RCD tripping mystery

[Ingwe]

Decided to have a few days away. -2° in York when we left and are now parked up at a site. Tried to hook up but no power. Site owners not here so assumed they hadn’t turned power on. When they arrived, they turned power on and on hooking up, RCD in van tripped. Tried three different cables (ours and theirs) same result.
So turned off both MCBs, left RCD on and pressed the test button. It tripped. So the van was getting mains hookup. Then turned on the first MCB (fridge, combi boiler&sockets) no trip. Then the second MCB (charger) turned on and no trip.
So, fingers crossed, everything working as it should. Any thoughts as to what is going on?

The hook up posts and leads on the site are lying in frosty grass and hence damp, but our lead, which was stowed in the van and is hopefully dry, also triggered the RCD.

 

[tonyidle]

It takes the tiniest amount of condensation in the van and in the wrong place to trip an RCD. Probably that rather than a fault as such.

 

[Odd-no-more]

Damp will reduce insulation values often to a point that they will trip an RCD or fail an insulation test. Years ago I used to fit the white goods on Wimpey sites, if we installed before the power was on and then there was a delay the electricity supplier's engineer would refuse to connect the power because the insulation reading was below 1m ohm. We would return and disconnect the oven and unplug the other appliances, the power would then be connected, we would go back again and connect the oven to a circuit without RCD protection run the oven up to temperature and retest the oven and it would pass any test as it had dried out.

 

[pappajohn]

Could have been a surge when you switched on with all circuits live (turned on)
Turning off MCBs then turning on one at a time would reduce the sudden load on the RCD.

 

[Fragle]

When I used to install new cookers in previous job the elements were always tripping the electric. Cookers sat for weeks/months in a cold warehouse. once you got some heat going they were fine. Probably the same issue. Then suddenly the electric is ok.

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