Cable size of tails supplying a CU

Whilst I think you have a point, mapj1, it does, of course, depends how "temporary" temporary supplies actually are. Less of a risk for individual electrical work of extremely short duration, and where combustible materials, dust etc, are not permitted to accumulate around the reduced csa conductors... and so on and so on.

Not something I'd recommend on a site with other trades working, and larger companies (bigger risk exposure at least on the numbers game vs increases in fines etc.) might see things very differently also.

I wondered whether Graham or anybody else would like to comment please?

Happy to comment ;-0 Folk can then point out the erroneous bits and we can converge on a more accurate description of what is going on.. If the loop impedance is increased, the fuse takes longer to blow and the 'really fast, adiabatic, constant energy I2t = constant' assumption breaks down. Actually, in extremis by time the PSSC is less than about  1,5 times the fuse rating, the heat arriving and leaving balance and the fuse wire reaches an equilibrium after several minutes but at temperature that may well be below its melting point of nearly 1000C, and that current flows forever, and the element within the fuse never blows - you just get a hot fuse and holder.

A similar thing happens to the calculation for wire damage, wire - but, and this may be important, because the permitted temperature rise is much less - we are browning off and embrittling the insulation, usually setting a danger limit , between 160 and 220C, nothing like  approaching the melting point of copper, the shape of the safe area of the I/T curve has a different aspect ratio. Also for 4mm copper the fuse highest no blow current exceeds the cable damage level. So, at low PSSC the cable is at risk, but at high ones it is not.

I agree it feels odd. If the cable is  at risk, (and if you care), also depends on other factors, the big one being the average load prior to the fault, which may pre heat the cable or not, and if damage is likely to go un-noticed.

Of course real faults are pretty much never  zero resistance - if they were there would be no heat, light or sound - there usually is so the fault has some voltage drop, but we will leave that dog asleep I think.

Mike.

Or to put it another way, as the fault current increases not only does the opening of the fuse get faster, the speed of opening accelerates such that I²t actually declines. Which is why all the old text books only considered max Zs as being the worst case (not true with MCBs now of course, where you might have to consider faults near the start of the circuit as giving the highest I²t and at the end of the circuit for the worst disconnection time).

    - Andy.

Chris Pearson said:

I wondered whether Graham or anybody else would like to comment please?

Apologies Chris Pearson  I missed this as it was embedded in a thread.

Yes, I think you've got it.

It does look counter-intuitive, but it's because the fuse acts a lot faster with more current - the fuse heats up a lot faster than the cable really.

The best way to look at this is on the line plot, so the [non-adiabatic] energy factor the cable can absorb is k²S², and what is plotted on the time-current curves is t=k²S²/I². Luckily, we don't need to actually calculate for smaller installations up to 100 A single-phase, as there are Tables in the OSG that already have the information.

With circuit-breakers, the same logic may not always hold, particularly in the 'instantaneous' (<0.1 s) region that we are normally operating in for BS 7671 disconnection times - and large current faults L-N. Hence, BS 7671 telling you to use the value of I²t (let-through energy) quoted by the manufacturer (or the standard) for circuit-breakers ... and so for circuit-breakers, we don't consider the actual fault currents involved at all, but calculate S_min=√([I²t]/k), where [I²t] is the let-through energy quoted by the manufacturer (or product standard).

gkenyon said:

Yes, I think you've got it.

Graham, thank you - that is reassuring. :-)

The above discussion almost but not quite addresses my own question. What is the minimum CSA I need if a sub-main of length < 1m is to be protected by my existing cut-out fuse - I assume to BS88 - and confirmed by the DNO to be rated at 80A?

Looking at the gG tables above, the 5s disconnection time for this fuse is achieved with a current of 430A. Presumably that is worst case, various manufacturer's data gives other results e.g. mersen give 300A.

The graphical method, looking at where the adiabatic curves cross the fuse characteristics, then shows for this "time to blow" I need somewhat under 10 sq mm conductors. Is that correct?

Also, can someone please explain where the figure for Zs of 0.27 came from in the original calcs above? The tables I have found e.g. these https://www.dungannonelectrical.co.uk/dun1-shop/pdf/loop-impedance.pdf show for 5 seconds a value of 0.425 ohms for the original 100A fuse and 0.525 ohms for my 80A case leading to rather different results. And why is a figure of 1 second then chosen (seemingly arbitrarily) to put in the adiabatic equation?

Hoping someone can enlighten me, I am more at home with electronics!

I have this table for maximum measured values (80%) of maximum permitted.BS 1361 or 1362, 100 Amp, 5 seconds 0.27 Ohms

Thanks, I wonder why it is not the same.

Meanwhile I may have found the answer to my original Q. My copy of the 17th Ed OSG has in Table 2B on p101 the smallest protective conductor allowed off an 80A fuse to BS88 is 4.0 sq mm, so the smallest permitted T&E cable is 6.0.

Also the max allowable Zs is 0.22 ohms which I think is OK. By observation the voltage dip on a 50A load is <2.5V which is about 50 mohm, and the length of cable involved is negligible. (This squares with the supply cable which is about 20m of 35 sq mm from the street, so 25 mohm according to Table 6E2.)

220 mohm worst-case would give a PSCC over 1000A so a time to blow of 0.2 secs, and the graphical method shows this is actually OK for 4.0 sq mm cable as well as 6.0. And the real PSCC looks as though it might be >4x this w/c figure so off the graph.

Now to get rid of the column of "helpful" IET drivel which has inserted itself to the right of the discussion screen!

Hi Chris, I’ve never come across a 6mm T&E cable with a 4mm cpc as a standard cable. 6mm T&E cable has a 2.5mm cpc, and a 10mm T&E cable has a 4mm cpc.