KYORITSU 4140 - Manual (Page 14)

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7. LOOP/ PSC/PFC Test
7.1 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 KEW4140 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;
●Impedanceofthepowertransformersecondarywinding.
●Impedance of the phase conductor resistance from the power
transformer to the location of the fault.
●Theimpedanceoftheprotectiveconductorfromthefaultlocation
to the earth system.
●Resistanceofthelocalearthsystem(R).
●Resistanceofthepowertransformerearthsystem(Ro).
The figure below shows (dotted line) the Fault loop impedance for
TT systems.
Fig.7-1