In article , dorothy
wrote:

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

[Snip]

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

Is it definitely a (current operated) RCD, or perhaps an old style
voltage operated ELCB (earth leakage circuit breaker)? If it's a voltage
operated ELCB it would trip if the earth voltage rose above a certain
level.

--
Mike Clarke