Ze & Ipscc

Sounds to me like UKPN have just quoted their usual worst-case values - which allows them an amount of leeway for future network changes. 0.15 Ohms sounds like 300A or more supply.


It's the same if you asked about a domestic single-phase PME supply you'd usually get the answer 16kA and 0.35 Ohms, which clearly doesn't stack up since the L-N and L-PE loop are identical (L-PEN) - what they're really saying is that the loop impedance could be anything between about 0.014 Ohms (16kA) (worst case for breaking capacity) and 0.35 Ohms (about 0.6kA) (worst case for ADS) and ideally your design could cope with anything between the two because you might wake up one morning to find the the DNO have changed things.


   - Andy.

As Andy has said I also think that UKPN has given you worst case generic data.


If that is realey the size and rating of the transformer then you can calculate PFC and Ze.

Reads as if they have not connected it yet, so the figures are a bit of a  PIDOOMA   (*   rather rude,  definition here) and the only fairly sure spec is the transformer will probably droop about 5,5% on full load. The rest is better treated as those are the test limits that if it is worse than that they will come and fix it.

Thank you for your responses.


Even if these are worst case scenario I would expect a lower value for the Ze. Ze @ 0.15 would give me around 1.5kA fault which is less than the flc of around 2165A??


John, thats what confused me. With 5.5% Z on the Tx I would expect the Ipscc to be around 39kA and the Ze around 0.007.

Yes - for a TNC-s connection, the Zs will be more like Vo/PSSC and  with a 5% transformer the PSSC  for a single phase fault could be 20 times the FLC (maybe twice that for a phase-phase fault of the worst kind.

(So in big hand fulls of no great precision

FLC = load current per phase of 2 to  2.5kA,

single phase PSSC of maybe 40kA  (20 times 2kA )

and a  Zs of half a dozen milli-ohms  (240V / that 40kA )

at the transformer, and resistance rising, current falling as you move away down the cable.)


I suggest you do check the  design for your own figures as well as theirs ?, yours are more credible. If Ze is TNS  and the earth so thin, that resistance is that high you may need an earth fault relay or similar as part of the design.