Will Dean wrote:
This is an abstract question - but I'm just interested to see what I've
misunderstood here...
Hi Will, sorry to come in a bit late to this. Hope you're still around...
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.
All figures agreed.
This seems to be well off the bottom of graphs for MCB response times.
[...]
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?
Yes, you raise some interesting points that a designer does need to be
aware of when the prospective fault level is high.
Firstly, the reason that the fault calculation is usually done at the
furthest point of the circuit is that that is /usually/ the worst case.
When we're not at the bottom of the graph, as you put it, a lower
fault current leads to higher I^2*t let-through and hence more risk of
cable damage.
There's a useful graphical approach he for any given cable CSA and
'k' value (k=115 for T&&E) you can superimpose an adiabatic withstand
line for the conductor on the operating characteristic plot for the fuse
or MCB. It's a log-log graph, so the cable's line is straight, with a
gradient of -2 (t proportional to 1/I^2). You then see at a glance the
minimum fault current at which the fuse or MCB will definitely protect
the cable, i.e. the point of intersection of the two lines. With some
(non-current-limiting) MCBs there may be a second intersection at high
fault level. At higher currents the device's curve is now above the
cable's line - meaning that the cable will not be protected.
So, yes, you do need to refer to manufacturer's I^2*t data, which will
almost certainly show that your bit of 2.5mm^2 wire is protected after
all. Most MCBs (and HRC fuses) now are 'current-limiting' - meaning
that at high current they will trip and quench the arc so quickly that
the instantaneous current never has time to build up to its full
prospective value. (Even if the fault occurs at the peak of the voltage
waveform the inevitable inductance in the circuit means that the current
has to rise from zero at a finite rate.)
Secondly, you might find that at 5kA the supplier's main fuse beats the
MCB - look at the characteristics for a 100A BS 1361 Type 2 fuse here
http://www.bussmann.co.uk/images/Dat...S1361/LR85.pdf and
extrapolate the line a little. Or use the I^2*t value given (57,300
A^2s) and conclude that this 100A fuse would protect a 2.5 mm^2 copper
conductor. This becomes an important point once the PSSC exceeds the
rated breaking capacity of the MCB - which is usually 6 kA or 9 kA (M6
or M9) - as the supplier's fuse is now acting as the backup protective
device.
Thirdly BS 7671 allows short lengths of conductor between a phase busbar
and the input side of a fuse/MCB to be excused from fault protection
provided that it's (a) 3 m long, (b) "erected in such a manner as to
reduce to a minimum the risk of fault current" and (c) "erected in such
a manner as to reduce to a minimum the risk of fire or danger to
persons". [Reg. 473-02-02]
OOI the highest PSSC I've encountered on a house installation is about
3.3 kA (Ze = 0.07 ohm). This is a property quite near the footway
(short service cable) with the substation a stone's throw down the road.
HTH
--
Andy