The 61,8% Rule; Lead To The Earth Electrode - Megger DET5/4R Guia Del Usuario

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 test lead (as shown in Fig. 8). The 3 pole push
button should be used to operate the instrument and
the single connection made 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
electrode and connect to the 'C2' and 'P2' terminals.
Press the 3 pole push button. The lead resistance can
then be deducted from the earth resistance
measurements. This procedure is not, of course,
necessary if the 'C1' and 'P1' terminals are connected
by separate test leads.
16
15m to 25m
Electrode Potential
3m
under test
spike
Fig. 8 Fall-of-Potential method using a single lead
to the earth electrode.

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, see the diagram of Fig.9. If these
requirements are not met, the Fall-of-Potential method
may give unsatisfactory results.
15m to 25m
Current
3m
spike
C1 P1 P2 C2
E ES S H
EARTH EARTH
®
MEGGER MEGGER DET5/4D DET5/4D TESTER TESTER
k
3 POLE 4 POLE
MEASURE R
E