This is an abstract question - but I'm just interested to see what I've misunderstood here...
Imagine a domestic installation, with a PSC of 6kA at the incoming supply, fused at 100A. Right next to the incoming supply, there's a consumer unit, which contains one 20A MCB. The CU is connected to the incoming supplies with very short 25mmsq tails (I've ignored their contribution to lowering the fault level.)
This 20A MCB supplies a short (let's say 0.5m) piece of 2.5mmsq T&E, which supplies a single socket next to the consumer unit.
My calculations suggest that the L-N fault current at the socket is going to be about 5kA, and for adiabatic I^2t or (kS)^2 compliance, we would need to break a short circuit in around 3ms max to avoid damaging the 2.5mm cable.
This seems to be well off the bottom of graphs for MCB response times. All examples I can find around the place of using the adiabatic temperature rise equations give answers which are conveniently above 0.1ms, which allows slightly glib commentary about MCBs always tripping in under 100ms, etc.
Although this was a completely contrived example, it actually applies to any S/C fault on any final circuit which occurs sufficiently close to the CU.
Something like this:
contains an example adiabatic compliance calculation, but it appears to me that it only considers a short occurring at the end of the final circuit, not close to the beginning. (It's also considering L-PE rather than L-N, but I don't think that's an important distinction here)
I'm clear why *load* is considered only at the end of a circuit, but not why fault conditions are being calculated for the end of the circuit.
Given that fuse and MCB time vs. current graphs don't tend to go down to single-millisecond levels, should one actually be looking at I2t let-through graphs and comparing that with k2S2?
Will (hoping that Andy and Andrew aren't both on holiday!)