Newish installation, wired about 4 years ago to 16th edition. Split load CU with most circuits on RCD side, lighting, smoke alarms etc on non RCD side. The problem is in the bathroom. There's a 9KW electric shower. There's also an extractor fan above the shower, an SELV fan with transformer in the loft space above the bathroom. The fan is fed from the output of the shower isolator via a switched FCU. The owner arrantly wanted it that way so the fan only comes on when the shower is in use, not every time the lights were turned on. Everything runs fine, lights, shower, fan all operate. But, when you have finished your shower and come out of the bathroom EVERY time you turn off the shower isolator switch, the RCD trips. RCD can be re set immediately and does not trip again until next time the shower isolator is turned off. If you turn the fan off buy turning off it's FCU, and then turn off the shower isolator, the RCD does not trip. I can't find any shorts or IR problems, and it has been working fine for a couple of years. It's only started tripping recently.