Zs and Ipfc

To my understanding, there's no way I can derive the voltage drop up to that DB from Zs/ Ipfc because I'll need the earth-fault current  or phase-phase impedance. Am I correct?

Ohm's Law relates voltage, impedance and current - so given one (impedance or current) and a decent assumption about the second (voltage) you should always be able to calculate the 3rd (current or impedance). Always take care to make sure you understand which ‘loop’ in involved and so which conductors the impedance relates to. For a 3-phase system the max PFC almost certainly relates to a bolted 3-phase (L-L-L) fault - so that in effect gives you the impedance of one of the line conductors (with Uo across it).

You might have to make some assumptions - such as the impedance of the N conductors is similar to that of the L ones - which may be reasonable for a modern installation, but could be out  for a much older one where reduced neutrals were common. But still you should be able to get some numbers that should be useful enough to suggest whether you're in the right general area or not.

That's not the whole story of course - when those values generally relate to the entire loop, and when it comes to voltage-drop you're usually only concerned with the part within your installation, and not about the part of the loop within the DNO's cables. If you're on a large site with its own HV transformer then it probably makes next to no difference, but even on a small domestic you should still be able to work out some ‘worst case’ values - and if they're sufficient to show your design is OK, then happy days.

Of course, most common loop impedance meters have less than perfect accuracy anyway - many can give readings several tens of percent different from a true value and still be within calibration, especially on relatively low impedance loops, so such readings need to be taken with large pinch of salt.

   - Andy.

To my understanding, there's no way I can derive the voltage drop up to that DB from Zs/ Ipfc because I'll need the earth-fault current  or phase-phase impedance. Am I correct?

Ohm's Law relates voltage, impedance and current - so given one (impedance or current) and a decent assumption about the second (voltage) you should always be able to calculate the 3rd (current or impedance). Always take care to make sure you understand which ‘loop’ in involved and so which conductors the impedance relates to. For a 3-phase system the max PFC almost certainly relates to a bolted 3-phase (L-L-L) fault - so that in effect gives you the impedance of one of the line conductors (with Uo across it).

You might have to make some assumptions - such as the impedance of the N conductors is similar to that of the L ones - which may be reasonable for a modern installation, but could be out  for a much older one where reduced neutrals were common. But still you should be able to get some numbers that should be useful enough to suggest whether you're in the right general area or not.

That's not the whole story of course - when those values generally relate to the entire loop, and when it comes to voltage-drop you're usually only concerned with the part within your installation, and not about the part of the loop within the DNO's cables. If you're on a large site with its own HV transformer then it probably makes next to no difference, but even on a small domestic you should still be able to work out some ‘worst case’ values - and if they're sufficient to show your design is OK, then happy days.

Of course, most common loop impedance meters have less than perfect accuracy anyway - many can give readings several tens of percent different from a true value and still be within calibration, especially on relatively low impedance loops, so such readings need to be taken with large pinch of salt.

   - Andy.