Mechanical Plug Timer in garage for christmas lights

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Hi. Hope somebody can help. Using a dry box outside i have put new outdoor led xmas fairy lights up. set up is ext lead from garage to dry box (garage power linked to house by armoured cable, garage has own consumer box). to manage these lights i have a cheap status mech plug timer inside garage where ext lead plugs into. lights went off at 9pm as set by timer. then at 2am the rcd trips the mains ring in the garage and house. Then next night same again but 1am. have now disconnected the lights and unplugged the timer. Can anybody suggest what may have happened? I suspect dodgy mech timer but unsure if this would trip the breaker. this has never happened having lived here for 10 years.
 
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Where supplying a consumer unit from a consumer unit using SWA cable the normal method today is the main consumer unit will only have MCB protection and the second consumer unit has it's own RCD. However the rules do allow a 30 mA RCD to feed a 30 mA RCD in fact with caravans you have to, but normally we look at factor of 3 so 10 mA fed from 30 mA fed from 100 mA fed from 300 mA fed from 1A etc. However with a short to earth rather than a leakage likely all RCD's will trip.

To answer question we would need to know sizes and where the RCD's are located.

A quick guess is moisture has got in some where. At night it is likely to condense onto contacts which will leak across. Once it leaks it generates a carbon track, so it will then do it again and again. Cure is with power off clean down all you can to remove and carbon track, and locate items where dew is unlikely to effect them.

Plug in 10 mA RCD's are rare, some camping and caravan shops sell them, but in the main 30 mA is all you can find, some consumer units have two neutral bars and some have three, those with three can have non RCD protected supplies and also use a few RCBO's if required. Those with only two neutral bars would need a whole bank of MCB's to be changed for RCBO's to allow circuits to have independent RCD's. (RCBO is a MCB and RCD combined)
 
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Thanks a lot, thats really appreciated. i will check tonight at home. there is also a pir light on side of garage, i will disconnect that tonight and try again. it could be contributing to problem and might explain the funney random hours of circuit trip.
 

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