4
allow automatic disconnection of the electrical supply by the
circuit protection device within a prescribed time interval.
Every circuit must be tested to ensure that the earth fault loop
impedance value does not exceed that specified or appropriate
for the over-current protective device installed in the circuit.
For a TT system the earth fault loop impedance is the sum of
the following impedances;
Impedance of the power transformer secondary winding.
Impedance of the phase conductor resistance from the
power transformer to the location of the fault.
The impedance of the protective conductor from the fault
location to the earth system.
Resistance of the local earth system (R).
Resistance of the power transformer earth system (Ro).
For TN systems the earth fault loop impedance is the sum of
the following impedances.
Impedance of the power transformer secondary winding.
Impedance of the phase conductor from the power
transformer to the location of the fault.
Impedance of the protective conductor from the fault
location to the power transformer.
