Prospective Short Circuit Current measurement (PSCC)
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 PFC. In a multi-phase
system phase-phase loops also need to be considered and these
can be measured using the
(Loop) L-L switch position.
When
CM500 measures the loop resistance, it also calculates
the PFC. After any loop test, this may be displayed by pressing
the
DISPLAY key.
The PFC is calculated by using the sum:-
Nominal supply voltage
Loop resistance
The nominal supply voltage depends on the actual measured
voltage and the configuration of the instrument. As supplied, the
CM500 is configured as follows:-
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.
Errors can be reduced by:-
• Using the 2 wire lead set with prods and making a firm
connection to clean conductors.
• Making several tests and taking the average.
• Ensuring that potential sources of noise in the
installation are isolated (switched off).
• Ensuring that the instrument is calibrated.
Earth Loop Resistance measurement at 15 mA
The LOOP 2 kΩ 15 mA range is a high resolution, low current
resistance range. The 15 mA current enables the earth loop
resistance to be measured without tripping all types of RCDs with
a rated current of 30 mA or higher. Tests may be made via
installation sockets with the plug terminated test lead, or at any
other point using the 2-wire lead set. Connections are required to
Line and Earth only.
15 mA - Phase to Neutral or Earth loop impedance
measurement - Power plug method
1. Select (Loop) 2kΩ 15mA.
2. Insert the plug into an installation socket.
3. Supply voltage and polarity are displayed.
4. Press the
TEST key. Test progress is displayed.
5. Measured loop value is displayed
If desired the test can be repeated by pressing
TEST again.
15 mA - Phase-Earth loop impedance measurement -2-wire
method
1. Select (Loop) 2 kΩ 15 mA.
2. Firmly connect the red lead to phase and the black lead
to earth. No connection to neutral is required.
17
Actual measured Voltage
150 V
>150 V and <300 V
>300 V
Nominal Voltage
110 V
230 V
400 V
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