—
17
—
○
Even though the test current in the 2000 ohm range (15mA test current) is
low some RCD's may trip due to sensitivity or where there may already be
additional leakage in the circuit being tested.
○
The Loop impedance in a TN system is small and therefore it is not
recommended to test in the 2000 ohm range. RCD's will have to be bridged
to avoid tripping when using other test ranges.
6. LOOP IMPEDANCE TESTS
DISCONNECT THE INSTRUMENT FROM THE CIRCUIT UNDER TEST
BEFORE OPERATING THE FUNCTION SWITCH
TO SELECT THE LOOP TESTING RANGE SELECT
“
LOOP
”
6.1
Voltage Measurement
Press the test button once. Then, the instrument switches on.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
five times every second. The voltage function operates whenever the test
button is in the up position.
6.2
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.
6.3
Automatic over-temperature cut-out
During the short test period the instrument dissipates power of about 6 kW. If
frequent tests are conducted over a prolonged period of time, the internal test
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