Induction Hob Tripping Rcd

Evening all,

i have a customer who has a meile induction hob fed via 32a MCB in consumer unit then through a seperate RCD.  most of the time when the hob (mostly one perticular ring) is turned on to between middle and highest heat setting the RCD protecting the sockets trips. 

The board is a double stack board where top row is off main switch bottom row through RCD.  originally the cooker circuit was protected by board RCD but previous electrician (who has now retired) moved it to none protected side and fed it through a seperate enclosure due to RCD tripping.  ive tested cooker circuit R1+R2, IR, Zs, RCD trip and ramp which were all fine, also done same tests for board RCD which again were all fine.  Had a clamp meter on cooker circuit to see if there was an inrush but didnt seem to be and at full load was below 32A.  ive double checked wiring at treble checked neutral bars but at a loss.  any ideas?

just to clarify the RCD protecting the cooker doesnt trip, its the board RCD which is after cooker circuit.  it only every trips when hob turned on, trips RCD in board all sockets etc trip but hob keeps working.  once rcd reset it'll work fine till next time, ie next time its used again.

sorry for long post but have probably still forgot something.

Are you saying the main consumer unit has a non-RCD protected 32A MCB that supplies the hob via an RCD in a stand-alone enclosure, but the RCD protecting other circuits trips? have you done IR tests between the hob circuit and the rest of the board. it sounds as though some part of the circuit is not fully isolated form other circuits. Shared neutral somewhere?

Doc H.

Yes none rcd protected mcb supplies hob. All tested and hob fully clear from rest of board.

Neutral pinched to fabric of building or to other circuit on RCD side ?

High resistance neutral connection before the RCD's.

i did tighten all connection in consumer but will check this again.

if circuit pinched with another circuit it should have shown up when IR testing it to RCD protected side? should it?