Lmax - Maximum Cable Length equation for TNS Vs IT system

Nick Parker: 
 

Since the equation is to determine the fault current at the end of line (before the downstream circuit breaker); In other words, minimum short circuit current (at the end of line) is used to verify whether the circuit breaker can be tripped for the magnitude of short circuit current at the end of line. 

you're correct … the formula you are quoting ignores the external earth fault loop impedance Ze or Zdb (or that calculated for L-L or L-N fault with the prospective short circuit current), which in effect makes it a volt-drop formula for a voltage drop of 20 % as I said, if you use Ib or In

In BS 7671 for the UK, we calculate the maximum permissible earth fault loop impedance from the worst-case instantaneous tripping current value using a voltage factor of 0.95. From this we subtract the external earth fault loop impedance Ze or Zdb, and then you can calculate the circuit length permissible for operation of the device from that. That's for earth faults.

WE then back-check adiabatic criterion from there.

Nick Parker: 
 

Since the equation is to determine the fault current at the end of line (before the downstream circuit breaker); In other words, minimum short circuit current (at the end of line) is used to verify whether the circuit breaker can be tripped for the magnitude of short circuit current at the end of line. 

you're correct … the formula you are quoting ignores the external earth fault loop impedance Ze or Zdb (or that calculated for L-L or L-N fault with the prospective short circuit current), which in effect makes it a volt-drop formula for a voltage drop of 20 % as I said, if you use Ib or In

In BS 7671 for the UK, we calculate the maximum permissible earth fault loop impedance from the worst-case instantaneous tripping current value using a voltage factor of 0.95. From this we subtract the external earth fault loop impedance Ze or Zdb, and then you can calculate the circuit length permissible for operation of the device from that. That's for earth faults.

WE then back-check adiabatic criterion from there.