Type A RCBO does not trip.

Oh wow.  I feel your pain - these can be a real !"$£ er  It is the sort of thing that can give RCDs a bad name ;-)

OK if the fan heaters and kettle are altering the clamp meter reading, and 2/3 of them are 2 wire loads (I guess we  could lift the green wire in the kettle plug to get 3/3 if you like living on the edge. Probably no need) then there has to be something getting off the L-N loop. Normally an NE confusion but the IR tests do not find it. So perhaps to the N of another circuit, or to something that is not really earthed but is neutralled in some way, so would pass the IR test. Or a cheeky backfeed from the neighbours ?

I presume your ultimate IR test would be  (L+N)  relative to (CPC+MET+ bonding+ other circuits neutral bar)  with the test loads plugged in.  (I know, ECSQR, but it is only a 10 minute diagnostic, and most other countries permit)

I presume there is no time of day, rainy days or heating on or off pattern to the RCD trips and that N and E are not many volts from the fork in the flower bed or screwdriver in the lawn depending on the type of garden.

Otherwise as you describe it, I am as stumped as you are.

Mike.

Oh wow.  I feel your pain - these can be a real !"$£ er  It is the sort of thing that can give RCDs a bad name ;-)

OK if the fan heaters and kettle are altering the clamp meter reading, and 2/3 of them are 2 wire loads (I guess we  could lift the green wire in the kettle plug to get 3/3 if you like living on the edge. Probably no need) then there has to be something getting off the L-N loop. Normally an NE confusion but the IR tests do not find it. So perhaps to the N of another circuit, or to something that is not really earthed but is neutralled in some way, so would pass the IR test. Or a cheeky backfeed from the neighbours ?

I presume your ultimate IR test would be  (L+N)  relative to (CPC+MET+ bonding+ other circuits neutral bar)  with the test loads plugged in.  (I know, ECSQR, but it is only a 10 minute diagnostic, and most other countries permit)

I presume there is no time of day, rainy days or heating on or off pattern to the RCD trips and that N and E are not many volts from the fork in the flower bed or screwdriver in the lawn depending on the type of garden.

Otherwise as you describe it, I am as stumped as you are.

Mike.