Hi joed, thank you for your response, yes I am in the UK. you are spot- on to suggest I could have equipment connected outside near a water source. I do keep a fish pond water scene in my garden, which today to my dismay I discovered the junction box located by the pond was full of rain water, which of course normally would suggest this would likely be the cause of the problem. I have now replaced the connection and made certain it is watertight and find it is not blowing the trip when I turn it on. Whereby I find other sockets when being turned on are still setting off the trip, for instance if I turn on the switch to my computer with say my Tumble dryer switch in the on position it will trip the breaker even with the pond socket pulled out. or if I disconnect the socket to the Tumble dryer I can then turn on the computer. this is really weird. could it be a possibility that maybe the junction box to the pond being full of water had perhaps damaged the RCD box. what do you think?