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Earthing design considerations
A correctly designed and installed earthing system will
safeguard both lives and equipment.
A good earth connection should have:
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Low
electrical resistance to earth |
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Good
corrosion resistance |
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Ability
to carry the required current repeatedly |
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A
reliable life of at least 30 years |
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The
crucial factors that determine the resistance to earth
of an electrode are: |
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Soil resistivity
Physical
composition |
| Different
soil compositions give different average resistivities: |
Effect
of soil type on resistivity
| Soil
type |
Typical
resistivity
Ohm-m |
| Marshy
ground |
2
- 2.7 |
| Loam
and clay |
4
- 150 |
| Chalk |
60
- 400 |
| Sand |
90
- 8,000 |
| Peat |
200
upwards |
| Sandy
gravel |
300
- 500 |
| Rock |
1,000
upwards |
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Moisture
|
Increased
moisture content of the ground can rapidly decrease
its resistivity.
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". |
Effect
of moisture on resistivity
| Moisture
content % by weight |
Resistivity
Ohm-m
Top soil |
Resistivity
Ohm-m
Sandy loam |
| 0 |
1,000
x 10^4 |
1,000
x 10^4 |
| 2.5 |
2,500 |
1,500 |
| 5 |
1,650 |
430 |
| 10 |
530 |
185 |
| 15 |
310 |
105 |
| 20 |
120 |
63 |
| 30 |
64 |
42 |
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Chemical composition |
Certain
minerals and salts can affect soil resistivity.
Their levels can vary with time due to rainfall
or flowing water.
Note that although the addition of salts can lower
soil resistivity, they are not recommended due to
corrosion and leaching.
(Click here from more information on soil conditioning.) |
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 |
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Temperature
|
When
the ground becomes frozen, its resistivity rises
dramatically. An earth that may be effective during
temperate weather may become ineffective in winter.
Please
note that, if your soil temperature decreases from
+20°C to -5°C, the resistivity increases
more than ten times. |
Effect
of temperature on resistivity for
sandy loam, 15.2% moisture
Temperature
(°C) |
Temperature
(°F) |
Resistivity
Ohm-m |
| 20 |
68 |
72 |
| 10 |
50 |
99 |
| 0 |
32
(water) |
138 |
| 0 |
32
(ice) |
300 |
| -5 |
23 |
790 |
| -15 |
14 |
3,300 |
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Electrode
dimensions
The most important dimension to consider when designing
an earth electrode is its length. The greater the length
of an electrode the lower the density of the current in
soil in the immediate vicinity of that electrode.
For
this reason a rod or strip type electrode will have a
much lower resistance to earth than a plate type electrode
of the same surface area.
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.
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Area available
Often a single earth rod, strip or plate will not achieve
the desired resistance alone. If a number of electrodes
can be installed in parallel the combined resistance is
then practically proportional to the reciprocal if the
number employed. This is true so long as each electrode
is situated outside the resistance area of any other. |
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For
rod electrodes this separation distance is considered
to be equal to the driven depth.
When an earth electrode must be composed of multiple
parallel electrodes the area available for earthing
becomes of major importance.
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Earth electrode materials
Quality earth rods are commonly made from either solid
copper, stainless steel or copperbonded steel.
Furse
can supply 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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