RCD Tripping for no apparent reason

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IF the RCD is 30 years old, then getting things properly tested may not be a bad idea.

Two thoughts for you (as an amateur myself)

1. a decent quality, 2nd hand RCD tester that would do you a ramp test and check the RCD is operating properly is £50 (linked below). Have a look on selling sites and Facebook marketplace as testers do go for very little money sometimes, and I believe are useful to have

2. a cheaper option for fault-finding could be a cheap 30mA plug in RCD. We could hope that with it's lower sensitivity it may trip and leave the main RCD on - helping you narrow things down.

Thanks for all the comments - practical and humorous.

No more tripping out since last reported, however, I've ordered the Kewtech RCD tester so watch this space for results.
I finally managed to trace the fault to one of the 2 pir outside lights while I was waiting for the rcd tester to arrive. It was a sealed unit but had condensation behind the glass. Not only that, when I cut the self-amalgamating tape from the connector block it was all wet!

Anyway, I took the light back, got a replacement and wired it through a little waterproof junction box. All now seems to be OK.

The rcd tester has now arrived and I am keen to test the 100mA RCD. However, before I do that I have a couple of questions:

1. The tester came without a mains lead. I have a spare computer lead that I can use with it but what rating fuse should I be fitting to the plug?

2. I am planning to unplug sensitive electronic equipment like computers & TV's before doing a ramp test but what about other appliances like oven, fridge, freezer, microwaves, door chime & bathroom fans etc. Should these be left connected/switched on?

Results of ramp test on 100mA rcd with all circuits off except for cooker radial which was needed for conducting the test since the test meter had just a 3 pin plug and no probes.

Tested 4 times:
70, 60, 60 & 70mA

Any comments?
did you turn off or isolate the cooker? I would have been inclined to make a lead with open end to 13a socket!