Loop Impedance Testing; Range Selection And Test Leads; Phase To Earth L-E Circuits - Megger MFT1800 Series User Manual

Loop impedance testing

7. Loop impedance testing
NOTE : IMPORTANT
This measurement requires both selector knobs to be set to the Loop testing mode (GREEN RANGES).
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.
Overview of the LOOP IMPEDANCE test
A Loop impedance test is the measurement of the impedance of a circuit whilst the circuit is electrically live.
Unlike a continuity test, a loop impedance test applies a load to the circuit and measures the change in the circuit
voltage, from which the loop "resistance" is calculated.
For those circuits protected by an RCD the load that is connected Phase to Earth must be small enough not to trip
the RCD.
Consequently there must be many tests performed to establish the loop impedance of the circuit. These are
automatically performed and the end result is displayed.
Test Lead Null:
The MFT does not need the resistance of the test leads to be nulled for this test. They are already calibrated into the
measurement circuit at 0.07 Ω.
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 "11.
Setup options" on page 53.
Circuit connection:
The MFT is designed to test the L-PE and the L-N (and L-L) part of the circuit. Selecting the L-PE range on the MFT will
enable testing of the Live to Earth circuit as below:
7.1

Range selection and test leads

7.1.1. Phase to earth L-E circuits:

The main range knob and test leads should be connected as shown:
L-PE Selected
The right hand knob should be set to "Z"
Connecting the 3rd (Blue) lead enables the "3 wire loop test 3Lo, as above and "reverse polarity detection".
26 MFT1800 series
Test performed
L1
E
N
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