The PFC is calculated by using the sum:-
Nominal supply voltage
Loop resistance
The supply voltage used in the calculation depends on the measured
voltage. The instrument uses the following voltage values:-
Actual measured
Nominal voltage
voltage
> 50 V and < 80 V
50 V
>80 V and <150 V
110 V
>150 V and <300 V
230 V
>300 V
400 V (LT320 & LT330 only)
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.
Warning messages
Noise Indication
The symbol
is displayed when excessive noise caused by other
equipment exists on the circuit under test. This noise can affect the
accuracy of the loop measurement.
The operator is advised to repeat the measurement or, if the noise symbol
continually appears, investigate the cause.
Voltages greater than 280V [>480V on LT320 and LT330]
LT310:
If a voltage greater than 280 V is detected, the display will
show >280 V.
LT320
/LT330:
If a voltage greater than 480 V is detected between phases the
display will show >480 V.
Over temperature
hot
To protect the instrument from over heating during Loop testing, thermal
protection is fitted. If the message [
hot
] appears in the display together
with the
G
symbol when loop testing, the instrument must be allowed
to cool down before further attempts are made at loop testing.
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:-
■
Use the 2 wire lead set with prods and making a firm connection to
clean conductors.
13
PFC=
