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A correctly designed and installed earthing
system will sageguard both lives and equipment.
A good earth connection should have:
Low
electrical resistance to earth
Good
corrosion resistance
Ability
to carry high currents repeatedly
A
reliable life of at least 30 years
Soil resistivity is a crucial factor in obtaining
a 'good earth'
a ) Physical Composition
Different soil compositions give different average resistivities:
Table
1 |
Effect of soil type on resistivity
|
Soil type |
Typical resistivity ohm-m |
Marshy Ground |
2 - 2.7 |
Loam and clay |
4 - 150 |
Chalk |
600 - 400 |
Sand |
90 - 8000 |
Peat |
200 upwards |
Sandy Gravel |
300 - 500 |
Rock |
1000 upwards |
b ) Moisture
Increased moisture content of the ground can rapidly
decrease its resisitivity.
It is especially important to consider moisture content
in areas of high seasonal variation in rainfall.
Wherever possible the earth electrode should be installed deep enough
to reach the "water table". or "permanent moisture level".
Table
2 |
Effect of soil type on resistivity
|
Moisture content %
by weight |
Resistivity ohm-m |
Top Soil |
Sandy Loam |
0 |
1,000 x 10 power 4 |
1,000 x 10 power 4 |
2.5 |
2500 |
1500 |
5 |
1650 |
430 |
10 |
530 |
185 |
15 |
310 |
105 |
20 |
120 |
63 |
30 |
64 |
42 |
c ) Chemical Composition
Certain minerals and salts can affect soil resistivity. Their levels
can vary with time due to rainfall or flowing water.
Table
3 |
Effect of Salt on Resistivity
For sandy loam, 15.2% moisture
|
Added salt (% by weight of moisture)
|
Resistivity ohm-m |
0.0 |
107.0 |
0.1 |
18.0 |
1.0 |
4.6 |
5.0 |
1.9 |
10.0 |
1.3 |
20.0 |
1.0 |
|
|
Note that although the addition of salt
can lower soil resistivity, they are not recommended due to corrosion
and leaching.
c ) Temperature
When the ground becomes frozen, its resistivity rises dramatically.
An earth that may be effective during temperate weather may become ineffective
in winter.
Table
4 |
Effect of Salt on Resistivity
For sandy loam, 15.2% moisture
|
Temperature
|
Resistivity ohm-m |
degC |
degF |
|
20 |
68 |
72 |
10 |
50 |
99 |
0 |
32 (water)
|
138 |
0 |
32 (ice) |
300 |
-5 |
23 |
790 |
-15 |
14 |
3300 |
Please note that, if your soil temperature decreases
from +20°C to -5°C, the resistivity increases more than ten
times.
By reaching permanent moisture and frost free soil
levels, low resistance should be achieved. Often these levels are some
metres below the surface and the most economical way of reaching them
is by extensible deep driven earth rod electrodes.
Furse recommend the use of deep driven earth rod electrodes
wherever conditions allow.
Where rocks lie just below the surface and deep driving
is not possible, parallel driven shorter rods, plates, mats or buried
conductors, or a combination of these can be used. However, these should
still be buried as deep as possible to avoid seasonal variations, damage
from agricultural machinery etc.
Often parallel rods are driven too close together;
this decreases their effectiveness. The distance between rods should
be greater than the rod length, L (see diagram below).
Quality earth rods are commonly made from either solid
copper, stainless steel or copperbonded steel.
Furse manufacture all three types, but the copperbonded
steel cored rod is by far the most popular, due to its combination of
strength, corrosion resistance, and comparatively low cost. Solid copper
and stainless steel rods offer a very high level of corrosion resistance
at the expense of lower strength and higher cost.
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