Electrical Properties of the Earthing System

I indicated that Earthing system in any installation is normally comprised of the following components:

1. Earth wells and accessories,
2. Earthing grid conductors,
3. Marshalling earth buses (earthing distribution buses),
4. Earthing wires and cables.

And, I explained all these components in the following Articles:

• Earthing System Components – Part One

• Earthing System Components – Part Two

• Earthing System Components – Part Three

Today I will explain the Electrical Properties of the Earthing System as follows.

You can preview the following Articles for more info:

• Introduction to Grounding System Design – Part One

• Introduction to Grounding System Design – Part Two
• Types of Earthing System – Part One
• Types of Earthing System – Part Two
• How to Select the Best Earthing System

Electrical Properties of the Earthing System Earthing systems should be constructed in such a manner and of such materials, that they perform correctly over the whole expected lifetime, at a reasonable construction cost. The electrical properties of earthing depend essentially on two parameters: Earthing resistance, Configuration of the earth electrode and favourable earth surface potential distribution, Adequate current carrying capacity, Long durability.

First:  Earthing Resistance Earthing resistance determines the relation between earth voltage and the earth current value. Under this item, I will explain the following points: Basic components of earthing resistance, Earthing resistance values, Soil resistivity (ρ).

1- Basic Components Of Earthing Resistance The earthing resistance of a ground electrode has 3 basic components: The resistance of the ground electrode itself and the connections to the electrode, The contact resistance of the surrounding earth to the electrode, The resistance of the surrounding body of earth around the ground electrode. 1- The resistance of the ground electrode itself and the connections to the electrode: The resistance of the ground electrode and it's connection is generally very low, ground rods are generally made of highly conductive/ low resistance material such as copper of copper clad. 2- The contact resistance of the earth to the electrode: The Bureau of Standards has shown this resistance to be almost negligible providing that the ground electrode is free from paint, grease etc. and that the ground electrode is in firm contact with the earth. 3- The resistance of the surrounding earth: The ground electrode is surrounded by earth which is made up of concentric shells all having the same thickness. Those shells closest to the ground electrode have the smallest amount of area resulting in the greatest degree of resistance. Each subsequent shell incorporates a greater area resulting in lower resistance. This finally reaches a point where the additional shells offer little resistance to the ground surrounding the ground electrode.

1.2 Earthing Resistance Values Typically, the site engineer or equipment manufacturers specify a resistance-to-ground number. The National Electric Code (NEC) states that the resistance-to-ground shall not exceed 25 ohms for a single electrode. However, high technology manufacturers will often specify 3 or 5 ohms, depending upon the requirements of their equipment. For sensitive equipment and under extreme circumstances, a one (1) ohm specification may sometimes be required. Other codes and standards (IEC , BS or Australian ) for earthing (see below image) have other requirements for the earthing resistance values , it is important to know the standard or code requirements for each installation’s earthing systems under design. The calculation of the earthing resistance requires a good knowledge of the soil properties, particularly of its resistivity. Soil properties are characterised by earth resistivity, which changes over a wide range from a few Ωm up to few thousand Ωm, depending on the type of ground and its structure, as well as its humidity. As a result, it is difficult to calculate an exact value of earthing resistance. All relationships describing earthing resistance are derived with the assumption that the ground has a homogenous structure and constant resistivity. However, earthing resistance should not exceed the values required by guidance or standards under the most unfavourable climatic conditions (long dry weather, heavy frost). If there are no exact requirements, the earthing resistance should be as low as possible.

1.3 Soil resistivity Electrical resistivity is the measurement of the specific resistance of a given material. It is expressed in ohm-meters and represents the resistance measured between two plates covering opposite sides of a 1 m cube. Soil resistivity is the key factor that determines the resistance or performance of an electrical grounding system. It is the starting point of any electrical grounding design. 1.3.1 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 Moisture content 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. d) 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 Temperature 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.

Second: Configuration of the Earth Electrode The configuration of the earth electrode determines the potential distribution on the earth surface, which occurs as a result of current flow in the earth. The potential distribution on the earth surface is an important consideration in assessing the degree of protection against electric shock because it determines the touch and step potentials. Earth surface potential distribution should be such that the touch and step voltages do not exceed the permitted values. The most favourable earth surface potential distribution concepts have horizontal earth electrodes, especially meshed ones, whose surface potential can be controlled relatively simply. The potential distribution of vertical electrodes is the most unfavourable, with high values of touch potential. On the other hand, vertical electrodes can easily reach low earthing resistance with stable values, largely independent from seasons. Vertical electrodes are also used in combination with horizontal ones in order to reach lower values of earthing resistance. Please review Article " Types of Earthing System – Part One " for more information about available Configurations of the Earth Electrode.

Third:  The Current Carrying Capacity It is the highest current value that can be carried through the earth electrode to the earth, without any excessive heating of the electrode elements and the surrounding soil itself. At too high current values and current densities, the water in the soil at the soil-electrode interface evaporates, leaving dry soil with high resistivity.

Fourth:  The Durability The durability of the earth electrode is its life from construction up to the time when, due to the corrosion of metallic parts, electrical continuity is lost. The durability of an earth electrode should exceed the expected lifetime of the installation. For the majority of power installations, lifetime can exceed 25 years and for power lines, 35 – 50 years. The earthing system should be included in repair and maintenance cycles. The durability of an earthing system depends mainly on its capability to withstand corrosion. The earth electrodes, being directly in contact with the soil or with water, operate in corrosive conditions. There are three main factors determining the rate of corrosion of metal objects in the soil: DC currents in the earth, Chemical contamination of the soil, Electrochemical (galvanic) phenomena between various metals located in the soil A- DC Currents In The Earth Corrosion due to DC currents occurs mainly in the neighbourhood of DC networks, (for example, DC railway supplies). There are standards and regulations covering the requirements in such cases. B- Chemical Contamination Of The Soil Corrosion due to chemical substances in the soil is not normally of great importance, affecting only those systems in chemical factories or near the ocean. In such cases, earth electrodes should be constructed from metals resistant to the specific chemical corrosion. In order to minimize the chemical corrosion it is recommended, in some cases, to measure the pH of the soil. For an alkaline soil (pH>7) copper electrodes are recommended, and for acid soil electrodes made from aluminium, zinc or galvanised steel are preferred. C- Galvanic Corrosion Galvanic corrosion is caused by a DC current flowing in a circuit supplied by the electrochemical potential difference between two pieces of metal in the damp soil, which in this case acts as an electrolyte. Of the commonly used electrode metals copper has the lowest potential. Other metals have a positive potential with respect to the potential of copper (Table 5). Metal Electrochemical potential to a copper electrode [V] Zinc or steel covered by zinc 0,9 – 1,0 Steel 0,4 – 0.7 Steel in concrete 0 – 0,3 Table 5 - Values of electrochemical potential of various metals to the copper electrode This small DC current flowing continually causes the metal ions from anode to flow to the cathode. Thus, metal is lost from the anode and builds up on the cathode. From this point of view, favourable metal combinations can be deduced. For example, steel covered by copper is a favourable solution because the amount of copper remains the same. An opposite example is steel covered by zinc, where zinc is always the anode and its amount continually diminishes. Note that the electrochemical potential of steel embedded in concrete is very close to that of copper. Thus, steel constructions in building foundations are cathodes in relation to other steel or zinc objects located in the soil (not only earth electrodes, but also, for example, water pipes). This means that the large foundations cause significant corrosion of these metal objects due to electrochemical corrosion. Mechanical strength and corrosion conditions dictate the minimum dimensions for earth electrodes given in Table 6 in below image to be: Copper 16 mm2, Aluminium 35 mm2, Steel 50 mm2,

In the next Article, I will explain Steps of Earthing Systems’ Design Process. Please, keep following.

Back To Course EE-5: Grounding System Design Calculations