vehicle charger tripping house rcd

Turning off will presumably open circuit the connection as a big contactor goes 'clunk' .

It sounds like it is modulating the voltage drop in the supply neutral a little, which alters the voltage between the PME side neutral which is the house CPC of course and the true earth voltage of water pipes etc.

Normally this would not matter, but if the N - terra-firma offset voltage is unusually high (as can happen if the supply neutral is higher resistance than normal) or the RCD is very close to tripping in normal use (due to accumulated leakage on many circuits) and just needs a mA or two more  to push it over the edge.


So two prongs of attack -


First, how close is that RCD to tripping without the EV charger ? A good 'headroom' woudl be if it can take an extra 10mA without tripping, less than 5mA and it might be time to wonder if the circuits it supplies really should be spread over a number of RCBOs rather than one RCD. May be worth looking if just one circuit is especially bad - induction hobs, outside power and some types of oven seem to cause more earth leakage than the rest.

Do not rule out an NE fault in the house that may be slowly forming but not  actually  a dead short, that makes the RCD more sensitive to Neutral earth voltage.


Second, what is the voltage offset between the true earth and the PME neutral - normally I'd say put a crocodile clip on a garden fork and wander a lead from  that to the MET, but  you have already fitted an electrode. More than single figure voltage  here should raise an eyebrow, and if so that is so be followed up by comparing PSSC L-N and L-E  - for PME should be the same, or the NE link at home is wobbly.

  Repeat that test with the house switched off and bonds to plumbing etc disconnected, so you  are only looking at the neutral path in the street cables, and not fooled by a duff supply neutral but the bonding and the piping taking all your neutral current down the pipes to use next door's NE bond. If the ZS is much lower or PSSC much higher with the bonding in place, then the bonding is a large part of the neutral path, and a call to the DNO may be in order - though maybe come back here with the numbers first to cross check and  avoid embarrassment of a DNO call for no fault found.)

Brilliant. Thanks for your help. I wasn't sure if the house rcd was slightly sensitive or the current from charger was tracking back down the earth spike, through the ground and onto the house oil pipes, then back to the met, or something like that. Will keep you posted.