The idea of the magnetic part of the trip is to disconnect very fast 0.01 seconds I think is memory serves me correct, under fault conditions, with a RCD this will disconnect within 0.03 seconds with a line - earth fault, so in real terms looking at line - neutral faults.
Although line - neutral could be measured in impedance Ω, it is normal to quote as prospective short circuit current A, and a B = 3 to 5 times thermal tripping current, C = 5 to 10 times thermal tripping current and D = 10 to 20 times thermal tripping current. So a C16 has the same tripping current as a B32 MCB. And the PSCC will need to be in excess of 160 amp, not hard to satisfy, but with a C32 looking at 320 amp this gets harder to satisfy.
It really does not matter if TN or TT supply, we want a supply with under the PSCC rated on the MCB's at the MCB's and over the PSCC that the MCB will trip at measured at the end of the circuit. So less than 4.7kA at consumer unit and more that 400A at end of circuit.
I have found some makes of reduced low voltage transformers do have a high in rush, with the same MCB plugged in close to consumer unit they will trip supply, but at centre of ring final or end of radial they will hold, the resistance of the supply cable is enough to reduce inrush current. So normally if they trip the supply then the supply has a high enough PSCC to allow the use of a C or D type. However we still need to measure, and a impedance or PSCC meter is not cheap, also they seem to be the first meter in the set to go out of calibration, I have not had a low impedance ohm meter go far out, or an insulation tester, but have had 3 impedance meters go well out of calibration, not talking about failing, had many meters fail, but talking about calibration.
I had a socket at home I knew the earth loop impedance was 0.21Ω and at start and end of a day where I would be using the impedance meter I would test it in that socket, better than finding out 10 inspections down the line, then one day it altered it was 0.28Ω, my son also did same as me, so went up to his house and checked it, one on his socket and two compared to his meter. Nothing wrong with meter, on retuning to my house noticed the road works, what was being done is anyone's guess, however the impedance is only guaranteed to be better than 0.35Ω with TN-C-S with a 100A supply, it is in real terms the point where they would have problems with volt drop. 230/0.35 = 660 amps approx, a 45A C type 450 amp so could in theory use one. However get it wrong and the supply fuse may pop before the MCB.