Types of Earthing System – Part One

In Article " Introduction to Grounding System Design – Part One ", I explained the following points:

• Differences between Grounding and Earthing Terms,
• Differences between Grounded and Grounding systems,
• Differences between Bonding and Grounding,
• Grounding and bonding systems definitions.

Also, In Article " Introduction to Grounding System Design – Part Two ", I listed the Factors that must be considered in the construction of the earthing system, which were:

1. Function,
2. Type,
3. Electrical properties,
4. Construction material.

And, I explained the Functions of the Earthing System in this Article.

Today I will explain the different Types of Earthing Systems as follows.





Types of Earthing System (on LV Systems and Within Premises)

Different Types Of Earthing Systems the Earthing systems can be divided according to the following factors: Function, System size, Neutral point connection to earth,  Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts.

Types of Earthing Systems according to its Function Generally, the earthing systems can be divided to Six types according to function as follows: Static grounding, Equipment grounding, System grounding, Lightning grounding, Electronic (including computer) grounding, Maintenance safety grounding. All of these systems are installed similarly. However, their purposes are quite different. Some of the systems carry little or no current. Others carry small to moderate currents at 50 or 60 Hz. Still others must be able to carry currents over a very broad range of frequencies in order to be considered effective. In the following paragraphs, I will discuss each type in detailed.

1- Static Grounding A static grounding is a connection made between a piece of equipment and the earth for the purpose of draining off static electricity charges before a spark-over potential is reached. The Static grounding is applied for more than just the comfort of the equipment operator. The possibility of an explosion ignited by an electrical spark must be considered. Dry materials handling equipment, flammable liquids pumps and delivery equipment, plastic piping systems, and explosive storage areas all need static grounding protection systems installed and functioning properly. Static grounding systems are generally not called upon to conduct much current at any given frequency. Smaller gauge, bare conductors, or brushes with metallic or conductive bristles make up most parts of the static grounding system.

2- Equipment grounding "Safety grouding" An equipment grounding pertains to the interconnection and connection to earth of all normally non-current carrying metal parts. This is done so the metal parts with which a person might come into contact are always at or near zero volts with respect to ground thereby protecting personnel from electric shock hazards. Equipment grounding consists of grounding all non-current carrying metal frames, supports, and enclosures of equipment. All these metallic parts must be interconnected and grounded by a conductor in such a way as to ensure a path of lowest impedance for the flow of ground fault current from any line to ground fault point to the terminal at the system’s source. An equipment grounding conductor normally carries no current unless there is an insulation failure. In this case the fault current will flow back to the system source through the equipment grounding conductors to protect personnel from electrical shock. The equipment grounding conductor must never be connected to any other hot lines. Equipment grounding systems must be capable of carrying the maximum ground fault current expected without overheating or posing an explosion hazard. Equipment grounding may be called upon to conduct hundreds to thousands of amperes at the line frequency during abnormal conditions. The Equipment grounding system must be sized and designed to keep the equipment surface voltages, developed during such abnormal conditions, very low. An example of this system is the bare copper wire (green conductor) connected to the frames of electric motors, breaker panels, outlet boxes, etc. Electrical supporting structures such as metal conduit, metal cable trays, or metal enclosures should be electrically continuous and bonded to the protective grounding scheme. Continuous grounding conductors such as a metallic raceway or conduit or designated ground wires should always be installed from the ground grid system to downstream distribution switchboards to ensure adequate grounding throughout the electrical distribution system. Part of the equipment ground is also formed by the switchgear ground bus.

3- System grounding "Functional earthing" A system ground refers to the condition of having one wire or point of an electrical circuit connected to earth. This connection point is usually made at the electrical neutral although not always. The purpose of a system grounding is to protect the equipment. This ensures longer insulation life of motors, transformers, and other system components. A system grounding also provides a low impedance path for fault currents improving ground fault relaying selectivity.  In a properly grounded system the secondary neutral of a power transformer supplying a building or facility is connected to a transformer grounding electrode. The transformer neutral is a part of the service entrance point that bonds to the grounding electrode system of the building. According to the National Electrical Code (NEC), NFPA 70, articles 250-81 and 250-83, metal underground water pipes, metal building frames, encased electrodes, rods, and plates are among the items that can make up the grounding electrode system of a building. The NEC article 250-83 requires that the size of the grounding electrode iron or steel rod must be at least 5/8 inches in diameter and driven eight feet deep. The resistance of the electrode to ground cannot exceed 25 ohms (NEC 250-84). Otherwise a second electrode should be added, and the distance between the two electrodes must be at least six feet. However, in some systems the 25 ohms resistance value cannot achieve the goals of grounding. They require ground resistance values below ten ohms. If the main building load is composed of computers or sensitive electronic equipment, the earth ground resistance should not exceed five ohms.

4- Lightning protection grounding Main lightning protection grounding requirements are dependent upon the structure, component, or system to be protected. This system will be discussed later in separate Articles.

5- Electronic and computer grounding Grounding for all electronic systems, including computers and computer networks, is a special case of the equipment ground and the system ground carefully applied. In fact, grounding systems for electronic equipment are generally the same as for system ground with an additional requirement, which is the degree of performance required. Electronic equipment grounding systems must not only provide a means of stabilizing input power system voltage levels, but also act as the zero voltage reference point. However, the need to do so is not restricted to a low frequency of a few hundred hertz. Grounding systems for modern electronic installations must be able to provide effective grounding and bonding functions well into the high frequency megahertz range. Effective grounding at 50-60 Hz may not be effective at all for frequencies above 100 kilohertz. There are several aspects to the requirement for good grounding performance for electronic equipment, all of which are due to electrical circuit behavior. Good electronic system grounding performance is achieved with a properly laid out distribution of multipoint, well bonded grounding connections. This system can use bare, braided, sheet, or stranded copper conductors for grounding or bonding functions. This system requires conduit and equipment enclosure bonding at all junction points. In other words, simple metallic contact between the enclosures, wiring conduits, and power panels is not enough. The multipoint bonding provides low impedance grounding for the electronic equipment. The low impedance between the separate items of electronic equipment keeps the noise voltages at or near zero between them and, therefore, provides an “equipotential plane.” This system is much easier to inspect and test. No special requirements must be met during modifications or expansion of the electrical system. All power panels and all supply transformers feeding an installation with this type of grounding system must be grouped and bonded together using short lengths of bare, braided, sheet, or stranded copper conductors in order to achieve the effective high frequency grounding performance described above. A single area of power entry with a large equipotential ground plane and short equipment grounding conductors forms the preferred grounding system for large automated data processing (ADP) and computer applications.

6- Maintenance safety grounding Grounds used for maintenance work are usually intentional, but temporary, connections between equipment power conductors and ground. These connections are always applied after the power source has been turned off and the circuit(s) have been tested and are known to be de-energized. The ground is intended to protect maintenance personnel from an inadvertent re-energization of the circuit. The ground is removed after maintenance operations have been completed.

Types Of Earthing Systems According To Its Size Generally, the earthing systems can be divided to Two types according to system size as follows: simple complex.

1- Simple Earthing Systems Simple consist of a single ground electrode driven into the ground. The use of a single ground electrode is the most common form of grounding and can be found outside your home or place of business. This single electrode can be: 1- Single Ground Rod One single ground electrode may be sufficient for an electrical installation in a built up area where the local supply authority utilizes a multiple or common multiple earth neutral system. However, it may not provide adequately low impedance for lightning current injection. 2- Single Strip End Connected This is a common option for installations where, because of rock, driving an electrode is impractical. It is not recommended for lightning protection systems as there is only one path. Very high ground voltages will be experienced at the injection point. 3- Single Strip Center Connected Since the connection to the strip is at the center, any fault/injection current travels in two directions. This layout has lower impedance, but it is generally not adequate for lightning protection systems.

2- Complex grounding systems Complex grounding systems consist of multiple ground rods connected, mesh or grid networks, ground plates and ground loops. These systems are typically installed at power generating substations, central offices and cell sites. Some types of complex grounding system are: 1- Radial Grounding, Single Radials A design that is well suited to lightning protection in areas of medium resistivity. The radials can run to 100 feet in length. 2- Radial Grounding, Multiple Radials Crows foot design. Well suited to lightning as it allows energy to diverge as each conductor takes a share of the current, offering lower impedance. Voltage gradients leading away from the injection point will be lower, reducing danger from step potentials. 3- Equipotential Mesh Electrodes Minimize the risk of step and touch potential hazard by positioning a mat and bonding it to the structure or operating handle at locations where personnel may be required to operate switchgear or stand in the course of their duties. Low ground impedance. 4- Grid Electrodes Grounding for installations where there is concentration of electrical equipment, such as electrical substations, are often designed to meet a specific value of resistance (typically 1 ohm). Under fault conditions, a grid can dissipate currents over a large area. 5- Grid with Ground Rods It may be advantageous to add ground rods to the grid. In doing so, it may be possible to access a low resistivity soil layer. Care must be taken to ensure each ground rod is spaced at least twice the installation depth. 6- Ring Electrode Installations, including communications huts, pad mount transformers and fences surrounding high voltage installations, are generally surrounded by a ground ring. This practice also reduces the hazard of step and touch potential.

In the next Article, I will continue explaining Other Types of Earthing Systems. Please, keep following.

Back To Course EE-5: Grounding System Design Calculations