), if there was a fault present it would inhibt this and act as a bleed resister

It would be a lot of cable and / or a very high impedance source in the meter to have a noticable ( by human eye ) charging time. If the display value increases slowly then t could be a small damp patch being dried out by the test current passing through it.

 

[ban-all-sheds]

Why am I the only one who finds a post like this:

Can someone please explain to me in lamans terns insulation resistance.

So the meter reads >200 on 500v which means everything is ok but at what point is it a fail is it at 100 or 50 etc?

And how does 1M ohm realtes to all this.

Ive always had a bit of a hard time taking this in

Cheers

from someone who has been working as an electrician for at least 2½ years really scary?

 

[ricicle]

Why am I the only one who finds a post like this:

Can someone please explain to me in lamans terns insulation resistance.

So the meter reads >200 on 500v which means everything is ok but at what point is it a fail is it at 100 or 50 etc?

And how does 1M ohm realtes to all this.

Ive always had a bit of a hard time taking this in

Cheers

from someone who has been working as an electrician for at least 2½ years really scary?

Never noticed, but now you mention it

 

[Taylortwocities]

With a RF circuit, if appliances are plugged in, you'll get a lower reading than the max. With some appliances, you will get a near-zero reading.

It is not recommended to do an IR test (500volts DC) across any appliance.

Your customer will quickly complain that his plasma TV seems not to work any more

 

[JohnW2]

Why am I the only one who finds a post like this: .... from someone who has been working as an electrician for at least 2½ years really scary?

You're not the only one. The absence of a response from me does not mean that I couldn't have given an answer, and I'm sure the same is true of others.

In my case, my problem was not based on the knowledge that the OP had been "working as an electrician for at least 2½ years" (if that's the case), but rather that I did not think that anyone needing to ask such questions should even think about doing IR testing, no matter who they were.

Kind Regards, John.

 

[holmslaw]

..