15
NOTE: Both the High current and No Trip loop tests will operate Phase to Phase.
5.4 High resolution loop test [High Resolut] (LTW425 only)
The [High Resolut] Loop test performs a 2-wire high current loop test, providing a loop test result to one
milliohm (0.001
Ω
) resolution.
This test is designed for non-RCD protected circuits.
A limited number of these tests can be done before the instrument will indicate over-temperature,
display the [hot] warning and inhibit further tests. In which case leave the instrument to cool down
for a few minutes.
5.4.1 All the high current tests in section 5.3 can be performed with the
instrument set to [High Resolut].
Note: In order to protect instrument from overheating, it is necessary to introduce pause time during testing
thus extending the [
High Resolut
] test to 15 seconds for measurements on supplies of less than 260 V a.c. and
30 seconds for measurements on supplies above 260 V a.c.
5.5 Prospective Fault Current display [PFC]
NOTES:
The PSCC of a circuit is the largest Prospective Fault Current (PFC). In a single-phase system, this would be the
larger of the earth loop PFC and the neutral loop PSCC. In a multi-phase system phase-phase loops also need to
be considered and are measured in the same way as single-phase supplies.
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.
Calculation of PFC (and PSCC) is based on the measured voltage divided by the measured loop resistance (in
ohms).
Measured Voltage
=
PFC
Loop resistance
The displayed value of PFC is calculated from the test results of voltage and resistance BEFORE they have been
rounded for display. Consequently the displayed PFC may not exactly correspond to the result of manual
calculation using displayed results of voltage and resistance.
5.5.1 PFC measurement
1.
On completion of any loop test press the [PFC] key.
2.
The prospective fault current is displayed in Amps.
3.
Press [PFC] key again to return to loop impedance result.
