Both rcd,s tripping with faulty light switch

[ebee]

Agreed - The Borrowed Neutral can potentially be dangerous , as in not only Electric Shock risk but also Electrocution risk (Electrical Execution)

 

[ericmark]

Update, this borrowed neutral is a spur from the nearest socket to this wall light, if I got an electrician to move the lighting circuit onto the same rcd as the house sockets, would this solve the issue?

I would hope the electrician would refuse. This has been the problem, until we went over the RCBO's it was easy for a borrowed neutral fault to go undetected for years, one good reason to use neon screwdrivers, although clamp on ammeter is better. My old clamp on only showed increments of 10 mA so 2 watt or less would not show, new one at 1 mA increments is likely to show a borrowed neutral with the lowest of loads. But still the neon screwdriver is more sensitive.

 

[ebee]

Eric has a very good point there.

The "Borrowed N" Scenario always was a potential for hazard and the Advent of seperate RCDs has, to some extent (Unless all RCBOs in which case best ever) has often been largely undetected - different circuits on different RCDs have a far better chance of warning that something is not as it should be.
To some, over many years, this scenario was not by any means uncommon.
I suppose another plus for RCDs especially RCBOs really.