HT instruments HT7051 - Manual (Page 22)

HT2055
EN - 21
7.3 EARTH RESISTANCE MEASUREMENT
The earth resistance measurement is performed by the HT2055S station unit only by using
the standard 3-wire method with two supplied auxiliary probes fitted at adequate distances
from the earth system under test according to the Fig. 24
Fig. 24: Connection of instrument for earth resistance measurement
1 Place the station unit close to the earth system under test (distance max. 10m)
2 Connect the input terminals C2/E and ES to the earth system under test by using the
cables supplied
3 Fit the auxiliary metal probe (1m length) on the ground at a distance of least 5 times
the maximum diagonal of the earth system under test (see Fig. 24). Connect the input
terminal C1/H to the probe by using the 50m long cable supplied (if enough) or use
another customized cable for longer distance. The nature of the measurement method
permits the use of different cables without any resistance compensation. Use also
more probes connected in parallel to each other if it’s necessary to reduce the contact
resistance on that point
4 Fit the auxiliary metal probe (60m length) on the ground at a distance of at least 2.5
times the maximum diagonal of the earth system under test (see Fig. 24). Connect the
input terminal S to the probe by using the 50m long cable supplied (if enough) or use
another customized cable for longer distance. The nature of the measurement method
permits the use of different cables without any resistance compensation. Use also
more probes connected in parallel to each other if it’s necessary to reduce the contact
resistance on that point
CAUTION
For large sized installations, place the ampere metric probe (H) at a distance
equal to the diagonal of the earth installation and the volt metric probe (S) at a
distance equal to about 0.5 times the diagonal of the earth installation. In the
latter case several measurements shall be carried out moving the volt metric
probe back and forth (along the measurement direction) with respect to the
middle point and checking that the result is nearly constant