dorothy wrote:

RCD is MEM 100A 30ma RCD - *with 3rd earth tail* connected

Not sure I follow that bit... what do you mean 3rd earth tail connected?
(we are talking about an RCD and not a ELCB I take it?)

o MCBs all OFF -- no L connection to house
o RCD-N-output disconnected -- no N connection to house

What type of CU is it? i.e. if its a split load unit, does it have a
dedicated N busbar for the RCD?

RCD still trips instantly.

I found this after:
o RCD would not reset - all MCBs off
o RCD would not reset - all MCBs off & all appliances dbl-pole off / unplugged
o RCD would not reset - all MCBs off & most cables L+N disconnected
o RCD would not reset - all MCBs off & RCD N-output to N-bar disconnected

So I'm sort of running out of options.
I don't understand how it can trip with no L connected or N connected.

knackered RCD would be a prime suspect.

The meter also powers a separate RCD for outside lights, skt, shed.
So I'm powering fridges & a few backup items off that for now.

Are you sure there is no possibility of for example one of the circuits
from the RCD being connected back to a N wire that returns to the main CU?

What happens if you turn off this secondary RCD, does the first one
still fail to reset?

Q: Does this sound like a faulty RCD - only 7-10yrs old?

Probbably. Could you swap over the RCDs to prove this?

--
Cheers,

John.

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