KYORITSU 6010B - Manual (Page 19)

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7. LOOP IMPEDANCE TESTS
DISCONNECT THE INSTRUMENT FROM THE CIRCUIT UNDER TEST
BEFORE OPERATING THE FUNCTION SWITCH
TO SELECT THE LOOP TESTING RANGE SELECT “LOOP”
7.1 Voltage Measurement
Power on the instrument. When the tester is set to the Loop test function, mains
voltage is displayed as soon as the instrument is connected for test. This
voltage display is automatically updated every 1 second.
7.2 What is earth fault loop impedance?
The path followed by fault current as a result of a low impedance fault occurring
between the phase conductor and earth is called earth fault loop. Fault current
is driven round the loop by the supply voltage, the amount of current depending
on the voltage of the supply and on the impedance of the loop. The higher the
impedance, the lower will be the fault current and the longer it will take for the
circuit protection (fuse or circuit breaker) to operate and interrupt the fault.
To make sure that fuses will blow or that circuit breakers will operate quickly
enough in the event of a fault, the loop impedance must be low, the actual
maximum value depending on the characteristics of the fuse or the circuit
breaker concerned. Every circuit must be tested to make sure that the actual
loop impedance does not exceed that specified for the protective device
concerned.
For a TT system the earth fault loop impedance is the sum of the following
impedances (See Fig 9):
Fig 9