Rcd fault not tripping

[Retroelectric]

Hi
I have only had a quick look at a fault so haven’t gone too far into it.
Lighting mcb tripped out, elderly lady wanted it checked out before it went back on so meggered it, all lights worked but rcd would not trip.
It’s a BG 3+3 split load installed by someone when the heating was done about 8 years ago, no paperwork.
Swapped the second rcd for one I had on board but still would not trip, plugged tester in first rcd and both tripped.
circuits are
Rcd
Cooker
Water heater - 2 twins
Sockets - 4 twins
Rcd
Sockets 4 twins
Garage - swa
Lighting - 4 twins
Is this likely to be a neutral in the wrong bar? No continuity between neutral bars when Rcds are off.
Any ideas where to start first?

 

[freddo]

You can disconnect the outgoing L and N of the RCD and do an RCD test at the terminals if this passes that rules out the RCD. I would expect a neutral in the wrong bar to cause immediate tripping rather than not tripping.

Insulation resistance tests would be where I would start to look for the RCD problem, on all circuits. Don't be mislead by the tripped MCB on the lighting circuit. There is likely another, separate issue causing the RCD not to trip. The MCB could have tripped due to a lamp blowing for example, though you mention all lights are working. How did you carry out the insulation test on the lights? If only the MCB tripped the fault could be between live and neutral conductors.