Right guys - it's all sorted. In spite of working happily for the last 6 years according to the electrician it's all wrong. The incoming neutral should be connected to the earth bus, and a short jumper from there to the RCB. The 65A 33ma RCB is the wrong thing for a main switch he told my wife and showed her a different breaker we should use. They then toddled off across the street and pulled the wires out of the meter on the post.

 

And would you believe that according to Sod's law, when one finds a fault it will mysteriously vanish when you try and demonstrate it - did the dangly bit and  it stubbornly refused to trip! However, the pulled everything off the board and rigorously checked things, found a chewed wire and taped it (for the last few years we've had rats getting in the ceiling void in the rainy season and we trap them regularly). Also had a snake before - gassed that out with an entire can of Chaindrite, house stank for weeks! Right now we are a hotel for Tokai geckos that don't do anything apart from being noisy at times and we never see a cockroach any more so they can stay.

 

Right - on subject again - fitted another new main breaker of a different type as ours no good, they never found the reason. Connected incoming neutral to our earthing system (cross-bonded to multiple deep earth rods around the house) and a jumper from earth to neutral pole on the breaker. Checked everything out and all works perfectly. Total cost including the new breaker 5800 baht.

 

So tomorrow back to Thai Watsadu to return the 2995 baht unwanted and unused breaker for credit for probably a new fan or two and some white paint for the new ceilings. Sorry I am unable to explain why it all went wrong after so many years but I got a feeling that there is a problem with the neutral outside - the Electric Board are changing poles and running both 3-phase and single phase lines all over the village so quite possibly they have introduced a fault.

You have a photo of what he installed?

 

Is it an RCBO?