KYORITSU 6050 - Manual (Page 15)

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5.1 Principles of Measurement
5.1-1 Measurement of Fault Loop Impedance and Prospective Fault
Current
If an electrical installation is protected by over- current protective devices
including circuit breakers or fuses, the earth fault 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.
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
local earth system.
● Resistance of the local earth system (R).
● Resistance of the power transformer earth system (Ro).
5. LOOP IMPEDANCE / PSC TESTS
Fig.9