Amprobe – Amprobe DMIII-Multitest Power-Quality-Recorder User Manual
Page 90
AMPROBE
DMIII MULTITEST
EN-88
16.4. Ground Resistance Measurement
PURPOSE OF THE TEST
Check if the Automatic protection is coordinated with the ground resistance value. It is not
possible to assume a ground resistance value as reference limit when controlling the test
result, while it is necessary to check every time that the co-ordination complies with the
requirements of the standards.
INSTALLATION PARTS TO BE CHECKED
The ground installation under working conditions. The check is to be effected without
disconnecting the earth plates.
Method for small earth plant
Let current circulate between the earth rod and a current probe positioned at a distance
corresponding to fivefold the diagonal of the area delimiting the earth equipment. Position
the voltage probe at half-way between the earth rod and the current probe, then measure
the voltage between the two.
Blac k
Green
Blue
Red
Use several rods in parallel and moisten
the surrounding ground if the instrument
does not manage to supply the current
necessary to perform the test because
o f a n h i g h e a r t h r e s i s t a n c e .
Ground resistance measurement for small Ground plant
Method for big Ground plant
Also this procedure is based on the voltamperometric method, but it’s mainly used when
it’s difficult to position the auxiliary current rod at a distance corresponding to fivefold the
diagonal of the area of the earth equipment. Position the current probe at a distance equal
to the diagonal of the area of the earth equipment. To make sure that the voltage probe is
positioned outside the area affected by the rod under test, take more measurements, first
positioning the voltage probe at half-way between the rod and the current probe, then
moving the probe both towards the earth rod and towards the current probe.
Blac k
Green
Blue
Red
Use several rods in parallel and moisten
the surrounding ground if the instrument
does not manage to supply the current
necessary to perform the test because
o f a n h i g h e a r t h r e s i s t a n c e .