I've been experiencing occasional nuisance RCD tripping for a few
years now. It's very occasional - sometimes going for up to six months
without tripping. On average I'm getting a trip around once every two
months.

I've done some equipment testing with a megger and basic RCD
sensitivity testing with some home-made leakage plugs (plugs with
resistors between live and earth to give a known leakage current).
Based on this, I think my RCD is over sensitive.

My plan is to replace the RCD, which is fairly cheap and simple. But
I'd like to test the old one and the new one with something better
than leakage plugs to see if there's any change in sensitivity.

Does anyone know if it's possible to hire RCD testers for short
periods at a reasonable rate? When I've looked, most hire shops direct
me to a PAT tester which isn't what I'm looking for. I don't want to
buy one for a single project.