HT instruments T2100 - Manual (Page 13)

T2000-T2100
EN – 12
4.5.3.2. System composed of a single rod
According to its operating principle, the instrument can only perform measurements on
resistive loops. This means that it is not possible to measure systems made of a single rod.
In these cases, it is still possible to assess whether the resistance of the rod under test is
lower than the maximum allowable value for earth resistance of the installation to be tested
(assessed with the traditional volt-ampere method) and whether it is suitable for the
installation, using an auxiliary rod placed close to the installation, thus creating an artificial
resistive loop.
Two different methods to perform such an assessment are described hereunder.
(A) Measuring the earth resistance of a rod with the 2-point method
As shown in Fig. 10 at an appropriate distance from the rod being tested with RA
resistance, an auxiliary rod is to be associated, having RB resistance and optimal features
in terms of earthing (e.g.: metal pipe, reinforced concrete building, etc…). These rods must
be connected from a conductor with an appropriate cross-section, in order to make RL
negligible.
Fig. 10: Assessment of rod resistance based on two-point method
Under these conditions, the resistance measured by the instrument is:
R
= RA + RB + RL ~ RA+RB (2)
CAUTION
The relationship (2) is to be considered valid only provided that it is possible to
neglect the effect of “mutual influence” among the rods connected in series,
i.e. with rods
placed at a sufficient distance between them (equal to at
least 5 times the length of a single rod or 5 times the maximum system
diagonal) so that they do not influence each other.
Therefore, if the value measured by the instrument is lower than the maximum allowable
value of the installation earth resistance to which the RA earth rod refers to (e.g.: where
30mA RCD
 RT < 50V / 30mA = 1667 ) the RA rod finally results to be optimal to be
qualified as an earth rod.