6 Loop impedance
tests
17
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
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 seconds. 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. The IEE Regulations (BS 7671) provides tables
showing the maximum permissible values of loop impedance in circuits
protected by various fuses and circuit breakers. 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 operates at high power. If
frequent tests are conducted over a prolonged period of time, the internal
test resistor will overheat. When this happens, further tests are
automatically inhibited and the over-temperature symbol appears in the
display. The instrument must then be left to cool down, after which testing
may be resumed.
6.4 The loop impedance test
Since the earth fault loop is made up of conducting path which includes the
supply system back to the supply transformer, it follows that loop testing
can only be carried out after the mains supply has been connected. In
many cases, any RCD in the circuit will be tripped by this test, which draws
current from the phase and returns it through the earth system. The RCD
