So this is an old post revisited. Some of you may remember but it was December 2018.
Father-in-Law had a new NEFF oven installed by John Lewis. Their installers said that the MCB needed changing to a 32A part. The electrician called and checked the wiring and swapped the MCB. He said that the method the JL installers had used to connect wasn't great; ideally the copper wires should be wrapped around the screw terminals.
This oven often trips the RCD when it is turned off using the oven controls. NEFF were called in sometime last year and they replaced the oven controls and left.
Same problem seems to have continued to plague this oven and NEFF were called before this lockdown hit. They've just visited and replaced ALL heating elements. The engineer (who spend about 4 hours there) also ran the oven from a 13A socket on a different RCD (the CU has two RCDs). It seemed to work fine, but would still trip the other RCD when it was wired back to the cooker point. He then tested the wiring and said (his words) "there is some continuity" and suggested a sparks should be called in to check the wiring.
Following my 1st post way back, someone suggested switching off all other MCBs on that half of the CU. My FiL did this and can report that it seems better, but still occasionally trips.
I'm not sure what wiring the NEFF man said "had some continuity" but when I checked with a simple multimeter I saw that when the cooker wall switch was off and the oven disconnected, then there was high impedance between all terminals at the oven end of the cable. With the oven MCB off I tried to measure the same impedance L to N from the cooker wall switch back towards the CU. The reading was low, like less than 1k. What was I measuring?
Anyway, he seems to be back to square one. What's the next step? Change the tripping RCD, move the oven MCB outside of the RCD zone, maybe using an RCBO? He's understandably near his wit's end!