insulation resistance testing

Hi All

Just a quick question on insulation resistance testing i know how to do it but what i dont get is i know its just an exam question and if you got readings like bs7671 further investigation would take place ie

250v------------->0.5Mohm

500v------------->1Mohm

etc

just been watching Niceic dvd and he gets >299Mohm @ 500v & >99.9Mohm @ 250v ! ive got a kewtech kt61 and mine wont give symbol > and how come in bs7671 the min is that low ?

The guy in the niceic dvd also on 250v test got 0.71Mohms on selv and said its of course a fail ! how come if its above 0.5Mohm ?

Regards

Kung.

Think all your m's should be M's.

Work out how much current would flow at 230V with 1Mohm. What does your meter show?

Ian.

Thanks so it must be above 299Mohm and on cert its down as >299Mohm ? and although mine dont do reduced test volatge of 250v so cant do selv/pelv but if it did it would have to give reading higher than 99.9Mohm and reading on cert would be >99.9Mohm ? as long as readings are these its a pass but if under its a fail so <299Mohm on 500v and <99.9Mohm on 250v are fails ?

Meter wouldn't show <299M ohms it would give you an actual figure

Meter wouldn't show <299M ohms it would give you an actual figure

Click to expand...

Yes warren what im saying is if ifs under 299 so 298/250/200/100/5Mohm etc its a fail ?

Hope this makes sense

My meter doesnt do symbols >< etc

No its still a pass as long as its above 1Mohm as you said in the first post.

Shame your meter won't do 250V tests as I always run that first to check if anything is still connected. Shouldn't fry it if it is at 250V but 500V may just. All those hidden sockets and sfcu's.

Ian.

No its still a pass as long as its above 1Mohm as you said in the first post.

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Thats what i thought and if 250v >0.5Mohm but tony cable got 0.71Mohm and said which of course is a failure but its higher than 0.5Mohm im confused !

I have not seen the video you refer to. Was it a LV system he was testing?

He just wired in a low energy light fitting and said now lets go and see what reading i get back at the CU and set his megger to 250v IR test and got 0.71Mohm then said which of course is a failure

well the part from the CU was LV wasn't it? so it should be tested at 500V.

Yes take your point but what im saying is then he said i could go through all the circuits and find out what one is causing the fail but i know its the upstairs lighting circuit ! dave can you switch off the upstairs lighting mcb ? then he retested it and got >99.9 Mohm then set it on 500v and got >299Mohm but what im saying is i would have passed the 0.71Mohm as still greater than 0.5Mohm how come he said its a failure ? i know mine wont do 250v test but will get another tester soon as cash allows

So he left a LE bulb in it to prove a point? If that is the case then i guess he was only testing at 250V to prevent the bulb being killed (as I mentioned above). The test at 500V should have had a similar reading anyway.

Hi Ian sorry for being a pain it was a low energy light fitting not bulb i know if it was on 500v test 0.71 would be a fail as under the min 1Mohm but 250v anything 0.5Mohm and above is a pass as he said selv/pelv will still require an insulation test in its own right so if your testing selv/pelv and on 250v dont understand 0.71Mohm being a failure ! or after 250v do you link L-N and 500v test it ?

Sorry for asking again just want to get head around it.

Regards

Kung.

Hi Ian sorry for being a pain it was a low energy light fitting not bulb i know if it was on 500v test 0.71 would be a fail as under the min 1Mohm but 250v anything 0.5Mohm and above is a pass as he said selv/pelv will still require an insulation test in its own right so if your testing selv/pelv and on 250v dont understand 0.71Mohm being a failure ! or after 250v do you link L-N and 500v test it ? Sorry for asking again just want to get head around it.

Regards

Kung.

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Hello Kung

You are right in saying above 0.5Mohm is a "PASS"

but if it is a brand new bit of cable you have installed on a new installation.

your IR reading should be ALL off your max scale

e.g.

>199 or >299

whatever your meter goes up to.

so a reading of 0.7Mohm on a bit of brand new cables imply you have dropped a big brown smelly one somewhere on your installation.

older installation will have figures getting lower

but in reality most are well above 50 or 60Meg ohm

as the Paragraph at the top of page 84 On site guide says ..

ANY reading below 2Meg Ohm require further in investigation.

HTH.

SL

Unless its a ELV fitting and not a LV fitting then the test at 250V is not valid anyway.

Thanks guys thats me done with questions on testing now your be glad to hear sorry for being a pain and thanks for your answers !

Regards

Kung.

Hello KungYou are right in saying above 0.5Mohm is a "PASS"

but if it is a brand new bit of cable you have installed on a new installation.

your IR reading should be ALL off your max scale

e.g.

>199 or >299

whatever your meter goes up to.

so a reading of 0.7Mohm on a bit of brand new cables imply you have dropped a big brown smelly one somewhere on your installation.

older installation will have figures getting lower

but in reality most are well above 50 or 60Meg ohm

as the Paragraph at the top of page 84 On site guide says ..

ANY reading below 2Meg Ohm require further in investigation.

HTH.

SL

Click to expand...

Likewise in IEE GN3

Hi All Just a quick question on insulation resistance testing i know how to do it but what i dont get is i know its just an exam question and if you got readings like bs7671 further investigation would take place ie

250v------------->0.5Mohm

500v------------->1Mohm

etc

just been watching Niceic dvd and he gets >299Mohm @ 500v & >99.9Mohm @ 250v ! ive got a kewtech kt61 and mine wont give symbol > and how come in bs7671 the min is that low ?

The guy in the niceic dvd also on 250v test got 0.71Mohms on selv and said its of course a fail ! how come if its above 0.5Mohm ?

Regards

Kung.

Click to expand...

if the SELV curcuit is fed from a Low Voltage supply ...ie an isolation transformer then the insulation resistance is done at the highest voltage.

Which in this case would be 230v with a result >1M

or

where protection is provided by SELV or electrical seperation the measured insulation resistance should be at least that required for the highest voltage present .

Guiness Drink

I maybe wrong but I get the impression that everyone is missunderderstanding the original post. Kung thought that by testing the circuit at 250v, then the 0.5Mohms reading was OK.

This is not the case. The circuit (if I read it correctly) is an LV circuit - 51v a.c to 499v a.c - and the insulation values given in table 61 of the bible are for the different voltage bands.

Because it is a 230v circuit, it should be tested at 500v d.c and the min. value of insulation resistance should be 1Mohm. However, doing a preliminary test at 250v d.c to protect the circuit STILL requires an insulation resistance of 1Mohm, not 0.5Mohm as has been suggested.

The 0.5Mohm value is only valid for a SELV circuit, regardless of the test voltage.

Probably as clear as mud..Blushing

J