HT instruments T2100 - Manual (Page 12)

T2000-T2100
EN – 11
4.5.3. Methods for resistance measurement on earth rods
1. Press and hold the multifunction key “
1” for more than 2s to switch on the instrument.
2. The displayed message “ OL ” indicates that the instrument is ready to carry out
measurements.
3. Open the jaws (the display will show the screen in Fig. 7) and clamp the rod to be
measured, then read the result on the display.
According to the type of installation found, refer to the cases described below.
4.5.3.1. Multiple-rod systems
Measuring the earth resistance of 1 rod making part of an earth system
In case of an earth system made of many rods connected in parallel (e.g.: high-voltage
poles, communication systems, industrial buildings, etc…) each of them individually
earthed, the instrument’s connection diagram can be sketched as shown in Fig. 9:
Instrument’s connection to a multiple-rod system
Fig. 9: Instrument’s connection to a multiple-rod system
The instrument provides the sum
R  R1 + R0 as measured value
(1)
in which:
R1 = resistance of the object under test
R0 = R2 // R3 // R4 = resistance equal to parallel among resistances R2, R3, R4
CAUTION
The relationship (1) is to be considered valid only provided that it is possible to
neglect the effect of “mutual influence” among the rods connected in parallel,
i.e. with rods
placed at a sufficient distance D between them (where D is
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.
In case the formula (1) is valid, the value of parameter R0 is usually much smaller than the
value of parameter R1, and an error is negligible assuming that R0
 0. In this way, we can
say that the resistance measured by the instrument corresponds to the resistance of the rod
under test, anyway increased for the sake of safety within the RCDs’ coordination. The
same procedure can be performed by moving the clamp on the other rods connected in
parallel in order to assess the values of resistance R2, R3 and R4.