This is an abstract question - but I'm just interested to see what I've
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!)