I know we measure the prospective short circuit current between L1,L2,L3 and N and then double the highest reading

Or, more accurately, establish Ipf between lines and divide by 0.87. Mind you, from my experience, its all fruit machine numbers anyway. As I understand it, the standard MFT instrument simply divides the measured resistance (rather than impedance) into 230. If you are out even by a small margin in the measurement of resistance, the resulting difference in KA can be substantial.

A candidate on the 2391 last week measured 0.33 ohms and 700A on his brand new Megger X, (the new tester that has little electronic diagrams to tell the operator how to conduct each test, heaven help us!) The new 1741 owned by the centre showed 0.26 ohms with an Ipf of nearly 900A. The impedance difference was only 0.07 ohms but the resulting current difference was substantial. The test rig is well into the installation, so you would expect a reasonable degree of accuracy at the higher resistance values than you would at the real intake position with a 1MVA tx in the next field. 

Why should we waste time with prospective earth fault current?

The Ipef should be taken with the earthing system connected so should be greatest at intake. So as you move to a single-phase board downstream and you know the PN and PE KA values at intake, you may choose to set aside duplicate re-testing at the SP board providing the Icn values of the OCPDs are grater that the values obtained at intake..

lyledunn said:

The Ipef should be taken with the earthing system connected so should be greatest at intake. So as you move to a single-phase board downstream and you know the PN and PE KA values at intake, you may choose to set aside duplicate re-testing at the SP board providing the Icn values of the OCPDs are grater that the values obtained at intake

Important to note, though, that unlike Ze, in most installations you can't calculate the prospective earth fault current at a distribution board in the same way, because of parallel earth paths.

Any calculation would have to ignore R2 (or Z2 if CSA of cpc > 16 sq mm, or SWA cables are used) to give you "worst-case conditions".

The resulting calculation will give you a reading higher than the actual prospective earth fault current ... sometimes much higher.

The alternative is measurement, but that's not accurate either !

lyledunn said:

The Ipef should be taken with the earthing system connected so should be greatest at intake. So as you move to a single-phase board downstream and you know the PN and PE KA values at intake, you may choose to set aside duplicate re-testing at the SP board providing the Icn values of the OCPDs are grater that the values obtained at intake

Important to note, though, that unlike Ze, in most installations you can't calculate the prospective earth fault current at a distribution board in the same way, because of parallel earth paths.

Any calculation would have to ignore R2 (or Z2 if CSA of cpc > 16 sq mm, or SWA cables are used) to give you "worst-case conditions".

The resulting calculation will give you a reading higher than the actual prospective earth fault current ... sometimes much higher.

The alternative is measurement, but that's not accurate either !