Why earth fault loop impedance test is necessary

Earth fault loop impedance is the path followed by fault current when a low impedance fault occurs between the phase conductor and earth, i.e. “earth fault loop”. Fault current is driven round the loop by the supply voltage. The higher the impedance, the lower the fault current will be and the longer it will take for the circuit protection to operate.
To make sure the protection operates fast enough, the loop impedance must be low. Every circuit must be tested to make sure that the actual loop impedance does not exceed that specified for the protective device concerned. It is recommended that the Ze test be done first. This test, done at the distribution board, gives the loop impedance of the circuit, excluding the installation. The Zs test, which includes the circuit tested in the Ze test as well as including the installation resistance, must be done next.

Disclaimer: As obtained from the Internet