IR readings problem? Confused newbie

[Steve34]

hi guys,

really hope you can enlighten me, i just did a periodic, and was going to connect L-N then test to earth for the insulation resistance test, client agreed to the Lim, i was getting no readings so i reverted back to standard 1 circuit at a time, L-N, L-E, N-E.

i started with the cooker as its the most simplistic circuit, L-N & L-E >500 but N-E no reading?

then when i came to do the ring final i got no readings at all??? yet they've never had any tripping problems.

i've only recently passed my 2391 which always explains what readings you should get but doesn't go into so much detail about iffy readings, and what causes them etc... hope someone can help me, and i'm sure this will be just 1 of many post from now on

ps. is there a website with a list of common testing problems with possible causes/solutions? that would be fantasic.

 

[Guest123]

When you say you didn't get a reading do you mean 0.00 or OL??? can you elaborate a bit.

 

[Steve34]

oops sorry about that, i mean't 0.00

 

[johnboy6083]

did you disconnect the cpc and neutral from the CU? it may be a TNCS system, and the neutral & earth linked atservice head?

 

[Steve34]

hi johnboy, must admit i didn't remove the cpc and neutral cables from cu, but will try that, the system is a TN-S, earthed to the supply sheathing.

 

[Guest123]

That'll be it then.

N&E are joined at the supply transformer.

 

[johnboy6083]

even in a TNS sstem, the earth and neutral will be linked at the supply transformer. in newer transformer installs ive seen, the star point of a transformer is connected to a busbar, which all of the main bonding of the transformer is connected to. this is then connected to an earth mat which is laid underneath the transformer, during the groundworks.

Ive never seen an old transformer that supplies domestic properties though!

 

[Steve34]

thanks guys, any idea why i'm getting a zero reading on all the ir tests on the ring?

 

[Des 56]

If this is domestic
The main isolator would be in the off position
The installation neutral would be isolated from the network
The earth -neutral wont be reading unless there is a fault or

Were the readings taken with an Rcd/Rcbo in circuit
Especially ones with a tail earth monitor,you will get false readings unless you dis connect those

Was the cpc continuity tests done before the insulaton resistance

 

[Steve34]

hi des, the cpc continuity was done and was fine, but the circuit was rcd protected, i know this is probably a stupid question but wouldn't the rcd give me a false(low) reading as apposed to 3 zero readings?

 

[Des 56]

Just put paint pot back down

The Rcd or Rcbo may give a virtual zero reading neutral - earth because the reading being in thosands or millions of ohms would be indistinguishable from a low reading on the insulation tester

If there is a load or neon or simillar in circuit the line -cpc may also give that same reading

Another test making sure nothing will be influencing the result would be my way forward

 

[Steve34]

good thing my missus didn't see that post, she'd have the roller in my hand so fast my head would spin.

cheers for the advice, i'll go through the tests again, making sure i haven't missed anything, it'll all come good with experience i suppose.

 

[Risteard]

reading being in thosands or millions of ohms would be indistinguishable from a low reading on the insulation tester

???

Insulation resistance ohmmeters measures in millions of Ohms (1 Megohm = 1 million Ohms).

 

[Des 56]

Risteard

I am well aware of the scale and value

Half a meghom = 500 thousand ohms

When we use an insulation tester,the readings obtained when getting a low value in perhaps thousands of ohms,when the tester is trying to measure in millions of ohms.the lower values whether dead shorts or low resistance cant be distinguished between that dead short or low insulation resistance

 

[Risteard]

Risteard

I am well aware of the scale and value

Half a meghom = 500 thousand ohms

Which means that it's distinguishable.

I thought you were saying that it wouldn't show up during an insulation resistance test.