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Appendix C – Installation category definitions
IEC 61010-2-030 defines measurement categories II to IV relating to transient over-voltages and locations within electrical installations.
Examples of electrical installation category rating are:
Category II - a mains socket outlet,
Category III - the wiring between the socket outlets and the consumer unit,
Category IV - the supply to the consumer cut-out from the distribution network transformer.
For further information on category ratings visit the relevant product page on www.megger.com.
Appendix D – Safe working practice
It is important that before the instrument is used, and when testing is completed, the functions of the instrument are proven to be
working. This is to ensure that a hazardous condition is not mis-reported by the instrument as being safe. For example:
By checking the voltage range correctly measures 230Vac on a separate electrical source, prior to measuring the circuit to be tested, and
then checking it at the end of testing, a live circuit is less likely to be mis-reported as dead.
The Megger MTB7671 test box is available for checking all electrical functions of the multi-function tester (excluding earth tests)
between calibration dates.
Appendix E – Cleaning and maintenance
The MFT1700 and 1800 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 warranty repairs see appendix E.
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
distance of approximately 62% between the electrode under test and the current stake, i.e. at A = 0.62 x B.
A
Earth electrode
under test
B
C (H)
P (S)
X (E)
V
I
schematic for three-terminal resistance measurement
