I'm switching on a ,aims supply with a relay via arduino. It all works fine, except very occasionally it trips the RCD device I have in the observatory. Im thinking this may be due to the relay arcing or something when switching on? Is thee anything I can do? I'm switching on 240v mains but the current is very small, I would say the total current drawn is still less than an amp, its only to power a couple of 12 v mains supplies that supply power to my telescope mount and the CCD camera so nothing too demanding!
If you think it is contact noise, change out the mechanical relay, of a SSR (Solid State Relay). It should give a lot less noise.