Functions and Advantages of Different Earth Rods
Earth rods or electrodes or grounding rods and their fittings are used for effective Earthing systems in overhead and underground electricity distribution and transmission networks. These are used singularly or in groups to:
- Form a ground field.
- Create a path to dissipate static discharge voltages (lightning or other forms) to the earth.
- Provide the required interface to ground in all soil conditions and provide high-fault current capacity on low, medium, and high voltage substations, towers, and power distribution applications.
The main functions of earth rods are:
Electrical equipment has exposed or covered metal parts with protective insulation. Earth rods protect humans from direct or indirect contact from exposed metal parts with damaged insulation even after the insulation wears off.
Protection to LV Installation
Reduces common-mode disturbances external to the LV installation, for example, 50/60 Hz overvoltage in the event of:
- MV/LV transformer breakdown
- Overvoltage due to lightning
In the case of LV breakdown, the potential of the power system rises concerning the earth. Without grounding, it creates a risk for the equipment in the TT system and/or for the workers in the TN system. Besides, during a lightning strike, the power system encounters pulsating overvoltage on all live conductors. It may result in high EMC risk; therefore, the Earthing system is required.
Design, Strength, and Effectiveness
For grounding, a metallic object is electrically connected to the earth using an earth electrode system. The strength of fault current varies based on the Earthing system in use. Hence with each electrical unit, the contact voltage safety convention needs to be followed. For example, UL (50 V) essentially be used in AC. The exposed metal parts of electrical equipment are connected to the PE protective conductors, which are also connected to the earth, forming the Earthing arrangement.
According to the National Electric Code (NEC) guidelines, the grounding electrodes must be under 25-ohms resistance-to-ground (Earth). Aluminium electrodes are not allowed in grounding [standards 364 and NF C 15-100 (1923).
Correctly specified, earth rods achieve safe, reliable, and long-term Earthing protection. This specification must be on the corrosive condition and electrical conductivity of the ground condition. The mechanical strength of the rod must withstand the abrasion and stress during installation. The head of the earth rod should not “mushroom” or spread when driven.
Make use of copper couplers to interconnect several earth rods for the required driving depth. They provide permanent electrical conductivity. The longer copper earth rods access lower resistivity soils at a lower level. The most effective are the vertically-driven earth rods. They are used in small area substations or when there are low soil resistivity ground conditions. For the latter, the rod can penetrate and lies beneath a layer of high soil resistivity.
An earth rod effectiveness is tested by placing a current probe around the conductor going to the earth or the ground rod. If there is a flaw, the current flow is visible on the conductor (leakage current).
Types of Grounding Rods, usages, and Capabilities
Standard driven rods or copper-clad rods are common used as grounding devices. These are made of relatively low carbon steel and are 8 to 10 feet long, and have 254 Microns as per UL Standard. NEC mandates that that driven rod must be at least 8 feet in length, and it must be in direct contact with the soil.
These thin copper plates are placed in direct contact with the earth under poles or supplementing counterpoises. NEC advises that these plates should have at least 2 ft. of surface area exposed to the surrounding soil. The ferrous materials must be at least .20 inches thick. The non-ferrous materials (copper) need a thickness of only .060 inches. These should be buried at least 30 inches below grade level. The surface area of grounding plates increases over a driven rod; the zone of influence is relatively small. The zone of influence of a grounding plate can be as small as 17 inches.
Ufer Ground or Concrete Encased Electrodes for surface and surroundings.
These electrodes channelize the faulty current to the earth through the concrete. These foundations are used, when the concrete of the structure is in direct contact with the earth, i.e. there are no plastic barriers. One example is the concrete surrounding ammunition bunkers, building foundations. For building foundations, they consist of any concrete-encased electrode, like, the rebar (0.500 inches in diameter) or a wire or wire mesh in the concrete. There would be a direct metallic connection from the service ground to the rebar buried inside the concrete. The process depends on the conductivity of the concrete and the large surface area, which usually provides a grounding system to handle very high current loads.
- As per NEC guidelines, they use 20 feet (minimum) long No. 4 AWG copper wire that is encased in a 2 inches (minimum) of concrete; the zone of influence is not increased. So the resistance to the ground is only slightly lower than the wire without the concrete. As concrete retains a substantial amount of water, the moisture in it superheats and expands rapidly. A high current may cause steaming creating cracks. Cracked concrete pieces do not let the copper wire contact the surrounding soil. It increases the resistance-to-ground of the electrode. Therefore, relatively small electrical faults may damage the electrodes, which is risky to the building.
- Besides, Ufer groundings are impossible to test unless there is a fault with a high current. Isolating the concrete slab to test resistance-to-ground is nearly impossible.
Water pipes with plastic insulators are in use as grounding electrodes for a long. Plastic insulators are mostly used by city water departments in the pipelines. It prevents the flow of current and reduces the corrosive effects of electrolysis. However, this type of earthing cannot be tested. Also, the plastic or tar coating makes them unreliable. Therefore, NEC guidelines mandate that at least one additional electrode must be installed with the water pipe.
Chemically charged Pipe Electrodes (ACRE)
The electrolytic electrodes have solved many drawbacks present in the other rods. This active grounding electrode consists of a hollow copper shaft filled with natural earth salts and desiccants. Their hygroscopic nature draws moisture from the air. This moisture mixes with the salts to form an electrolytic solution that continuously seeps into the surrounding backfill material, keeping it moist and high in ionic content.
The electrolytic electrode is installed into an augured hole. It is also backfilled with a highly conductive speciality product to protect the electrode from corrosion and improve its conductivity. The electrolytic solution of salt, desiccants, and the specific backfill material together provide a solid connection between the electrode and the surrounding soil; Itis free from the effects of temperature, environment, and corrosion. This active electrode is the only grounding electrode that improves with age.
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