HT instruments M71 - Manual (Page 11)

M71
EN – 10
4.3.2. EARTH 2W- Measurement procedure
CAUTION
The instrument can be used for voltage and current measurements on
installations with overvoltage category equal to CAT III 240V to earth and
maximum voltage of 415V between inputs. Do not connect the instrument to
installations whose voltages exceed the limits indicated in this manual.
Exceeding such limits may cause electric shock to the user and damage the
instrument.
Fig. 3: Example of instrument's wiring for two-wire earth measurement
Fig. 4: Example of instrument's wiring for two-wire earth measurement (only for TT
systems).
¾
Whenever it is not possible to drive rods into the ground to take a three-wire
measurement (i.e. In historical centres) it is possible to use the simplified two-wire
method which gives an excess value for the sake of safety
¾ To carry out the test a suitable auxiliary rod is necessary; a rod is defined “suitable”
when its earth resistance is negligible and is independent of the earth installation under
test
¾ In Fig. 3 a lamp post is used as auxiliary rod. However a water pipe or any metal body
driven into the ground is suitable provided that the above mentions requirements are
met