Maximum Earth Fault Loop Impedance Calculation

The calculation of max Zs is made in order to verify that the protective device functions within time limits during earth fault.

The maximum impedance during earth fault, max Zs, is calculated from the time-current curve of the protective device.

After the user has defined the protective device’s operating time limits during earth fault, the software finds from the time-current curve of the protective device the corresponding current ImaxZs, and calculates max Zs, as below:

where:

Uo	The nominal ac rms line voltage to Earth in volts (V) ,
Cmin	The minimum voltage factor to take account of voltage variations. Cmin = 0.95
	The current causing the operation of the protective device at the maximum time threshold, calculated from the time-current curve of the device.

Maximum Earth Fault Loop Impedance Check

with:

where

	the external impedance,
	the impedance of the phase conductor,
	the impedance of the protective conductor.

Note: For TT Systems Zs includes also the impedance of the earthing electrode.