Earth 2W - Two Wire Earth Resistance Measurement - Amprobe GP-2A Manual De Uso

4.4 EARTH 2W – Two Wire Earth Resistance Measurement
�CAUTION
• The instrument can be used for voltage and current measurements on installations with
over voltage 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
• Always connect the cables to the instrument and to the alligator clips when the latter are
not connected to the plant under test
• Always respect the Hand-held area of probe (see 4.2)
• If the length of the supplied cables isn't suitable for the plant under test (see Par. 11), You
can create your own extensions following indications in Par. 11.2.1
Whenever it is not possible to drive rods into the ground to take a three-wire measurement (i.e.
historical centres), it is possible to use the simplified two-wire method which gives an excess
value for the sake of safety. To carry out the test a suitable auxiliary rod is necessary; an auxiliary
rod is deemed as suitable when its earth resistance is negligible and independent of the earth
installation under test.
In Fig. 3 a lamp post is used as auxiliary rod, however any metal body driven into the ground can
be used provided that the above mentioned requirements are met.
�CAUTION
The instrument displays the sum value of RA+RT as result (see Fig. 3 and Fig. 4). Therefore
the measurement achieved is the closer to RA (prospective value) the more negligible is the
value RT of the auxiliary rod with respect to RA itself. In addition the measurement will be
increased "for safety sake" by RT, i.e. if RA+RT results to be coordinated with protective
conductors, RA alone will be far more coordinated
Fig. 3: Two-wire earth resistance measurement using an auxiliary rod
In the TT systems (see Fig. 4) it is possible to perform a two-wire earth measurement using the
NEUTRAL conductor provided by the national Energy Board taken directly from a socket or panel
board as an auxiliary rod; if also the earth connection is available, the measurement can be
taken on the socket directly, between NEUTRAL and EARTH.
�CAUTION
If you wish to effect the measurement using the neutral and earth conductors of an ordinary
socket, you may accidentally connect to phase; in this case the detected voltage as well as the
warning symbol
for wrong entering will be displayed and no measurement will be effected even though the
GO key is pressed.
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