mapj1:

You may on a good day get 0.04 ohms at the origin to a block of flats or an industrial unit.

You probably will  not in most cases. A house is likely to be at the end of 10s of metres of something that is 35mmsq aluminium or copper, or smaller, fed by a fatter street main that may be 95 or 185mmsq but is also probably quite a bit longer, perhaps from a transformer with a 5% droop at 500KVA load. (750A per phase, ish, so  ~ 20 time that PSSC of 16kA  at TX ~ 15 milliohms  = 0.015 ohms before you leave the TX)

A fairly short single phase line of 18m or so of 35mm2 will add 20 milliohms straight away (0.02 ohms) that is without any allowance for the main cable in the street or other effects included. (your BS1361 service fuse will only add a very small resistance  - the makers claim it dissipates 5watts at full load so a 100A one is half a milliohm, and a 63 A one is more like 1.2 milliohms - not affecting the PSSC that much  and can probably be nelected.)

I realise I have skipped lightly over the effect of reactive and resistive impedances, but my point is that on a UK housing estate, its not that uncommon to see a PSSC around or under 1kA, so more than 1/4 of an ohm at the origin.


Note that the resistance of the substation transformer CPC to true earth, the transformer LV electrode resistance could be as high as 20 ohms, though in a built up area it is likely a lot lower, but for a 'pole pig'  serving  a group of houses in a small village, it would not be that surprising and those are more likely to be TT.

Mike.

Well...if I may... ?

https://www.alabamapower.com/content/dam/alabamapower/Business/Services%20by%20Industry/Architects%20&%20Engineers/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf






You are correct for a mean average, as those nearest to the trafo will have the very high fault current with the value sharply falling to a valley as distance progresses. However code must take into account worse case conditions likely to be found in the real world such as a 50kva pole pig feeding a home 8 meters away via overhead drop or a high rise with a 1000kva unit directly below in the basement. And of course you're got London's meshed networks. In these cases the bulk of the drop will be along R1+R2.


 



 

mapj1:

You may on a good day get 0.04 ohms at the origin to a block of flats or an industrial unit.

You probably will  not in most cases. A house is likely to be at the end of 10s of metres of something that is 35mmsq aluminium or copper, or smaller, fed by a fatter street main that may be 95 or 185mmsq but is also probably quite a bit longer, perhaps from a transformer with a 5% droop at 500KVA load. (750A per phase, ish, so  ~ 20 time that PSSC of 16kA  at TX ~ 15 milliohms  = 0.015 ohms before you leave the TX)

A fairly short single phase line of 18m or so of 35mm2 will add 20 milliohms straight away (0.02 ohms) that is without any allowance for the main cable in the street or other effects included. (your BS1361 service fuse will only add a very small resistance  - the makers claim it dissipates 5watts at full load so a 100A one is half a milliohm, and a 63 A one is more like 1.2 milliohms - not affecting the PSSC that much  and can probably be nelected.)

I realise I have skipped lightly over the effect of reactive and resistive impedances, but my point is that on a UK housing estate, its not that uncommon to see a PSSC around or under 1kA, so more than 1/4 of an ohm at the origin.


Note that the resistance of the substation transformer CPC to true earth, the transformer LV electrode resistance could be as high as 20 ohms, though in a built up area it is likely a lot lower, but for a 'pole pig'  serving  a group of houses in a small village, it would not be that surprising and those are more likely to be TT.

Mike.

Well...if I may... ?

https://www.alabamapower.com/content/dam/alabamapower/Business/Services%20by%20Industry/Architects%20&%20Engineers/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf






You are correct for a mean average, as those nearest to the trafo will have the very high fault current with the value sharply falling to a valley as distance progresses. However code must take into account worse case conditions likely to be found in the real world such as a 50kva pole pig feeding a home 8 meters away via overhead drop or a high rise with a 1000kva unit directly below in the basement. And of course you're got London's meshed networks. In these cases the bulk of the drop will be along R1+R2.