intermittent RCD trips - any advice?

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
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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.

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.

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.

If earth leakage resistance tests out OK, this can point to an intermittent fault connection to ground, rather than a low current leakage path. You could try flexing, prodding, and otherwise moving things around to see if you can trigger it manually to help in identifying the location. Obviously, beware you are handling a known faulty installation, where something nasty might be waiting to happen, such as a bare cable end pulling out of an accessory.
Another possibility could be condensation forming inside one of the accessory boxes, in which case it's going to be dependent on temperature, humidity, and rate of change of temperature, but can completely vanish later if the sun warms up the accessory. You might want to check any condensation drain holes have not become blocked (and were provided in appropriate locations in the first place).
As another point, it's a bad idea to have indoor and outdoor circuits on the same RCD, for the reasons you've discovered. I would move the outdoor circuit to the non-RCD side, and add it's own RCD to the circuit.
--
Andrew Gabriel

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