Measurement Methods And Sources Of Error - Megger MFT1800 Series User Manual

Loop impedance testing
7.6

Measurement methods and sources of error

Method of measurement
During a loop test the instrument measures the difference between the unloaded and loaded supply voltages.
From this difference it is possible to calculate the loop resistance. The test current will vary from 15 mA to 5 A,
depending on supply voltage and the loop resistance value. The volt drop from a 15 mA load is exceptionally small,
consequently the instrument performs many measurements automatically. This test takes a long time to complete,
typically 20 seconds
Possible sources of error
The reading depends on the stability of the supply voltage during the test. Therefore noise, harmonics or transients,
caused by other equipment during the test could cause an error in the reading. The instrument will detect some
sources of noise and warn the user.
It is recommended that more than one test is performed on the circuit to ensure the measured value is repeatable,
especially when performing a 3Lo measurement.
Capacitive loads across the Phase-Earth circuit can affect the accuracy of the Non-trip loop test. For this reason the
P-E (non-trip) loop test should not be used on the P-N circuits.
Errors can be reduced by:-
Use the two-wire lead set with prods and making a firm connection to clean conductors.
„
Make several tests and take the average.
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„ Ensure that potential sources of noise in the installation are isolated (switched off), eg: automatically switched
loads or motor controllers
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