Megger DET2/2, User Manual

Page: 21 / 116

21
Suppose the distance from the earth electrode system
to the current spike is EC. From the curve equivalent
resistance readings to Potential positions 0,2EC, 0,4EC
and 0,6 EC can be found. These are called R1, R2 and
R3 respectively.
Calculate the slope coefficient µ, where
µ = (R3-R2)
(R2-R1)
which is a measure of the change of slope of the earth
resistance curve.
From the table commencing on page 36 obtain the
value of Pt / Ec for this value of µ.
Pt is the distance to the Potential electrode at the
position where the ‘true’ resistance would be
measured.
Multiply the value of Pt / Ec by Ec to obtain the distance
Pt.
From the curve read off the value of resistance that
corresponds to this value of Pt. The value obtained is
the earth electrode system's resistance.
Note:- (i) If the value of µ obtained is not covered in
the table then the current spike will have to be moved
further away from the earthing system.
(ii) If it is necessary, further sets of test results can be
obtained with different values of E C, or diff e r e n t
directions of the line of EC. From the results obtained of
the resistance for various values of the distance EC.
Example of possible results from several Slope
method tests.
This shows how the resistance is decreasing as the
distance chosen for EC is increased.
The curve indicates that the distances chosen for EC in
tests (1) and (2) were not large enough, and that those
chosen in tests (3) and (4) were preferable because
they would give the more correct value of the earth
resistance.
(iii) It is unreasonable to expect a total accuracy of
more than 5%. This will usually be adequate, bearing in
mind that this sort of variation occurs with varying soil
moisture conditions or non-homogeneous soils.