Kyoritsu 4140 User Manual download pdf (Page 14)

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;

●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.7-1

1 2 ... 9 10 11 12 13 14 15 16 17 18 19 ... 31 32

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Kyoritsu 4140 User Manual Page 14