Handbook of Electrical Installation Practice - BeKnowledge

used to produce the 430V or 650V hot zone contours required by telecommunication
authorities.
Artificial method of reducing earth resistivity
Ground Earthing 173
Low resistivity materials
It is widely thought that adding low resistivity materials to the soil in the immediate
vicinity of the earthing system or rod will have a dramatic effect on reducing its
resistance. This is not normally true. There is always a contact resistance between
the electrode and the soil. Where a new rod has been driven into the ground, the
sideways movement will have increased the width of the hole in which the rod lies.
The gap between the rod surface and the compressed soil to its side will introduce
a large contact resistance which will be apparent when testing the resistance of the
rod. However, experience shows that this resistance falls over time as the soil
becomes consolidated around the rod due to rainfall, etc. One way to accelerate this
is to add a low resistivity material, such as bentonite slurry, as the rod is driven in.
Bentonite is a fine, naturally occurring clay powder. It is mixed with water to form
a slurry. As the earth electrode is driven into the soil the bentonite is drawn down
by the rod. By continuously pouring the mixture into the hole as driving proceeds,
a sufficient quantity is dragged down to fill most of the voids around the rod and
lower its resistance. Bentonite is thixotropic in character and therefore gels when
in an inert state, so should not leach out. It has a low resistivity and does not dry
out under normal conditions. However, its use in this manner does not necessarily
produce a significantly lower earth resistance than that which would occur naturally
over time. In some cases, installing the rods a little deeper can achieve a better result
than using low resistivity material.
Adding bentonite and similar material, such as marconite, in a trench or larger
drilled hole around the electrode has the effect of increasing the surface area of
the earth conductor, assuming its resistance is lower than that of the surrounding
soil. Figure 7.8 shows the effect on the resistance value of one rod when its radius
(and hence surface area) is increased. Clearly there is an improvement, but this falls
off rapidly with increasing radius, and the cost quickly becomes prohibitive. Additional
reasons for increasing the surface area may include a requirement to reduce
the build up of electrochemical deposits on the electrodes which may reduce their
efficiency, or for high frequency earthing (lightning protection, etc.).
Chemical treatment
Treating the soil surrounding the earth electrode with chemicals normally only provides
a temporary improvement as the chemicals will usually be dispersed over time
by rainwater. The environmental effects may also mitigate against this undesirable
practice. In addition, one must ensure that the chemicals used do not have any
corrosive effect on the electrode material.
One interesting technique used in rocky sites in Hungary was to drill a hole
several metres deep into the rock. Explosives were introduced and when detonated,
Fig. 7.7 (a) Potential rise on the soil surface above an earth grid and terminal tower; (b)
touch voltage contours above a buried substation earth grid.