Handbook of Electrical Installation Practice - BeKnowledge

174 Handbook of Electrical Installation Practice
Fig. 7.8 Curves of rod resistance with increasing rod radius.
formed a cavity and fissures. Low resistivity material was then introduced into the
cavity and fissures and the earth electrode installed within it. This forms a relatively
large electrode at a lower cost than drilling.
Marconite, concrete or similar materials are also now increasingly used around
structures where these are to be used as part of the earthing system.
RESISTANCE MEASUREMENT OF ELECTRODE SYSTEMS
Provided it has sufficient noise immunity (or filtration), the same instrument that is
used for soil resistivity surveys can also be used for measuring the resistance of an
earth electrode system. The most common technique used is the ‘fall of potential’
test. It should be noted that safety precautions are necessary while carrying out this
test if the earth system is connected to live equipment; for example, fused leads and
an appropriate isolation routine are necessary. This is to ensure that an earth fault
occurring while testing does not subject the individual handling the test probes to
a transferred potential.
Potential and current terminals on the tester are connected to the earth electrode
under test. The remaining current terminal is connected to a current probe which is
inserted into the ground some distance away. The remaining potential terminal is
connected to a voltage probe which is situated at 61.8% of the distance between
the current probe and the rod (or centre of the earth grid). Current is circulated
between the electrode under test and the current probe, via the current circuit and
the ground. From the voltage detected at the 61.8% position, the apparent resistance
is displayed on the tester. Under ideal conditions, this apparent resistance
reading corresponds to the actual resistance value of the earth electrode.The reason
for choosing this position is based on mathematical theory applied to uniform soil.
However, the test electrode and current probe must be sufficiently far away from
the electrode under test that they do not interact. For earth rods the current lead
would be about 100m long and the voltage lead 61.8m. For longer leads the same
ratio must be maintained.
The most common cause of error arises from placing the current electrode too
close to the electrode under test. The effects of the two electrodes overlap and the