Iso-Tech IMT-3300 Manuel D'instructions page 14

5.3.2 Loop impedance measurement at a single-phase
distribution board or equipment:
5.3.2.1 Preparation
a) Ensure the pre-use checks at Para 5.1 above have been carried out.
b) Slide the test-lead connector cover to expose the IEC test socket.
c) Connect the distribution board test lead (See Fig. 11 above)
to the IEC socket of the instrument.
d) Replace the red and black test probes with the crocodile
clips if required.
e) Set the function switch to the "20Ω Loop" range.
f) Press the test button momentarily and release it. Check "V"
and "Lo" appear on the display.
g) Connect the test probes to the circuit in the following sequence:
Connect the green crocodile clip to the earth connection; the
black neutral lead to the neutral of the distribution board and
the red phase lead to the phase of the distribution board.
5.3.2.2 Wiring check
 Caution! After connection, ensure the LEDs are as shown below:
P-E red LED is lit
P-N red LED is lit
Red LED is not lit
If the above sequence is not correct, do not continue the test as the
wiring is incorrect. Investigate and rectify the cause before continuing.
5.3.2.3 Loop test
a) Note the mains voltage as shown on the display.
b) Press and release the test button. The value of the loop
impedance will be shown on the display. If the display shows
'OL', the measured resistance is greater than the range
selected. Select the 2000Ω range and repeat the test.
Note:
Tests carried out with the function selector switch in the 2000
Ω position pass a test current of 15mA which is less likely to
trip an RCD.
If any in-circuit RCD trips, set the phase-angle selector switch
to the alternative position, reset the RCD and repeat the test. If
the test cannot be carried out without tripping the circuit RCD,
temporarily replace the RCD with a suitably rated MCB for the
duration of the tests.
5.3.3 Loop impedance measurement at a 3-phase
distribution board or equipment:
a) Use the same procedure as at (5.3.2) above, but connect the
phase test lead to each phase in turn to obtain 3 separate
results. i.e. Connect the green crocodile clip to the earth
connection; the black neutral test lead to the neutral and the
red phase test lead to phase 1. Carry out the first test.
b) Move the red phase test lead to phase 2 and carry out the
second test. etc.
c) Disconnect the test leads in the reverse order of connection.
Note:The procedures given at 5.3.1, 5.3.2 and 5.3.3 above are
for measuring the phase to earth loop-impedance of a circuit. To
measure the phase to neutral loop-impedance of a circuit, connect
the earth test probe to the system neutral (i.e. the same point as
the neutral test probe). If the circuit has no neutral, connect the
neutral test lead to the system earth (i.e. the same point as the
earth probe). Testing in this manner may trip any RCD in the circuit.
5.3.4 RCD testing
Note:
Testing of an RCD is carried out in stages to establish correct
operation. The first test is the no-trip test which simulates a
residual fault current of half the rated tripping current of the
RCD. The no-trip test is designed to ensure that the RCD will
not trip at low or nuisance current levels.
The next test is to simulate a residual fault current at the rated
tripping current of the RCD and measure the time taken to
break the circuit. The tripping time should comply with the
manufacturers' specifications, typically within 40mS.
Depending on the type, location and purpose of the RCD,
further tests may be required such as a fast-trip test which tests
at 5 times the rated trip current of the RCD. Again, the tripping
time should comply with the manufacturers' specifications.
Consult the RCD manufacturers' data for further information.
— 14 —