To give more context.

During my training my instructor said that houses with sockets wired using 2.5/1.0 T+E would probably fail on the adiabatic equation and need rewiring. In my opinion if the circuits are RCBO protected, due to the quick switching time of the RCD element they are ok. I have yet to do some calculations but wanted to explore the general principles first.

Fault current discussed earlier is flowing through live and neutral, size of which is checked against appendix 4 and voltage drop calculations, therefore not a major concern. I wouldn't condone designing new circuits that fail on the standard adiabatic equation calculation method but don't believe a rewire with 2.5/1.0 T+E in good condition is justified.

well, even if it fails the adiabatic analysis, one may as well wait until the insulation has actually been cooked by a short circuit fault, before re-wiring - the effort is the same, and it may allow you to keep the installation running for several years longer, maybe pretty much for ever. You will be far enough away from melting the copper that there is no risk of that, just that the insulation around the CPC will be taken over the recommended temp during a 'silver spanner' short circuit.

Typical figures (Hagar)

The let-though of an MCB varies with available PSSC, as unlike a fuse it does not get faster and faster with increasing fault current but we can say something fairly intelligent.
Mike.

IIRC, my tutor said that we needn't worry about T&E because somebody cleverer than us had done the calculations, although I do take the point about 2.5/1.0.

I think that you only really need to worry about it for substantial circuits in singles.

Let's not forget that a particular device may go faster than 0.1 s and that in a domestic installation, PFC may be relatively modest. In any event, if the most likely place for a fault is at the plug or the appliance's flex, the actual fault current will be smaller still.

Alan B said:

Fault current discussed earlier is flowing through live and neutral

But also in a fault to an exposed-conductive-part, the cpc.

The issue with 1.0 sq mm is likely the use of circuit-breakers instead of fuses (and one potential issue with "upgrading to modern devices").

Alan B said:

In my opinion if the circuits are RCBO protected, due to the quick switching time of the RCD element they are ok.

But for higher fault current, the magnetic trip of the mcb part of the RCBO operates in < 0.01 s (in fact, < 0.001 s is also not unheard of) - far quicker than the RCD 0.04 s. Same, though, happens with cartridge fuses and RCDs as I've seen many times - BS 88-3 fuse blown on earth fault, RCD still "closed".

Alan B said:

In my opinion if the circuits are RCBO protected, due to the quick switching time of the RCD element they are ok.

But for higher fault current, the magnetic trip of the mcb part of the RCBO operates in < 0.01 s (in fact, < 0.001 s is also not unheard of) - far quicker than the RCD 0.04 s. Same, though, happens with cartridge fuses and RCDs as I've seen many times - BS 88-3 fuse blown on earth fault, RCD still "closed".