Weird IR results? At a Switched Fcu in RFC

[JaegerZero1984]

So I am doing an insulation resistance test on a new flat and I get a dodgy L-E reading on the IR. I won't go above 20Meg and it fluctuates. Identified where the problem is and its a switched spur. When cables are removed all test +299 including the load which is 2 sockets. When all are put back in spur, the load gives the same dodgy reading. Replaced the spur and still get the reading. Any thoughts?

 

[Pete999]

Cables touching the back box?

 

[westward10]

Could well be the spur, damp possibly. Only one way to find out, replace it or Wago all conductors together at the spur point and retest.

 

[JaegerZero1984]

Plastic back box ...

 

[JaegerZero1984]

Replaced the spur and tested the cable legs and all come up clear in every combination. Supervisor has just suggested to pit a simulated leg in to see if its the cable or not.

 

[westward10]

So replacing it has resolved it?
Have you IR tested the spur itself with nothing connected.

 

[Lucien Nunes]

Load side L leaking to real earth but not to its own CPC, which is well insulated from earth. So you only see it when supply and load CPCs are connected together. Try testing between load L and supply CPC with the FCU removed.

 

[marconi]

Re#7: I may be a but thick today (in pain with my new knee after its total replacement) but should your explanation read:

'you only see it when supply and load Ls are connected together.'

Then your final sentence makes sense.

 

[Lucien Nunes]

I prefer my version. Suppose the load L is damaged somewhere along the cable and leaking to the building structure or ground, but the CPC is not in contact with ground as the fittings are in dry studwork. Measuring L-E in the load cable alone will not reveal the fault due to the floating CPC. But the supply CPC, in contact with the building and ground through the bonding / main earthing, will be connected to the fault. Thus when the FCU is installed, even with the switch off the load cable will now read faulty as the CPCs are connected together.

BTW good luck with the knee. Do the exercises! Gave my mum her mobility back, has been marvellous, after ten years she can't tell it's not her own.

 

[marconi]

I think a take-away from this interesting problem is the importance of not only doing pair-wise IR readings for the supply cable and then the load cable but also to do the cross-pairs between conductors of the supply and load cables .

 

[Deleted member 26818]

Does the SFCU have a neon?

 

[Madocks]

Neon

 

[DPG]

Neon would not cause the results he's getting. Unless I've misunderstood.