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Grounding
Professor Ahdab Elmorshedy

The objective of a grounding system are: 1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential. 2. To assure correct operation of electrical/ electronic devices. 3. To prevent damage to electrical/electronic apparatus. 4. To dissipate lightning strokes.

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5. To stabilize voltage during transient conditions and to minimize the probability of flashover during transients.

6. To divert stray RF energy from sensitive audio, video, control, and computer equipment.

1. CLASSIFICATION OF ELECTRICAL SYSTEMS An electrical system comprises a source of energy and an electrical installation. According to the relationship between the source and Earth and between the exposed conductive parts and Earth, a system can be classified as follows:

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a. TN System The system has one or more points of the source of energy directly earthed and the exposed and extraneous conductive parts of the installation are connected only by means of protective conductors to the earthed point(s) of the source.

Extraneous: not forming an essential or proper part:

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TN systems are subdivided into the following: TN-C systems where the neutral and protective functions are combined in a single conductor throughout the system. TN-S systems where there are separate neutral and protective conductors throughout the system. TN-C-S systems where the neutral and protective functions are combined in a single conductor but only in a part of a system.

b. TT System The system has one or more points of the source of energy directly earthed and the exposed and extraneous conductive parts of the installation are connected to a local earth electrode that are electrically independent of the source earth(s).

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c. IT System The system has the source either unearthed or earthed through a high impedance and the exposed conductive parts of the installation are connected to an electrically independent earth electrode

2. PURPOSE OF GROUNDING The purpose of grounding is to connect together all metalwork, other than that intended to carry current, to the earth, so that dangerous potential differences cannot exist, either between different metal parts, or between metal and earth.

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The system is connected to earth at the secondary winding of the supply transformer, where one conductor, which is usually the neutral, is connected to an earth electrode, buried in the mass of earth.

Advantages of Grounding The whole system is tied to the potential of the general mass of earth, and cannot 'float' at another potential. For example, we know that the neutral of our system is at or very close to zero volts (reference potential) and not above or below it when becoming charged.

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By connecting to earth metalwork not intended to carry current, a path is provided for leakage current which can then be detected, and if necessary, can be cut.

Disadvantages of Grounding Cost - the provision of a complete system of protective conductors, earth electrodes, etc., is expensive. Safety - the argument is made that complete isolation from earth will prevent shock from exposed conductive parts because there is no complete path for the shock current.

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This approach ignores the effect of leakage resistance and of phase-to-earth capacitance (the insulation behaves as the dielectric between the lines and earth). In many situations, the combined impedance of leakage resistance and earth capacitive reactance is low enough to allow significant shock current to flow.

GROUNDING FOR TN-S SYSTEM

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a. Principle The principle of 'earthed equipotential bonding and automatic disconnection of the supply in a TN-S supply system' is adopted here to explain the typical arrangement. In addition, the grounding practices are based mainly upon the IEE Wiring Regulation (BS7671) and the Code of practice for Grounding (BS7430).

This method is generally applicable to prevent the occurrence of a voltage of such magnitude and duration between simultaneously accessible conductive parts that danger could arise. With this method, the characteristics of the protective devices for automatic disconnection, the earth arrangements for the installation and the relevant impedances of the circuits concerned shall be coordinated.

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A system is satisfactorily earthed if the protective gear operates to remove danger in the event of a fault to any metalwork having a continuous metallic connection to the system neutral.

b. Earth Mat Earth mats are installed in each substation. The overall earth mat design including number and location is to meet the maximum allowable step and touch potentials. Earth electrodes are jointed together and brought out to be the principle earth conductor. A bolted copper link is normally provided for connecting and disconnecting the principle earth conductor from the grounding network to facilitate testing.

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c. Earth Electrode Common types of earth electrodes or means of grounding include: (a) Vertical plates, rods & pipes (b) Horizontal strip or round conductor (c) Uninsulated metallic sheaths and armour of cables (d) Underground structural steelwork (e) Sheet steel piling and steel reinforcement of concrete piling

Water and service pipes, gas or drainage, should not be used as the means of grounding but should be bonded to the protective conductors. Material for an earth electrode shall be resistant to corrosion in the type of soil in which it will be used. Copper is one of the better and commonly used material. The resistance to earth of an electrode depends upon the size and shape of the conductor and the electrical resistivity of the soil in which it is installed.

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d. Grounding Network From the test link, a vertical grounding conductor rising main with an grounding terminal at a convenient location in each level is provided. From the grounding terminal in each level incorporating electrical equipment rooms, a copper tape protective conductor is routed through all electrical equipment rooms. For substation incorporating more than one earth mat, a grounding conductor of copper interconnecting the various earth mats is provided.

e. Equipotential Bonding Main equipotential bonding conductors shall connect to the earth network for the large extraneous conductive parts not already earthed by circuit protective conductor. They include, but do not limited to, the following: (a) all external metallic service pipes. (b) fire main pipes. (c) ventilation, air conditioning and chilled water ductwork. (d) steel floor plates. (e) cable tray.

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f. Grounding Conductor The grounding conductor of the earth mat should be (a) sufficiently sized and supported to carry without danger the greatest earth fault currents (b) sufficiently robust to withstand mechanical damage and corrosion in the ground, and (c) compatibility with the material of the earth electrode

g. Residual Current Device (RCD) RCDs are to be installed when earth fault current in a circuit is insufficient to cause operation of the overcurrent protective devices within the time required. RCD should be installed for every socket outlet circuit.

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The RCD should have the product of the rated operating current (in A) and the earth fault loop impedance (in ) not exceeding 50 V and be capable of disconnecting all the phase conductors of the circuit. RCDs for socket outlet circuit, in addition to requirements above, should have a rated residual operating current not exceeding 30 mA.

The RCD range offers both 2 and 4 pole devices in 30mA - 500mA trip sensitivities, with main current ratings of 25A, 40A, 63A and 100A The RCD protects against residual currents (earth leaks), where a small current may leak from the circuit due to bad insulation. Critically the residual current device will react to leakage currents as low as 30mA. This is an essential protection requirement.

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Residual Current Device (RCD)

h. System Grounding High Voltage Power Distribution (1) The secondary star point of the distribution transformers is solidly earthed by direct connection to the substation earth network with copper conductors of sufficient size. (2) The sheath and armour of all high voltage distribution cables is bonded to the earth bar or terminal of the switchgear panel or transformer at each termination.

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Low Voltage Power Supply (1) The star point of the service transformers is earthed by connection to the local earth network with copper tapes of appropriate size. (2) The phase and neutral conductors of the distribution circuits are provided with a circuit protective conductor. The protective conductors are bonded to the grounding terminal or earth bar of the switchboard, control panel or to which they are connected.

Equipment Grounding The objective of electrical equipment grounding is to ensure effective operation of the protective gear in the event of earth fault currents that might otherwise cause damage to property, and to protect against danger to life through shock due to installation metalwork being maintained at a dangerous potential relative to earth.

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HV & LV Switchgear (1) All switchgear should be provided with a copper earth bar of sufficient size, running the full length of the switchboard. (2) All metal parts, other than those forming part of an electrical circuit should be connected to the earth bar. The protective conductors of incoming and outgoing cables should be bonded to the earth bar. (3) Busbar and circuit grounding devices are provided for all HV switchgear.

4) The earth bars of HV and LV main switchboards are bonded to the substation earth network via separately routed copper grounding conductor connected at each end of the earth bar. (5) Fuseboards and MCB boards are equipped with a single earth terminal for connection of all conducting parts, which do not form part of an electric circuit. The terminal is bonded to the earth network by copper conductor.

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5. SPECIAL APPLICATION AND SITUATION OF GROUNDING a. Lightning Protection Protection of electrical equipment in a building against induced voltages and possible side flash can be provided by bonding the grounding conductor of the lightning system to the main grounding terminal of the electrical installation.

b. Clean Grounding If required, it should be directly connected to the principle earth conductor isolating link using insulated copper conductor and separated from the grounding network and power cables to minimize interference. The distribution of the insulated clean earth conductor should be in the form of star topology or alike in order to avoid electrical noise loop back phenomenon.

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c. Corrosion Due to Interconnection With Another Metallic Item The possibility of damage to underground services in the vicinity of earth electrodes, to which the grounding system is to be bonded, due to electrolytic action between dissimilar metals should be considered.

The rate of corrosion depends on the metals involved and to some extent on their relative surface areas. Galvanized steel is strongly electronegative to both copper and steel in concrete so that an earth electrode of bare galvanized steel should not be bonded to either of them.

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d. Grounding Cathodically Protected Structures Cathodic protection is normally applied to wholly or partly buried ferrous structures in order to counteract electrolytic corrosion. Such a protection system relies on the metalwork being maintained at a slightly more negative potential with respect to the ground than it would exhibit if it were unprotected.

If the cathodically protected structure has to be earthed for any reason, earth electrodes of bare copper should not be connected directly to the structure. The bare copper is strongly cathodic to ferrous materials and may require a quite unacceptable current drain if the protection is to be maintained.

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In the event of failure of the source of protective current, a copper electrode will accelerate the rate of corrosion of the structure. If copper electrodes are used, the connection to the structure should be made through a polarization cell. This will drain only a small current from the cathodic protection source, but will pass alternating current with a low voltage drop.

TESTING AND MONITORING Tests should be carried out on the earthing system to determine its effectiveness at the completion of installation and periodically after installation at a preset interval. The tests include the following items: (a) Earth resistance of each earth mat. (b) Earth fault loop impedance measurement. (c) Touch voltage and step voltage measurement. (d) Interference voltage measurement. (e) Inspection on the integrity of earthing conductors and all associated connections.

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