TESTING AN RCBO AT A LIGHT SWITCH

What sort of fault are you testing for? - if it is to be sure the light switch is earthed, then that is a a sensible thing to verify, especially if you are working on the lihgting circuit in that room, or there is a problem with it.

However, so long as there is CPC contiuity, and the RCD trips at some point, I'd not too get hung up  on a specific accurate answer - for example if the RCD still trips at In/2, well that may just mean there is already a lot of leakage present, quite possible with a large office full of electronically ballasted lamps, and the tester pushes it over the top.

Equally a large capacitive load may make the power seem to hold up longer than expected after tripping, so it may fail the 40ms test and appear  a few tens of msec slow.

To verify the RCD functions perfectly  on its own yes it needs to be isolated from such effects, but unless you have good reason to suspect it of being faulty, this is is just a lot of extra aggro for  something that may introduce more new problems than it solves.

Of course, if the RCD never trips, even at X2 or X5 or if there is no CPC to the fitting at all, then you have found something that will need looking at, and as part of that an RCD test is then worth it.

What sort of fault are you testing for? - if it is to be sure the light switch is earthed, then that is a a sensible thing to verify, especially if you are working on the lihgting circuit in that room, or there is a problem with it.

However, so long as there is CPC contiuity, and the RCD trips at some point, I'd not too get hung up  on a specific accurate answer - for example if the RCD still trips at In/2, well that may just mean there is already a lot of leakage present, quite possible with a large office full of electronically ballasted lamps, and the tester pushes it over the top.

Equally a large capacitive load may make the power seem to hold up longer than expected after tripping, so it may fail the 40ms test and appear  a few tens of msec slow.

To verify the RCD functions perfectly  on its own yes it needs to be isolated from such effects, but unless you have good reason to suspect it of being faulty, this is is just a lot of extra aggro for  something that may introduce more new problems than it solves.

Of course, if the RCD never trips, even at X2 or X5 or if there is no CPC to the fitting at all, then you have found something that will need looking at, and as part of that an RCD test is then worth it.