Readings not adding up!

[luminaire]

Doing a job today and replaced a twin skt using a dual box for two FCUs.
The 13A FCU supplied a 2.5 SWA cable o/s to a w/p box containing a 4-way skt. The 3A FCU supplied 1.5 cable in conduit for two o/s wall lights.
Both circuits went through the wall into a 5-way JB, to keep them separate.
When I did an insulation test for the skt circuit, I got the following results:
L-PE >299MOhms
L-N 245kOhms - I couldn't understand why only 245kOhms!
N-PE >299MOhms
ELI(0.4Ohms) & RCD both ok.
On the other circuit, Insulation tests all >299MOhms
ELI 2.1Ohms
Rcd tests would not trip and showed up as >299ms
I thought afterwards perhaps I should have tried a ramp test, this I can still do. I am using a Megger CM500.
Any ideas or help please?

 

[securespark]

There must be something in circuit to give a low IR.

As far as the RCD goes, did you test it in isolation?

 

[Lectrician]

IR and RCD's not good. Can give dodgy readings, and can damage RCD's.

 

[Spark123]

Neon in the 4way socket??

 

[luminaire]

There must be something in circuit to give a low IR.

As far as the RCD goes, did you test it in isolation?

The skt cct was live at the time and the 13A FCU feeds the 3A FCU in the dual box.

 

[luminaire]

Neon in the 4way socket??

Yes, that's right there was a neon.

 

[Spark123]

Did you disconnect the Neon? Is possibly why you were getting a low P-N IR result.

What sort of RCDs were they, RCD fused spurs or something?

 

[luminaire]

Neon in the 4way socket??

What size RCD and which test you doing? Can you test it at the CU?

30mA RCD and I normally do the auto sequence RCD test ie ½I, 0°, 180°,fast trip 0° & 180°. I usually only have to stand by the CU and reset it when it trips. But this time it failed to trip.

 

[Spark123]

So the RCD is a mechanical type installed in the CU? Can you try testing it in isolation at the CU? If it fails it is broken and needs replaced.

 

[securespark]

What about the functional test?

I have heard of RCD's not tripping before but testing OK in isolation & from the test button.

If it's not the RCD, it must be something in the circuit...

 

[luminaire]

Did you disconnect the Neon? Is possibly why you were getting a low P-N IR result.

What sort of RCDs were they, RCD fused spurs or something?

No I didn't disconnect the neon, but I could try that and test it again.
The skt cct comes off the ring main which is RCD protected as you would expect, so there was no need to use RCD fused spurs.

 

[ColJack]

5-way JB, to keep them separate

i hope you mean 6 way JB, otherwise you have a bridge on it ( probably the earth.. )

neons give reading across L+N..

can you describe your RCD testing methodology?

 

[luminaire]

So the RCD is a mechanical type installed in the CU? Can you try testing it in isolation at the CU? If it fails it is broken and needs replaced.

I can try that, it is only protecting the ring main and a shower cct.

 

[luminaire]

5-way JB, to keep them separate

i hope you mean 6 way JB, otherwise you have a bridge on it ( probably the earth.. )

neons give reading across L+N..

can you describe your RCD testing methodology?

The earth is at the moment common to both ccts, but if that is the case I can separate them.