PFC measurement accuracy
An accurate PFC measurement requires an accurate measurement of the
loop resistance. The difference of a few digits in the loop resistance
measured will have a large effect on the PFC displayed.
Noise Indication
The symbol is displayed when excessive noise caused by other
equipment exists on the circuit under test. This noise can effect the
accuracy of the loop measurement. The operator is advised to repeat the
measurement or, if the noise symbol continually appears, investigate the
cause.
Method of measurement
The phase-earth, phase-neutral or phase-phase loop resistance can be
measured. The instrument takes a current from the supply and measures
the difference between the unloaded and loaded supply voltages. From this
difference it is possible to calculate the loop resistance. The test duration
will depend on the loop resistance value and the presence of noise on the
supply.
The NO TRIP loop test range performs a test with a current of up to 25 A
flowing Line to Neutral to measure the resistance of the source and line
wires, followed by a current of 15 mA flowing Line to Earth to measure the
resistance of the earth wire .
Possible sources of error
The reading depends on a measurement of the supply voltage and
therefore noise or transients caused by other equipment during the test
could cause an error in the reading. One way to check for these is to do
two tests and look for any difference in value. The instrument will detect
some sources of noise and warn the user, where other instruments may
give an incorrect reading. Any leakage current as a consequence of other
appliances connected to the supply under test may affect the reading. If the
Phase-Earth loop is being measured, this leakage may be due to filter
capacitors, etc.
Test results may be adversely affected by supply voltage fluctuations or
electrical ‘noise’ during a measurement. It is recommended that tests are
repeated and the results verified, if measurement results are considered
abnormal.
Errors can be reduced by:-
■ The two-wire lead set with prods and make a firm connection
to clean conductors.
■ Several tests and take the average value.
■ Ensure that potential sources of noise in the installation are isolated
(switched off), eg: automatically switched loads or motor controllers
■ Ensuring that the instrument is calibrated.
Thermal Protection
To protect the MFT from overheating during Loop testing, thermal
protection is fitted. If the message ‘HOT’ appears in the display when loop
testing, the instrument must be allowed to cool down before further
attempts are made at loop testing.
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