26
L
N
E
MCB
g
RCD
g
L
N
E
MCB
.4 Press and release the TEST button to start the test.
.5 On completion of the test, the display will show the loop resistance on the large display segments, and the fault current on the
small display segments.
7.4
Prospective Fault Current and Short Circuit calculation (PFC & PSCC)
The prospective fault current and short circuit current of a circuit is automatically calculated when making a loop impedance test. The
calculation uses a nominal circuit voltage, not the actual circuit voltage, and is displayed above the loop impedance measurement, as
below:
The fault current is calculated using the expression:-
PSCC or PFC = (Nominal supply voltage in Volts / Loop resistance in Ohms
Example PSCC or PFC = 230V / 0.13
W
= 1769VA (displayed on the MFT as 1.77kA)
The nominal supply voltage used in the calculation is automatically selected depending on the actual circuit voltage. The instrument uses
the following voltage values:-
Actual measured voltage
Nominal voltage
< 75V
55V
≥ 75V and <150V
110V
≥ 150V and <300V
230V
≥300V
400V
7.5
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 15mA to 5A, depending on supply voltage and the loop
resistance value. The volt drop from a 15mA 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
