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Appendix E - Cleaning and maintenance
The MFT1700 should only be opened or repaired by an approved Megger service or by Megger Instruments Limited.
To clean the instrument, use a damp cloth or isopropyl alcohol if available. To clean the display window only use a lint free cloth.
For repairs and warranty, see page 40.
Appendix F - Earth resistance testing – Basic principles
F.1 Principle of operation (three-terminal resistance measurement)
The classic “fall of potential” test is used to accurately measure the resistance of an earth electrode using auxiliary stakes driven into the
soil, which form a circuit for the test current injection and voltage measurement as used for the two-terminal method.
The MFT injects an a.c. current of known magnitude into the system under test and measures the voltage developed across it as shown
in Figure 4. The system resistance is a simple ratio as per hm’s Law. In this case, the potential stake is moved by fixed increments in a
straight line between the electrode under test and the current stake. At each location, the resistance is calculated as R=V/I. A graph of
resistance versus potential stake position is plotted and the resistance of the electrode under test is taken to be the point at which the
curve is flattest.
Empirical testing has shown that with suitably positioned stakes, this method can be shortened by placing the potential stake at a dis-
tance of approximately 62% between the electrode under test and the current stake, i.e. at A = 0.62 x B.
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