I have a domestic consumer unit fitted with an 30mA RCD. I'm getting intermittent, almost random trips from the 32A line that feeds an external installation running pond equipment - it trips twice in one day, then nothing for six weeks and so-on. When it does trip the MCB sometimes goes, sometimes doesn't. Circuit will reset if disconnected, but not always immediately. I figured it was just a twitchy RCD, but I, 5I and 1/2 I tests averages out at
11msec and 19mA trip current.External installation is fed from RCD side of main consumer unit via 32A MCB and
42A rated armoured undergorund cable to a subsidiary consumer unit and mains switch houses in a small cupboard. The subsidiary consumer unit feeds a pond heater and pump and some lighting. All external equipment is rated at IP67 or better. All cabling checks out, all megger readings and cable impedances within regs.My best wild guess is that the pond pump is intermittently stalling when it chews on a lump of crud, causing a large surge current and the current phase shifting relative to the voltage phase, tripping the RCD. I would normally put a rotating machine on the non-RCD side of the supply, but Regs insist that it's in the garden (and worse still in the pond) so it must have RCD cover.
Any advice of suggestions? Anyone come across anything like this before? Anything else that might be causing it?
Steve