Very strange fault, I have a couple of catering ovens which plug into normal 13A sockets. Ovens are very basic - normal oven elements, electric fan, neon indicators, rotary switch & electromechanical timeswitch, no electronics or surge suppressors etc. Insulation resistance is up in the many megohms, so problem is not an earth fault in the ovens themselves. All works fine, except that occasionally the rcbo serving the circuit trips when the switch on the wall outlet is turned OFF. As I say, during operation, no problems, it's only when they are turned off.
Any ideas on this rather mystifying problem?