Amprobe GP-2 Geo-Test - Manual (Page 36)

GP-2 GeoTest
EN - 36
EXAMPLE OF EARTH RESISTANCE MEASUREMENT
You must test equipment protected by a RCD at 30 mA. Measure the earth resistance
according to one of the above-described methods. In order to check whether the
resistance complies to the standards in force, multiply the value for 0.03A (30 mA). If the
result is lower than 50V (or 25V for special places) the equipment is to be considered as
coordinated as it meets the above stated requirement.
In case of RCDs at 30 mA (used for nearly the totality of civil equipment) the maximum
earth resistance admitted is 50/0.03=1666###. This also permits use of the simplified
methods described, which give an approximate value (although not extremely precise) for
the calculation of the coordination.
VOLTAMPEROMETRIC METHOD
For small earth plant
Let a current circulate between the earth rod and a current probe positioned at a distance
from the earth equipment outline corresponding to five-times the diagonal of the area de-
limiting the earth equipment (see Figure 5). Position the voltage probe halfway between
the earth rod and the current probe, then measure the voltage between the two.
Black
Green
Red
Blue
Figure 5: Earth resistance measurement (voltamperometric method
for small earth plant)
Use several rods in parallel and
moisten the surrounding ground if
the instrument is not able to supply
the current necessary to perform the
test because of an high earth
resistance.