7. Loop impedance testing
IMPORTANT
This measurement requires both selector knobs to be set to the loop testing mode (GREEN RANGES) on the MFT1720 and 1730.
This is a live circuit test. All precautions relevant to working on live circuits, to ensure the safety of the operator and any other personnel should be in place.
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The MFT does not need the test leads to be nulled for this test. They are already calibrated into the measurement circuit.
However if using fused leads or 3rd party test leads, the resistance of these leads may be different. In this case they can be measured using the continuity test and the resistance can be compensated for in the SETUP options, see section 10.
Overview of the LOOP IMPEDANCE test
The MFT has two primary loop test options
(a) Phase to Earth test mode
(b) Phase to Neutral (Phase to Phase) test mode.
When the rotary knob is changed from L-E to L-N (L-L) the tester automatically switches the test between the Phase/Earth terminals and the Phase/Neutral (Phase/Phase) on the instrument.
In addition, the test current changes from a non-trip test between L-E to a high current test between L-N (L-L).
Consequently there is significantly less risk of accidently testing a Phase-Earth circuit with a high test current and tripping the RCD.
(a) Phase to Earth testing:
The Phase to Earth test mode uses a non-tripping loop test. This will not trip RCDs.
(b) Phase to Neutral (and Phase to Phase) testing:
The Phase to Neutral (and Phase to Phase) test uses a higher current test. This is a more accurate test and should always be used in preference to the non-trip test where there is no risk of tripping RCDs.
Note: RCDs can still trip when performing a “non-trip” loop test if there is an existing high level of fault current flowing in the Earth conductor, or the RCD is not operating within specification.
7.1 Making a loop impedance measurement
7.1.1 Phase to earth loop-testing (non-tripping)
Use either the 2 wire test lead or 3 wire mains plug test lead.
.1 Set the LEFT rotary range knob to the range
.2 On the MFT1720 and 1730, set the RIGHT rotary knob to
The tester automatically uses the Phase and Earth terminals.
.3 Connect test leads to the Phase (L1) and Earth (L2) terminals on the tester.
If using the mains plug test lead, connect all three leads to the instrument.
.4 Connect the test leads to the circuit Phase and Earth conductors. If using the mains plug test lead, plug the test lead into an outlet point and ensure it is switched on.
.5 Press TEST to start the test sequence. This can be automated in SETUP so the test starts when contacting the circuit.
NOTE:
The 3rd test lead can be connected to Neutral (L3) but is not used in the Phase-Earth measurement. However some models will display Phase-Neutral reversed connection if present.
A warning is displayed if there are any disturbances to the circuit under test during the test sequence. The display will show the
symbol. The loop impedance reading may have been compromised by the circuit interference. Repeat the test.
7.1.2 Phase to neutral (or Phase to Phase) loop-testing – high current
Warning: This test will trip an RCD if the test leads are connected to the Phase and Earth conductors. The RCD will not trip if they are connected to Phase and Neutral.
.1 Set the LEFT rotary range knob to the
range.
On the MFT1720 and 1730, set the RIGHT rotary knob to .
.2 Connect the test leads to the Phase (L1) and Neutral (L3) terminals on the tester.
The 3rd test lead can be connected to Earth (L3) but is not used in the Phase-Neutral measurement. However some models will
display Phase-Neutral reversed connection if present.
If using the mains plug test lead, connect all three leads to the L1, L2 and L3 terminal.
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.3 Connect the test leads to the circuit Phase and Neutral conductors.
If using the mains plug test lead, plug the test lead into an outlet point and ensure the outlet is switched on.
.4 Press and release the TEST button to start the test.
