HT instruments GEO416 - Manual (Page 10)

GEO416 - GEO416GS
EN - 9
4.4. EARTH 2W – 2 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
§ 11), You can create your own extensions following indications in § 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. 4 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 R
A
+R
T
as result (see Fig. 4 and Fig.
5). Therefore the measurement achieved is the closer to R
A
(prospective
value) the more negligible is the value R
T
of the auxiliary rod with respect to
R
A
itself. In addition the measurement will be increased “for safety sake” by
R
T
, i.e. if R
A
+R
T
results to be coordinated with protective conductors, R
A
alone
will be far more coordinated
Fig. 4: Two-wire earth resistance measurement using an auxiliary rod
In the TT systems (see Fig. 5) 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.