The assumption is as per Grahams explanation above, and if it helps you with the visualization of why, consider that  for any fault current down any one line is set by the voltage at the driving end of the line - 230V to ground 400V to any other phase- and the difference between that and whatever the voltage is at the fault end.

In this case the fault end voltage is the point of 3 wire junction - which, if all 3 fault currents are equal is exactly zero. So the full 230V appears along each line.

In contrast in the single phase fault, the fault end voltage rises by the drop in the return path - and usually L and N are assumed the same cross-section so half the voltage (115V) on the line and the other half on the way back,

Handily then having done a single phase PSSC test, (with 115V along the line) and double it and say 'calculated 3 wire fault current'

But be aware that all this assumes all 3 phases lines and the neutral return path, are all the same impedance, are the same. That is not always quite true but is close enough for most purposes.

And for fun, the fault current between any 2 phases puts the fault point midway along the phase to phase voltage, so 200V along each line, and a fault so the current  is  200 / 115 = 1.7 or so of the single phase test result.

Mike.

The assumption is as per Grahams explanation above, and if it helps you with the visualization of why, consider that  for any fault current down any one line is set by the voltage at the driving end of the line - 230V to ground 400V to any other phase- and the difference between that and whatever the voltage is at the fault end.

In this case the fault end voltage is the point of 3 wire junction - which, if all 3 fault currents are equal is exactly zero. So the full 230V appears along each line.

In contrast in the single phase fault, the fault end voltage rises by the drop in the return path - and usually L and N are assumed the same cross-section so half the voltage (115V) on the line and the other half on the way back,

Handily then having done a single phase PSSC test, (with 115V along the line) and double it and say 'calculated 3 wire fault current'

But be aware that all this assumes all 3 phases lines and the neutral return path, are all the same impedance, are the same. That is not always quite true but is close enough for most purposes.

And for fun, the fault current between any 2 phases puts the fault point midway along the phase to phase voltage, so 200V along each line, and a fault so the current  is  200 / 115 = 1.7 or so of the single phase test result.

Mike.