insulation resistance testing

[ianmacd]

Yep that's what I was saying too.

 

[SPECIAL LOCATION]

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.I don't think he did ?

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.

Which is what Kung said but he questions why the Nic bod said 0.71Mohm was a fail on SELV when 0.71Mohm is greater than 0.5Mohm?

Probably as clear as mud..Blushing

J

TBH

I think you have got it wrong Janice and have misread the OP ..

Although he was also questioning some of the readings on his particular meter & the max ranges etc.. :|

IMHO Kungs 0.5Mohm IS relating to an SELV circuit!.

If you re-read Kungs post and specifically the last sentence?

(although it could have been worded better if he swapped his '250V test' & the 'SELV' over?)

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------------->0.5M

500v------------->1M

etc

just been watching Niceic dvd and he gets >299M @ 500v & >99.9M @ 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 0.71M on selv and said its of course a fail ! how come if its above 0.5M ?

Regards

Kung.

"The guy in the niceic dvd also on 'SELV' got 0.71M on '250V test' and said its of course a fail ! how come if its above 0.5M ?"

That was my reading of the question? :|

But I can go and wait by the cloak room if you want?

:coat

 

[AK-UK]

Based on my experience, a healthy circuit will read, in most cases, above 50Mohm (a new circuit will read above the meter's range). A healthy circuit will give much lower readings if you left something connected. If the reading for a specific circuit is less than 5Mohm, I will still investigate, although it is officially pass.

As mentioned the meters do not have the > sign, but you should know the measuring range of your meter; my meter can read up to 500Mohm, so I know that if it reads 500Mohm it means > than 500Mohm.

Again I agree with the idea of testing with 250V first, if something is wrong or there is something still connected you will get a very low reading anyhow, if it reads above the max value for 250V (in my meter 200MOhm) I know that i can test with 500V without causing damage. The other acceptable way is to test E to L, E to N and than L+N to E.

 

[ianmacd]

Again I agree with the idea of testing with 250V first, if something is wrong or there is something still connected you will get a very low reading anyhow, if it reads above the max value for 250V (in my meter 200MOhm) I know that i can test with 500V without causing damage.

I always do that but BRB does say can test at 250V instead of 500V if SPD equipment or other such equipment can not be removed but has to be at least 1Mohm not 0.5Mohm (612.3.2).

The other acceptable way is to test E to L, E to N and than L+N to E.

612.3.3 says that L+N to E is allowed. Why test E to L and E to N in this case as they will also be tested in L+N to E?

Ian.

 

[AK-UK]

I always do that but BRB does say can test at 250V instead of 500V if SPD equipment or other such equipment can not be removed but has to be at least 1Mohm not 0.5Mohm (612.3.2).612.3.3 says that L+N to E is allowed. Why test E to L and E to N in this case as they will also be tested in L+N to E?

Ian.

I said that L+N to E is acceptable, but when you test this way how do you know that there is no problem between L and N? it does not have to trip the CB but still be too low to be safe. There for I always test all with 250 v and if the result is hight enough, I test with 500V.

 

[SPECIAL LOCATION]

I said that L+N to E is acceptable, but when you test this way how do you know that there is no problem between L and N? it does not have to trip the CB but still be too low to be safe. There for I always test all with 250 v and if the result is hight enough, I test with 500V.

I do what you do as well AK!Applaud Smiley

Although...

theoretically if there was a problem between L&N

On T&E it would show up between L+N & E because the E is physically between the L&N.

Obviously not true if singles or conduit wiring though!! :|

 

[AK-UK]

I do what you do as well AK!Applaud SmileyAlthough...

theoretically if there was a problem between L&N

On T&E it would show up between L+N & E because the E is physically between the L&N.

Obviously not true if singles or conduit wiring though!! :|

And when it is a flex...? or the T&E is twisted? but is general I think that we agree...

 

[SPECIAL LOCATION]

And when it is a flex...? or the T&E is twisted? but is general I think that we agree...

Ah!

Flex!!!!

forgot that one...

Bonus point to MR AK please admin!!!Applaud SmileyGuiness DrinkGuiness Drink

I blame the beer BlushingGuiness DrinkGuiness Drink]

 

[green-hornet]

I just think you all make it sound more complicated than it actually is.

I still use the anomaly of big pipe more water, little pipe more pressure.

However at that age 1970's was a good time in my education.

 

[wozz]

forgive me if im wrong but hasnt the low IR reading been caused by not disconnecting the sensitive equipment he had just installed?

Shoving 500v up a low energy fitting (if its one of those pendants) will surely ****** it.

 

[ianmacd]

Depends on how you describe "low". If its something still connected then you would expect <<0.5 Mohm.

Ian.

 

[tom1]

so best way is to test on 250v, low readings <.5 M ohm

check everything is disconected and retest. if still low

test on 500v but phase + nutral to earth if still low then cable fails, if high >50 M ohm then cable pass and there is prob a pendent with bulb hidden in the loft.

so this way cables are tested and any sensertive equipment including selv and fluresent control, dimmers is left undamaged???