KYORITSU 6016 - Manual (Page 29)

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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 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. 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