Earthing Design Considerations

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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'

Factors Affecting Soil Resistivity

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
Moisture content % by weight	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.

Selecting the Correct Earth Electrode

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).

multiple driven rods

Earth Rod Electrodes

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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