Any electricians on the forum

[Oddjob]

I asked when I was buying them Al, not sure if this was the correct answer but in reply to my question about curves and the different letters that they were, he replied these were B curve and that denoted the tripping time, is that correct. As far as I could gather B is ok for domestic use with the higher letters required for commercial use etc.

From what I've read they trip at 3-5 times the current (In), so a 16a would trip at 48-80a, does that sound right?

[BigAl (Alan)]

The 16amp & 20amp RCB & RCBO's with a B rated tripping curve is the most popular option. C & D tripping curves MCB/RCBO's could be required in domestic properties...you may have a fridge/freezer/compressor that is nuisance tripping on a B rated MCB/RCBO. But in all cases the Earth Fault Loop Impedance of the circuit must be confirmed by test or design when choosing B,C,D rated RCD/RCBO's.
When your friend comes around ask him to check the Ze & Zs of the installation.
Image attached of a combined installation tester