measurement is better than calculation lol
Not always!
Consider the situation where supply transformer is close to the domestic installation intake and Ze is likely very low. The best BS EN 61557 EFLI testers are typically +-5% accurate +-3 least significant digits (lsd), so when Ze (or Zs) is less than say 0.1Ω the least significant digits become the increasingly significant error term as Z reduces.
Few, if any, EFLI testers actually measure the fault current, they in fact use the ‘measured impedance' as the divisor in a fault current calculation in which the measured voltage (or 230V nominal voltage, depending on the design of the tester) is the dividend and then it just displays the result in A or kA as appropriate. So it is easy to see that if the divisor is a very small number with a significant error, this can result in a very large current with a significant error.
To illustrate the point:
So if the actual Ze =0.05
Ω,TN-C-S
, then the PFC =230V/0.05
Ω =4.6kA
However, our ELFI tester with a resolution of 0.01Ω, may be indicating a wildly different reading…
At increasing low ‘impedance’ values the +-5% tolerance becomes less important to the point where it becomes a second order term and we can effectively ignore it, however the +-3 lsd becomes increasingly dominant.
So, if the EFLI tester reads -3 lsd, Ze becomes approximately 0.05 – 0.03 Ω =0.02Ω, therefore PFC would read 230V/0.02
Ω =11.5kA
So, if the EFLI tester reads +3 lsd, Ze becomes approximately 0.05 + 0.03 Ω =0.08Ω, therefore PFC would read 230V/0.08
Ω =2.88kA
… do you still have blind faith in your beguiling digital EFFI tester, I hope not.
This is before we further complicate the issue with large CSA line conductors and their significant reactive components and the very low impedances we might expect in some commercial and most industrial installations, where the testing methods often have to be significantly different and more complex.