Okay gotcha.As @westward10 pointed out your usual RCD test would not reliably test-trip a VOELCB but if it dumps enough current in to the CPC system to get raise that to the required potential above the Earth rod then it will trip.
Is the system TT at it appears from the picture of the cut-out? Typically that was the use-case for them but not a very good or reliable system compared to RCD style protection.
Probably best to have RCBO boards, but they could be replaced by 2 * 100mA delay RCDs if those boards and the old time-switch are being kept. You could use a 4P to do the same job the same way, but it is a bit of a mess, with red tail cable used for one of the neutrals as well!
Yes it's a TT system, but a good reading, was around 6.5ohms iirc, though it's a very old installation as you can see, so the rod has well and truly bedded in.
There isn't an option to carry out any further work at present as the chap is elderly, and dare I say, lives in squalor. It was a very grim visit and felt the need to run home to shower afterwards.
Nice chap though, just very old school and set in his ways I think.
I replaced the ELCB with an RCD, then actually found further issues including a faulty kitchen ring circuit.
The whole house was an accident waiting to happen.
He'd had a water leak from the bathroom pipes, soaked the downstairs ceiling causing the lights on the ground floor to fail, and the previous electrician just left it to dry out but still powered up and not working...
That was 2 years ago, lights still don't work, I suspect a break somewhere.
I digress, I really hope to never get a call back there.