Most of the discussion in this thread so far is actually covered in Section 8 of the IET Electrical Installation Design Guide - the current 2022 Edition even covers the issue of why you can't (and shouldn't ) rely on the RCD element of RCBOs even in TT systems.

Does this mean that a circuit must comply with maximum ZS on the mcb portion of an rcbo? How would that work in a TT ?

Does this mean that a circuit must comply with maximum ZS on the mcb portion of an rcbo? How would that work in a TT ?

That's how I read the text from the Guidance Note, but not how i read BS 7671. 

Newfutile said:

Does this mean that a circuit must comply with maximum ZS on the mcb portion of an rcbo?

Please don't confuse ADS (that is, meeting the Zs value for mcb) with Adiabatic for overcurrent.

So, no, Zs for mcb as per Chapter 41 does NOT need to be met.

Newfutile said:

How would that work in a TT

Guidance Note 6 (Section 6.4.3 in 2022 version) and the IET Electrical Installation Design Guide (Section 8.11 in the 2022 version) recommend that the maximum prospective fault current L-N is taken into account for earth faults in TT systems (in most cases - there may be instances this is not necessary) and that is used to calculate adiabatic using the OCPD disconnection time (which it is accepted may be far longer than the RCD).