Megger DET2/2, User Manual

Page: 20 / 116

20
Measuring Techniques - Testing Earth Electrodes
THE SLOPE METHOD
This method is more applicable to larger earth
electrode systems or where the position of the centre of
the earthing system is not known or inaccessible (e.g.
if the system is beneath the floor of a building). The
Slope method can also be used if the area available for
siting the earth electrodes is restricted. It can be tried if
the previous methods prove unsatisfactory and
generally yields results of greater accuracy than those
methods.
Connections for the Slope method
The equipment is set up as shown. The remote Current
spike is placed 50 metres or more from the earth
electrode system to be measured and connected to the
'C2' terminal. The Potential spike is inserted at a
number of positions consecutively, between the earth
system and the Current spike, and connected to the
'P2' terminal. The test spikes and the earth system
should all be in a straight line.
The ' C1' and 'P1' terminals are connected separately
to some point on the earth electrode system.
The earth resistance is measured at each separate
position of the Potential spike and the resistance curve
is plotted from the results. At least six readings are
needed. Drawing the curve will show up any incorrect
points which may be either rechecked or ignored.
Example Resistance curve from Slope method
tests.