20
7.
Loop Impedance testing
IMPORTANT
This measurement requires both selector knobs to be set to the Loop testing mode (GREEN RANGES) on the MFT1825 and 1835 or just
the left knob on the 1815
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
W
.
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.
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 below:
L-PE
Selected
Test
performed
L1
N
E
The right hand knob should be set to any of the “RCD” or “Re” ranges.
Connecting the 3rd (Blue) lead enables the “3 wire loop test” 3Lo, as below and enables “reverse polarity detection”.
Test Options in L-PE mode:
In L-PE mode the MFT1800 series offer 3 types of loop test: