Low Insulation Resistance reading of 0.63 MOhms , what does this indicate?

[TheSilverSpider]

on a ring testing Insulation Resistance L-N shows 0.63 Mohms
but L-E is ok.
I checked with the home owner that nothing was switch on, nor plugged in still.( its a 3 bed semi, looked every nook and cranny)

Is that kind of reading tell tail of anything causing that in particular, or is it a case of next visit to investigate each socket and feed to/between it individually?

Reason is they are getting random trips of the RCD for that circuit. BUT on a different circuit (hot water tank) MCB it was tripping out the RCD only when switching on the heater element , BUT NOW isolated that, and it still ocasionally trips same RCD when i.e. turn on toaster (happened once) or garage lights (happend once), also a few times when no appliance/switche were touched, both on different circuits but same RCD.

I've checked the trip out times and manual function of that RCD:

1/2: a/c tripped out, reset and never tripped on this setting after re-trying later numerous times.
1/2: d/c didnt trip

1: a/c 34ms
1: d/c 29

5: a/c 9
5: d/c 7

Its an MK RCD standard usual, Megger 1552 on 30mA setting

Heater tank circuit tested ok, and kitchen ring tested ok.

Thanks in advance

 

[sajeel]

What is approximate age of wiring ? I would say it's due a rewire if ir reading are that low, commonly the l-n fail first.

 

[telectrix]

lift the sockets off the boxes and check all. . even a bit of damp in a wall can cause tracking across terminals. if still low readings, then split ring and test each leg, then split faulty leg and so on.

 

[sajeel]

As telectrix said any damp would cause a problem with ir readings, as explained the only way to diagnose is to split ring up then keep adding until fault is found.

 

[telectrix]

As telectrix said any damp would cause a problem with ir readings, as explained the only way to diagnose is to split ring up then keep adding until fault is found.

well, at least that solution is less drastic than your previous post of a rewire. LOL.

 

[sajeel]

Lol if no damp is found and wire is faulty due to age then a rewire is due

 

[telectrix]

that's something that can only be determined by on site inspection/test.

 

[derek]

What is approximate age of wiring ? I would say it's due a rewire if ir reading are that low, commonly the l-n fail first.

Just wondering how l-n fails before l-cpc or n-cpc considering it has no insulation and is between l and n?

 

[sajeel]

Of course I agree

 

[sajeel]

As live to neutral always carries voltage and cpc does not therefore the insulation pvc can break down and voltage leaks can occur.

 

[telectrix]

and if the wiring is not too old: has not been attacked by rodents, 90% of the time, the fault will be at an accessory. damp, trapped/squashed cable, faulty accessory. even a loose connection could cause the fault.

edit: included with rodents are plumbers, builders, wood butchers with their blowlamps, hammers, and nails.

 

[Silly Sausage]

As live to neutral always carries voltage and cpc does not therefore the insulation pvc can break down and voltage leaks can occur.

So you would expect L-E to fail first. Assuming T&E.
No voltage on the Neutral

 

[derek]

As live to neutral always carries voltage and cpc does not therefore the insulation pvc can break down and voltage leaks can occur.

Would it not show a low IR reading to the cpc seeing as it is between the live conductors?

 

[telectrix]

not always. cores are split at accessories, so there could be a L-N fault which does not affect the cpc.

 

[derek]

Good point, but personally I wouldn't say that damaged insulation at an accessory meant a rewire was required as had been previously suggested,but perhaps that is why I'm not rich.