came across this today. washing machine repaiman had been and was getting problems with RCD tripping. the set up is 16th Ed. CU. front end 30mA RCD. MEM. 8 circuits, 2 of which are socket RFCs. in the kitchen is a 13A FCU feeding a socket under for washer. so far , so good. however, a dishwasher has been added, the socket under the worktop now feed an extension lead for both the washer and dishwasher. not ideal if they are both in use simultaneously. checked behind both the FCU and socket for trapped/skinned cables. IR tested at CU global readings L-E 5Meg. N-E 5Meg. tried a no-trip loop. .... RCD tripped. ramp tested RCD 19mA. then a normal RCD test. came up trp, with no msec. value. i have condemned the RCD> as it's an old MEM, going to replace CU for a dual RCD unit. customer is old and disabled so i think it's the best way, split the sockets and the lighting between the 2 RCDs. as it's a straightforward job ( easy access and can't see any snags) quoted him £300 tops.