Overcurrent fault on socket ring

[SimonH2]

Could it be that you've got some aluminium/carbon fibre grinding dust inside the socket - and that's triggered a flashover ?

 

[AndrewDavie]

No one suggested that the breaker is faulty and is being too sensitive?
I'd try swapping it out for a new one and see if that helps - they aren't exactly expensive.

 

[endecotp]

No one suggested that the breaker is faulty and is being too sensitive?

There are two separate MCBs that both trip.
And it doesn't trip if some parts of the wiring are disconnected.
So it's pretty unlikely to be faulty MCBs.

 

[SimonH2]

I'm going to pull the wiring out of the trunking and inspect it to those sockets tomorrow, I can't imagine how the wiring is damaged such that L&N are shorting without the earth getting some current (to trip the RCD), but I suppose that must be what's happening...

Well they could short to each other, at least if singles, and both L&N were damaged at the same place - but I'd find it highly unlikely that you'd get the level of damage to cause an intermittent problem without causing a permanent one. . If T&E then it's unlikely.
My money is on the sockets themselves. If you've been grinding aluminium and carbon fibre, then you'll have had conductive dust around - and it won't take much of that to cause a flashover inside a socket. When it's happened once, it tends to leave a conductive film on the insulators, so it's more likely to happen again. Having a plug inserted means that more insulator is involved - so if you unplug (or just switch off the socket) stuff that's not in use that could explain why it apparently only happens when an appliance is in use

As an aside, in the IRAQ operations, the allies used carbon fibre loaded air-burst weapons to take out electrical systems. Basically, air-burst a load of conductive strands over an outside substation, it causes a flashover and trips everything out. It was claimed that this was non-damaging and the power could be "just turned back on" afterwards - but I recall reading or seeing on telly that it didn't work out that way. The flashover's caused a film of carbon to deposit on the insulators, sufficiently tenacious to prevent it being easily cleaned off - and thus requiring replacement of all the insulators.