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The principal application of the DET62D is in the testing of single or small earth electrodes by the 3 terminal test
technique. The instrument will directly measure the resistance to earth of an earth electrode, in the presence of electrical
mains supply 'noise' in the soil, provided it is not excessive. The instrument may also be used equally well for the testing of
Continuity of conductors; conduit coupling joints etc.
MEASURING METHOD - 3 TERMINAL RESISTANCE MEASUREMENT
A measured current is passed between the electrode under test 'X', and the remote Current electrode 'C'. The voltage drop
between 'X', and the remote potential electrode 'P' is measured. The resistance of the electrode under test is then the
CURRENT SOURCE
Electrode
X
Under Test
THE FALL OF POTENTIAL METHOD
This is the basic method for measuring the resistance of earth electrode systems. It is most useful on small single or double
earth electrode systems. To obtain a reliable measurement with the 'Fall-of-Potential' method the Current spike must be far
enough away from the Earth electrode. In general,
reliable results may be expected if the distance
between the Current spike and the Electrode under
test is at least ten times the maximum dimension of
the electrode system, e.g. 30 m for a 3 m electrode
system. Both possess resistance areas, and if they
are positioned too close together their resistance
areas will overlap.
The optimum position for the Potential spike will be
located between these resistance areas. To locate
this point the Potential spike is moved between 'X'
and 'C', and readings are taken at each location.
The more readings that are taken, the more
accurate the final resistance reading will be. Test
results are plotted, on a curve. The optimum position
is the point on the curve where reading variation is
minimal. Provided there is reasonable homogeneity
of the soil, the distance apart is adequate, and the straight line of the electrodes is maintained, the shape of the curve will
ideally be as shown.
Auxiliary
X
Electrode
Potential
E
Under Test
Electrode
X
E
R
A
Auxiliary
V
Potential
P
Electrode
Resistance Areas
(Not Overlapping)
Auxiliary
P
C
Current
S
H
Electrode
P
C
S
H
D
EARTH
TESTER
APPLICATIONS
voltage between 'X' and 'P', divided by the current
flowing between 'X' and 'C', provided there is no
overlap of the resistance areas of the electrodes.
Auxiliary
Current
The resistance area is where the incremental
C
Electrode
resistance is still significant, owing to localised
current flows.
By Ohm's Law:
INITIAL TEST SET UP AND CONNECTIONS
1. Connect a short lead from the Earth electrode to be tested to the
instrument terminal 'X' ('E') (The resistance of this lead will be included
in the measurement).
2. Insert the Current test spike into the ground 30 to 50 metres away
from the Earth electrode to to be tested. Connect this spike to the
instrument terminal 'C' ('H').
3. Insert the Potential test spike into the ground midway between the
Current test spike and the Earth electrode, and in direct line with them
both. Connect this spike to the instrument terminal 'P' ('S').
4. When running the test leads out to each remote electrode, avoid
laying the wires too close to each other.
V
R = I
OPERATION
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