Earth Electrode: What it Is, Requirements, Resistance

Earth electrode (or ground electrode (US)): a conductive part that is in electric contact with local earth, directly or through an intermediate conductive medium [defined in the IEC 60050-195-2021].

Independent earth electrode (or independent ground electrode, US): an earth electrode located at such a distance from other earth electrodes that its electric potential is not significantly affected by electric currents between Earth and other earth electrodes.

When performing a separate (independent) earthing switch for functional earthing under the conditions of information or other interference-sensitive equipment, special measures must be taken to protect against electric shock, excluding simultaneous contact with parts that may be under a hazardous potential difference if the insulation is faulty.

Earthing arrangement
Figure 1. Example of an earthing arrangement (including vertical and horizontal earth electrodes are shown in the figure)

Requirements

The type, materials and dimensions of earth electrodes shall be selected to withstand corrosion and to have adequate mechanical strength for the intended lifetime.

NOTE 1. For corrosion, the following parameters may be considered: the soil pH at the site, soil resistivity, soil moisture, stray and leakage a.c. and d.c. current, chemical contamination, and proximity of dissimilar materials.

For materials commonly used for earth electrodes, the minimum sizes, from the point of view of corrosion and mechanical strength, when embedded in the soil or in concrete, shall be as specified in Table 54.1.

NOTE 2. The minimum thickness of protective coating is greater for vertical earth electrodes than for horizontal earth electrodes because of their greater exposure to mechanical stresses while being embedded.

Where a lightning protection system is required, IEC 62305-3:2010, 5.4 also applies.

Table 54.1 – Minimum size of commonly used earth electrodes, embedded in soil or concrete used to prevent corrosion and provide mechanical strength
Material and surface Shape Diameter, mm Cross sectional area, mm2 Thickness, mm Weight of coating, g/m2 Thickness of coating/enclosure, μm
Steel embedded in concrete (bare, hot galvanized or stainless) Round wire 10
Solid tape or strip 75 3
Steel hot-dip galvanizedc Stripb or shaped strip/plate

– Solid plate – Lattice plate

90 3 500 63
Round rod installed vertically 16 350 45
Round wire installed horizontally 10 350 45
Pipe 25 2 350 45
Stranded (embedded in concrete) 70
Cross profile installed vertically (290) 3
Steel copper-sheathed Round rod installed vertically (15) 2000
Steel with electrodeposited copper coating Round rod installed vertically 14 250e
Round wire installed horizontally (8) 70
Strip installed horizontally 90 3 70
Stainless steela Stripb or shaped strip/plate 90 3
Round rod installed vertically 16
Round wire installed horizontally 10
Pipe 25 2
Copper Strip 50 2
Round wire installed horizontally (25)d 50
Solid round rod installed vertically (12) 15
Stranded wire 1,7 for individual strands of wire (25)d 50
Pipe 20 2
Solid plate (1,5) 2
Lattice plate 2
a Chromium ≥16 %, Nickel ≥5 %, Molybdenum ≥2 %, Carbon ≤0,08 %.
b As rolled strip or slit strip with rounded edges.
c The coating shall be smooth, continuous and free from flux stains.
d Where experience shows that the risk of corrosion and mechanical damage is extremely low, 16 mm2 can be used.
e This thickness is provided to withstand mechanical damage of copper coating during the installation process. It may be reduced to not less than 100 μm where special protective provisions to avoid mechanical damage of copper during the installation process (e.g. drilled holes or special protective tips) are taken according to the manufacturer’s instructions.
NOTE. Values in brackets are applicable for protection against electric shock only, while values not in brackets are applicable for lightning protection and for protection against electric shock.

The efficacy of any earth electrode depends on its configuration and upon local soil conditions. Earth electrode(s) shall be selected according to the soil conditions and the required value of resistance to earth.

When selecting the type and embedded depth of an earth electrode, consideration shall be given to possible mechanical damage and to local conditions to minimize the effect of soil drying and freezing.

Consideration shall be given to electrolytic corrosion when using different materials in an earthing arrangement. For external conductors (e.g. earthing conductor) connected to a concrete-embedded foundation earth electrode, the connection made from hot-dip galvanized steel shall not be embedded in the soil.

Earth electrodes shall not be directly immersed in water of a stream, river, pond, lake or the like (see also [3], 542.1.6).

Where an earth electrode consists of parts that must be connected together, the connection shall be by exothermic welding, pressure connectors, clamps or other suitable mechanical connectors.

NOTE. Connections made by a wrapped iron wire only are not suitable for protection purposes.

In Germany, every new building shall have a foundation earth electrode, erected in accordance with DIN 18014.

In Austria, in a TN system every installation shall have an earth electrode. In Austria a foundation earth electrode shall be erected according OVE/ÖNORM E 8014.

In Switzerland a foundation earth electrode shall be erected according to SNR 464113.

Earth Electrode Resistance

Where the earthing system incorporates an earth electrode as part of the installation, the electrode resistance to Earth shall be measured.

NOTE: Where a measurement of RA is not practicable, the measured value of external earth fault loop impedance may be used.

Earth electrode resistance depends on its dimensions, its shape and on the soil resistivity in which it is embedded. This resistivity often varies from one place to another and in accordance with depth.

Resistivity of a soil is expressed in Ωm: numerically, it is the resistance in Ω of a cylinder of ground with a cross-sectional area of 1 m2 a length of 1 m.
The aspect of surface and vegetation may give some indication of the more or less favourable characteristics of a soil for the implementation of an earth electrode. Where available results of measurements on earth electrodes installed in similar soil provides a better indication.

Soil resistivity depends on its humidity and on its temperature, both of which vary throughout the year. Humidity itself is influenced by the soil granulation and its porosity. In practice, the soil resistivity increases when humidity decreases.

Ground layers where water streams may go across, as found close to rivers, are rarely appropriate for the implementation of earth electrodes. In reality, these layers are composed of stony ground, are very permeable and become easily waterlogged by water itself purified by natural filtration and presenting high resistivity. Deep rods should be driven in order to reach deeper soils that may have better conductivity.

Frost considerably increases soil resistivity, which may reach several thousands of Ωm in the frozen layer. The thickness of this frozen layer may be 1 m or more in some areas.

Dryness also increases the soil resistivity. Drought effect can be found in some areas up to a depth of 2 m. Resistivity values in such circumstances can be of the same order as those occurring during times of frost.

Table D.54.1 gives information on resistivity values for certain types of soil [3].

Table D.54.2 indicates that resistivity may vary in large proportion, for the same type of soil [3].

Nature of groundResistivity Ωm
Marshy groundFrom some units to 30
Alluvium20 to 100
Humus10 to 150
Damp peat5 to 100
Malleable clay50
Marl and compact clay100 to 200
Jurassic marl30 to 40
Clayey sand50 to 500
Siliceous sand200 to 3 000
Bare stony soil1 500 to 3 000
Stony soil covered with lawn300 to 500
Soft limestone100 to 300
Compact limestone1 000 to 5 000
Cracked limestone500 to 1 000
Schist50 to 300
Mica-schist800
Granite and sandstone according to weathering1 500 to 10 000
Granite and very altered sandstone100 to 600
Table D.54.1 – Resistivity for types of soil

To enable a first approximation of the earth electrode resistance, a calculation may be made, using the average values indicated in Table D.54.2 [3].

It is obvious that calculations made from these values only give a very approximate result of an earth electrode resistance. After having used the formula given in Clause D.3 [3], the measurement of this resistance may allow an estimation of the average resistivity value of the ground. Such knowledge may be useful for further works done in similar conditions.

Nature of soilAverage value of resistivity Ωm
Slimy arable soil, damp compact embankment50
Poor arable ground, gravel, rough embankment500
Bare stony ground, dry sand, impermeable rocks3000
Table D.54.2 – Variation of the resistivity for different types of soil

Assessment of Earth Electrode Resistance

According to Clause D.3 (IEC 60364-5-54-2021):

a) Horizontally buried conductor

The earth electrode resistance (R) realized with a horizontally buried conductor (see 542.2.3 and Table 54.1 [3]), may be approximated from the formula:

R = 2 * ρ / L

where ρ is the resistivity of the soil (in Ωm) and L is the length of the trench occupied by the conductors (in m).

It should be noted that the laying of conductor with a sinuous path in the trench does not noticeably improve the resistance of the earth electrode.

In practice, these conductors are laid down in two different ways:

  • foundation earth electrode of the building: these earth electrodes are made of a foundation loop around the whole perimeter of the building. The length to be considered is the building perimeter;
  • horizontal trenches: conductors are buried at a depth of about 1 m in trenches dug for this purpose.

Trenches should not be backfilled with stones, cinders or similar materials, but with earth liable to retain moisture.

b) Buried plates

To maintain good contact of the two surfaces with the soil, full plates should preferably be arranged vertically.
Plates should be buried in such a way that their top edge is situated at approximately 1 m depth.
The resistance (R) of a buried plate earth electrode at a sufficient depth is approximately equal to:

R = 0.8 * ρ / L

where ρ is the resistivity of the soil (in Ωm) and L is the perimeter of the plate (in m).

c) Vertically buried electrodes

The resistance (R) of a vertically buried earth electrode (see 542.2.3 and Table 54.1) may be approximated from the formula:

R = ρ / L

where ρ is the resistivity of the soil (in Ωm) and L is the length of the rod or pipe (in m).

Where risk of frost or dryness exists the length of rods should be increased by 1 m or 2 m. It is possible to reduce the value of the earth electrode resistance by driving several vertical rods connected in parallel, separated from each other by one rod length, in the case of two rods, and by more if there are more than two rods.

Attention is drawn to the fact that, where extra long rods can be driven, as the ground is rarely homogeneous such rods may reach ground layers with low or negligible resistivity.

Metallic Pillar as Earth Electrodes

Metallic pillars interconnected by a metallic structure and buried at a certain depth in the ground, may be used as earth electrode.
The resistance (R) of a buried metallic pillar may be approximately calculated with the formula:

R = 0,366 * (ρ / L) * log10 (3L / d)

where:

  • L is the buried length of the pillar (in m);
  • d is the diameter of the cylinder circumscribed to the pillar (in m);
  • ρ is the ground resistivity (in Ωm).

A set of interconnected pillars located around a building has a resistance of the same order as that of a foundation earth electrode.
The eventual embedding of concrete does not prevent the use of pillars as earth electrodes and does not appreciably modify the earth electrode resistance.

Examples

The following are examples of earth electrodes which may be used:

  • concrete-embedded foundation earth electrode;
  • soil-embedded foundation earth electrode;
  • metallic electrode embedded directly in soil vertically or horizontally (e.g. rods, wires, tapes, pipes or plates);
  • metal sheath and other metal coverings of cables according to local conditions or requirements;
  • other suitable underground metalwork (e.g. pipes) according to local conditions or requirements;
  • welded metal reinforcement of concrete (except pre-stressed concrete) embedded in the earth.

The earthing arrangement shall not rely on a metallic pipe for flammable liquids or gases as the earth electrode and their buried length shall not be considered when dimensioning the earth electrode.

NOTE. This requirement does not preclude the protective equipotential bonding via the main earthing terminal (541.3.9) of such pipes for compliance with IEC 60364-4-41.

Where cathodic protection is applied and the exposed-conductive-part of an item of electrical equipment supplied by a TT system is directly connected to the pipe, a metallic pipe for flammable liquids or gases may act as the sole earth electrode for this specific equipment.

References

  1. IEC 60050-195-2021
  2. IEC 60364-4-41
  3. IEC 60364-5-54-2021
  4. BS7671