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8.13 Measurement methods and sources of error
RCD testing - Method of measurement
A two wire lead, or mains plug lead should be used for this measurement. A constant current source is connected across the supply and
the time taken for the supply to trip is measured by the instrument in ms.
RCD testing - Possible sources of error
Measurement results can be affected by the following:
• Significant operating errors can occur if loads, particularly rotating machinery and capacitive loads are left connected during tests.
• A poor connection to the circuit under test.
8.14 Useful information
It is only necessary to test the 10 mA and 30 mA at 1/2xI, 1xI and 5xI. All other RCDs only need to be tested at 1/2xI and 1xI.
Always press the RCD TEST button on the RCD to ensure the function works.
It is recommended the RCD test button is tested AFTER the timing tests above are complete. This can identify RCDs that may stick or fail
if not checked periodically.
9. Earth resistance measurement
The Megger MFT family of test instruments offers a unique solution to the measurement of earth or ground electrode (rod) supporting 2
and 3 wire measurements:
For the principles of earth resistance testing go to Appendix F
9.1 Connection terminals
The terminal references used on the MFT are:
MFT1700 series
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