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Strange fault, any bright ideas?

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A silly fault in a circuit only 4m long is currently eluding me!

This is ECIR remedials of a landlords supply in a block of flats.
Circuit 2 = hall heater (just one). IR L+N > E of 1.23Mohms.

It's a very short circuit.
1 - Cable from CU up to Controller FCU. (about 50cm)
2 - Cable from Controller FCU to heater FCU, other side of wall. red dots show where. (about 3.5m)
3 - Cable from Controller FCU load to controller, right next to it. (about 20cm)
4 - filpilote control wire from controller to the heater. (about 4m)

The heater FCU is very creatively wired up as really it needs a 3 pole switch with fuse. But leaving that aside for now....

Heater completely disconnected > Same results.
All cables separated > every cable independently tests ok. Hmmmm.
N from the cable which runs from control FCU to heater FCU (i.e. the 2nd half of the radial) has 1.23Mohms IR to the CPC bar.

After a bit of head scratching the only thing I could thing of was one of the earthed metal clad boxes might have a fixing screw that had caught the cable, because the heater is directly behind where the CU is. But with all fixing screws removed for all of the boxes it's still showing this reading.

I ran out of time tonight, but I guess next step is to find out which CPC it has contact with to hopefully provide a clue.
Any other bright ideas? It's a plastic CU and plastic trunking.


This happens to be one of two identical buildings, and the the other one didn't have this fault.
Have you ruled out a fault within any of the accessories?
Thanks for the thought - I think so!
I just checked photo's...the core circled in red is faulting to the CPC bar.
The only CPC connected here is the load.


(the installer used the N half of the spur to connect the control cable and put the N in a choc block, it won't be going back like that)
Or a fault on a seemingly undamaged length of cable?
Every cable independently passes all IR tests.
There is some fault on the 2nd leg of cable causing a N to installation earth fault, but not a N-E fault on that cable.

Honestly, It would easy enough to run a new cable but I've become a bit obsessed with what is causing this when there is so little going on in such a small area!
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