Insulation resistance woes again!

[Benny_Boy]

Hi all,

Just completed a minor works + CU change on a property and found that the circuit which has had the addition (up ring) has a doorbell tranny buried between floors. I was initially getting 0.00M L-N and 0.32M L-CPC and 0.32M N-CPC.

I found the section of the ring which the junction box must be coming off and disconnected both ends at their sockets and retested and got >299M for all tests on that circuit.

The section of the ring I've added to for the minor all tests ok once the doorbell section is disconnected, however, for the STR to accompany the EIC I look like having to LIM the upstairs ring as even if I do L+N - CPC it's still only reading 0.32M when the ring is back in place!

Is it normal to get such low readings from live conductors to earth with a bell tranny? or am I looking at a dangerously deteriorated fire hazzard?

any thoughts greatly appreciated chaps

 

[acat]

Have you tried live and neutral on one clip and earth on the other clip for test.

Have a look at 612.3.2 and 612.3.3 of BRB page 158


Chris

 

[Benny_Boy]

yeah, and I get that same 0.32M reading as I do when I go from L- CPC or N- CPC

that's what was worrying me really, I wouldn't have been bothered if I could have got a nice >299 for L+ N linked to CPC

 

[acat]

Sorry re read your post and missed that little bit. Everything else tests out ok so I would say its got to be the tranny thats causing the problem hopefully someone will confirm that has actually had this fault before.

Chris

 

[elec1984]

Hi i'm new to this forum and have my elecsa assessment coming upina few weeks. I have a question about doing an insulation test on a radial circuit (domestic upstairs lights) i have tested the circuit and am getting a very low reading under 0.28 mohms. Anyone got any ideas why this is so low

 

[Jason Smith]

Hi mate, I would assume that you have good knowledge if its almost assesment time for you, however thers not a lot of detail in your post as to how you did the test, did you take all the bulbs out etc?

 

[elec1984]

Yeah sorry conducted a cu change, for the radial circuit i removed all bulbs and checked all the wiring visually and returned to the cu and tested for insulation using a fluke 1652 line-neutral and line-earth. The line to earth reading was fine over 500mohms but the line-neutral was low 0.28mohms

 

[Guest123]

Then theres definately still a load on the circuit somewhere....TV booster in attic, ex fan, shaver light, attic light.......

 

[elec1984]

There are none of these things as i have been into the attic and checked i agree it sounds like a load but have checked and their is no sign of one. Any other ideas?

 

[Jason Smith]

do a visual on every termination you can find, and nip up a bit. spread the wires apart a bit, leave all exposed (ban customers from house ) Test again. If this has not greatly improved your readings, check the location of every screw etc, thats used to fix accesories and make sure no wires being pinched together by them.
If the rest is as you say, with no surprises anywhere, the test may of reveled a fault in the wires that you cant see,as that is what the test is for. Then its a matter of breaking down into legs untill the faulty section is found.

 

[elec1984]

Thanks for the advice. Greatly appreciated

 

[lj.l]

agree with Jason split the circuit in half and test both halfs then split the low reading side again and test again,you will either find a fault or a load

 

[Jason Smith]

just for your info I was getting 2 MO on a circuit that fed a conservatory the other day, I left the face plates off all night and tested again- up to 250MO this time, so damp does play a big part, perhaps drop the bathroom lights down as well and have a few minutes with the hairdryer pointing at em.
You can only do so much though before you have to admit you have a fault, hopefully some of the above will prevent "invasive" fault finding.

 

[Spudnik]

Yup.

Gotta love those trapped cables behind faceplates

Also, neons are a fave, and PIR sensors.

 

[Alan ralph]

Had problems with insulation resistance tests.seems my hands are sweaty causing lower readings ie not 500 meg.when wearing rubber gloves readings seem to be ok.anyone else had these problems ?