I have a consumer unit with an RCCB and 10 MCBs, one of which runs my upstairs lights. The bathroom has 4 low voltage spots, a wall light and an extract fan (with an isolating switch). Just lately, when the bathroom lights are switched on, the RCCB trips out, but the MCB does not. All the other upstairs lights seem fine. The problem is that this does not happen every time - about 50% of the time everything works okay. The wiring is not subject to movement as it is in the loft. I have checked the wiring as best I can and cannot see any problems, except that it is a pretty messy job. The electrician seems to have used junction boxes everywhere and some of these have up to 4 or 5 cables meeting, making them very crowded. Can anybody suggest what may be causing the problem and why it does not happen all of the time. A friend has already suggested to me that I could replace the RCCB with an isolating switch as the RCCB could be too sensitive. Is this a bad idea? Any ideas welcome, as my wife is getting fed up with having to reset all the clocks.