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Calculation of Earth Rod using BS 7430 and IEC 62305 Standards

Earthing plays a vital role in any substation. Earthing maintains the potential gradient throughout the substation while keeping step and touch potentials within safe limits in a substation and dissipates the fault current through the low resistance path into the earth. Earthing is done by combining earth electrodes, clamps, conductors and equipotential bonding bars. The equipotential bars provide lightning and transient over-voltage energy an effective low resistance path from the lightning protection system to the earth. Earth electrodes generally comprise two overlapping groups of buried, parallel, horizontal electrodes laid at right angles to each other, where the electrodes are bonded together at each intersection. The fundamental requirement for any electric structure or system at all voltages would be to install a well-designed earthing system. The national and international standards define that are applicable for earthing system design parameters for structures, electrical equipment and systems. These standards include:

BS EN 50522: Earthing of power installations exceeding 1kV ac
BS 7430: Code of practice for protective earthing of electrical installations
BS 7354: Code of practice for the design of high voltage open terminal stations
IEEE Std 80: IEEE Guide for Safety in AC substation grounding
ENA TS 41-24 Guidelines for the design, installation, testing, and maintenance of earthing systems in substations

 

BRITISH STANDARD BS 7430 enables guidance on the design of earthing of electrical installations includes:

  • Low voltage installation earthing and equipotential bonding for all applications like general, industrial and commercial buildings, locations with increased risk, rail systems, etc.
  • Earthing of generators and uninterruptible power supplies (UPS) supplying low voltage installations.
  • The interface between low and high voltage substation.

IEC/BS EN 62305 takes care of protection against lightning and switching transients. It focuses on the protection measures of metallic service lines (typically power, signal and telecom lines) using transient overvoltage or surge protective devices (SPDs) against both direct lightning strikes and more common indirect lightning strikes (often described as the secondary effects of lightning) and switching transients.

Electrodes are constructed by combining earth rods, earth plates, flat tape, stranded cable or any combination of the materials. There are three types of Furse earth i.e. copper bonded steel cored rod, Solid copper rod, and Stainless steel rod. The copper bonded steel cored rods are made by molecularly bonding 99.9% pure electrolytic copper onto a low carbon steel core. The copper bonded steel rod is used due to the strength, corrosion resistant properties and comparatively economic nature. The solid copper rods are made of low carbon steel with molecularly bonded pure electrolytic copper. They offer great resistance to corrosion and are used in applications where the condition of the soil is very aggressive or its high salts concentrate. Stainless steel rods are made of iron and chromium alloy with at least 10.5% chromium. The stainless-steel rods overcome many problems that occur due to galvanic corrosion that is caused by the burial of dissimilar metals in close proximity. These rods are highly resistant to corrosion. Solid copper rods and stainless-steel rods are highly resistive to corrosion but provide lower strength at a higher price. Copper bonded earth rods provide the installer with the best and most economical earth rods compared to the others. Earth rods not sheathed type, they have high tensile strength and can be driven by a power hammer to great depth. The couplings used in the earth rods are made from high copper content alloy that provides excellent corrosion resistance and high tensile strength.

 

According to BS 7430, an electrode or an earth rod placed in the substation is likely to be buried immediately adjacent to the plinth, which is to be surface mounted copper tape attached to the floor or at the low level around the internal wall. To minimize the touch potential, the copper earth mats or earth rods are installed in front of the switchgear where the substation operator stands.

 

The selection of the type of soil plays a vital role in the placement of the electrodes. Generally, wet marshy ground, clay soil, clay and loam mixed soil with varying proportions of sand, gravel and stones or damp and wet sand are preferred for placing the electrode. In highly resistive locations where long-term performance is required, it is necessary to utilize conductive concrete to improve the contact of the earth resistance around an earth rod. Therefore, all precautionary measures should be taken to ensure that the electrodes remain in contact with the concrete and do not shrink or swell away after drying out. The resistance o the electrode depends upon the electrical resistivity of the soil. Soil temperature has some effects on the upper layers of the earth’s strata. Hence, the electrode system that is less than 0.5m below the ground level is considered ineffective.

 

The predetermination of the resistance or the impedance of the electrode can be measured using the four-probe method or Weber method. In homogenous soil conditions, the average soil resistivity can be measured as:

probe method or Weber method

Where,
a - Spacing between electrodes
R - Resistance measured between the electrodes

Environmental conditions have major impacts on the resistivity of the earth as the soil temperature rises with respect to the decrease in the resistivity of the soil.

The approximate resistance of earth rods can be calculated as:

resistance of earth rods

Where,

resistivity of the soil  - resistivity of the soil in ohmmeter
L - length of the reinforcing rod below ground level
d - diameter of the rod

 

The resistance to earth of the earth rod is calculated by assuming that the vertical reinforcing rods are bound to the building structure or the earthing system. The effect of other bonding may be attached by wire ties or can be neglected. The earth rods are assumed to be equal spaced in a symmetrical pattern. The resistance of an electrode encased in low resistivity material.

For example - conducting concrete can be calculated as:

resistance of an electrode

Where,
resistivity of the soil - resistivity of the soil in ohmmeter
resistivity of concrete in ohm meter  - the resistivity of concrete in ohmmeter
L - length of the reinforcing rod below ground level
ẟ - thickness of the concrete between the rods and the soil
z - a geometric mean distance of the rod cluster

 

Earth rods can be arranged in different patterns, and the resistivity of the electrode can be calculated using different safety standards available like BS 7430 AND IEC 62305.

The earthing arrangement should be robust to ensure it lasts longer. The lifetime of the installation should be protected, from any mechanical damage and corrosion, so it carries the maximum expected current under both normal and fault conditions.

BS 7430, therefore, defines selection parameters for the earthing arrangement as the size and material for conductors, earth electrodes etc., and makes a clear requirement for the need for careful consideration of site conditions (soil composition and resistivity).

 

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