insulation resistance explained

[iamelectric]

can someone please explain insulation resistance testing to me i have read GN3 and brian scaddans book , i know it is to ensure conductor insullation has not deteriorated or damaged but i read posts on here about how you can use it to find borrowed neutrals and short circuits , and why does coils and neons effect the value , as you can probably tell i dont do testing

 

[telectrix]

basically, you test between conductors to ensure there's no leakage through the insulation. if you test from . say, the N of the upstairs lights to the N of the downstairs lights and get a reading, then that indicates a borrowed neutral. if there is anything in circuit when you test , e.g. a neon onn a FCU, then that neon has a finite resistance which will bugger up your IR test.

 

[iamelectric]

so say i am testing between live and neutral of a circuit 50m long and around the middle of the circuit the insulation of the live conductor has been sheared off for about a foot from being pulled through steel trunking bend but is not touching the trunking would an IR test show up anything

 

[telectrix]

not if you're IR testing L -N it won't. think of your IR tester as a multimeter on ohms, but it puts a much higher voltage across whatever you are testing, thus will detect a high resistance short between conductors and/or metal containment.

 

[Mr Mark Sparks]

That's right
i once found by chance a section of t and e under a floor with all cores completely bare for about a foot having been eaten by mice or similar but It still tested ok as no cores touching

 

[Richard Burns]

If the cable was very close to the trunking and the trunking was earthed then it is possible you would get a reading, similarly if the area was damp slightly and there were a moisture path then you would get a low value reading.
If the exposed line conductor were totally clear of any earthed metal work and other live conductors then unfortunately the resistance of the air would mean that you would have a satisfactory reading. (however there would also be no risk (unless things were moved))
This is where inspection before testing is important and this may then be picked up before it became a problem.

 

[iamelectric]

thats what i dont quite understand how can the IR test show if insulation is damaged

 

[telectrix]

it can't always. only if there is a resistance between conductors. touching would give you a 0.00 Meg. reading. water in a fitting may give you a reading of anywhere between 0.00 and 5 Meg. takes a lot of experience to understand IR readings, and even then, you can be led up the garden path.

 

[iamelectric]

yeah i once tested a swa in garden came up with 0.27 or somthing like that , it was water in an enclosure

 

[Richard Burns]

The insulation resistance test is effectively checking the state of the insulating properties between two conductors. Not necessarily the physical plastic covering.
However if the cable's insulation is deteriorating either because of age or physical damage, such that two conductors are starting to get within electrically conductive range then the insulation resistance test will show this.

 

[Mr Mark Sparks]

A little off topic but we tested an installation 5 yrs old and all external lighting circuits were .5 meg or below they were wired in lsf singles in metal pipe that had let water in over time


see note below


Further, the construction of the cable affects water ingress. Low smoke zero halogen (LSOH) cablestend to have a lower resistance to water ingress because the sheathing materials used are evenmore hygroscopic than PVC.


My point to the op is that even a new installation can suffer ir problems early into its life and why we ir test