I've recently rewired my house, so when the time-delayed RCD, which
acts as my main switch started tripping, I assumed I had made a stupid
mistake somewhere. My installation is TT, all circuits (apart from
distribution circuits) are on RCBOs.

I noticed that the tripping occurred at about 8am or about 8pm on
weekdays. At weekends it occurred at other times. This lead me to
suspect an outside cause.

To cut a long story short, my next door neighbour is in the habit of
switching her electric shower on/off via the isolation pull switch.
When she does this, my RCD trips. When she turns the shower on/off
normally, it doesn't trip.

Her shower circuit is 2.5mm cable and the insulation looks a bit
perished and frayed in the pull-switch. It's also on a 40A breaker!
The pull switch is in "zone 1" - I kid you not! The cord is reachable
from outside the shower enclosure and quite convenient to use.

Her house appears to be protected by RCDs - a fairly ancient type, but
they don't trip.

She also promises to control her shower in the normal way until we get
to the bottom of this...

Any ideas?

T