The 61,8% Rule - Megger DET2/2 Guide De L'utilisateur

Measuring Techniques - Testing Earth Electrodes
Fall-of-Potential Method with Short 'E' Lead
Another way of making connections to the earth
electrode is to connect to the earth electrode using only
one single connection to the 'C1' terminal. This should
only be done if the test lead can be kept short because
its resistance will be included in the measurement.
Note:- Earth electrode test lead resistance can be
determined separately. First remove it from the the
electrode and connect to the 'C2' and 'P2' terminals.
Press the Test push button. The lead resistance can
then be deducted
measurements. This procedure is not, of course,
necessary if the 'C1' and 'P1' terminals are connected
by separate test leads.
15m to 25m
3m
Electrode
under test
Fall-of-Potential method using a single lead to the
earth electrode.
18
from the earth resistance
15m to 25m
3m
Current
Potential
spike
spike

THE 61,8% RULE

To obtain an accurate reading using the Fall-of-
Potential method the current spike must be correctly
sited in relation to the earth electrode. Since both
possess 'resistance areas', the Current spike must be
sufficiently remote to prevent these areas overlapping.
Furthermore, the Potential spike must be between
these areas. If these requirements are not met, the Fall-
of-Potential method may give unsatisfactory results.
Electrode
E
Under Test
Resistance areas associated with an earth
electrode and current spike.
Theoretically, both the Current and Potential spikes
should be at an infinite distance from the earth
electrode. However, by graphical considerations and by
actual test it can be demonstrated that:-
The 'true' resistance of the earth electrode is equal to
the measured value of resistance when the Potential
spike is positioned 61,8% of the distance between the
earth electrode and the Current spike, away from the
earth electrode.
Auxiliary
Auxiliary
Potential
Current
P
C
Electrode
Electrode
Resistance Areas
(Not Overlapping)