Owner manual

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9. LOOP/ PSC/PFC
9.1 Principles of measurement of fault loop impedance and PFC
If an electrical installation is protected by over-current protective devices including circuit
breakers or fuses, the earth loop impedance should be measured.
In the event of a fault the earth fault loop impedance should be low enough (and the
prospective fault current high enough) to allow automatic disconnection of th
e 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. The
KEW6016 takes a current from the supply and measures the difference between the
unloaded and loaded supply voltages.
From this difference it is possible to calculate the
loop resistance.
TT System
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).
The figure below shows (dotted line) the Fault loop impedance for TT systems.
Fig.22