(2) Earthing lead or Earth Conductor
(3) Earth Electrode
(4) Distance of Earth Pit from a Building
(5) Requirements for Earthing (1) Earth Continuity Conductor The conductor by which an electric appliance or instrument or metal sheath of conductor of a wiring system remains connected to earthing lead is known as Earth Continuity Conductor. With the help of this wire earth continuity of a complete wiring system and earthing of appliances are maintained. In case of conduit wiring the metal conduit and in base of metal sheathed wiring the outer sheath may be used as earth continuity conductor. Similarly, armouring of an armoured cable may also be used for the same purpose. But in case of cleat, casing, C.T.S. or P.V.C. wiring a separate earth wire is to be drawn everywhere along with the wiring line.
The following articles of a wiring system are to be connected to earth continuity conductor. The outer metallic covers of electric supply line and appliances are to be earthed. In all junction boxes, switch boards and similar other places where a wiring line terminates, special arrangement must be provided to maintain electrical continuity of earth continuity conductor. Such conductor is sometimes kept within the same sheath along with current carrying conductors, sometimes again within the cover of flexible cord. (2) Earth Lead or Earth Conductor The earth conductor is the conductor, which provides connection between earth electrode and earth connecting point (Main Earth Point). The earthing conductor must be short and straight with the minimum number of joints. Two forms of earthing conductors, depending upon the load, are commonly used. These are copper wire and copper strip. Copper strip is used on very large installations. The usual practice in Pakistan is use to hard drawn copper wire. In case of copper wire duplicating earthing conductor should be run up to the earth electrode in order to increase the protection of installation. If there are two earth plates there should be four earthing conductors. When copper wire is installed, it should be enclosed in the G.I Pipes. This gives protection against mechanical damage and corrosion and also can b used to guide water to the plate in order to keep the plate and surrounding artificially damped. (3) Earth Electrode An earth electrode provides connection between the installation metal work and general mass of earth. For domestic installation main water pipe was used as earth electrode. Fig installation (over 21 kW) has to have its own earth electrode. The most common type of electrode is plate of either galvanized iron or copper. Cast iron pipes can also be used in place but are not as efficient as plates. The main purpose of an electrode is to provide good conductivity with earth at all time. The way is to install the electrode well below water level. The plate should be placed in an upright position and surrounded by a bed of at least on ft. of charcoal mixed with lime or salt. The plate must rest, at least one foot below the permanent water level. (4) Distance of Earth Pit from a Building The distance of an earth pit from the nearest building shall not be less than 1.5 metres (5 ft). Each and every earth wire shall be connected to an earth electrode. If the phase wire or live wire of a consumer’s circuit comes in contact with a bare conductor, the earthing arrangement shall be such that, the circuit current will rise up to 3 1/2 times the rated current of the main fuse or 1 1/2 times the setting of the over load earth leakage circuit breaker, As a result the fuse will melt or the circuit breaker will trip. After the completion of earthing arrangement, test shall be carried out to see that the arrangement satisfies this condition, otherwise an earth leakage circuit breaker is to be installed there. Besides, so long the leakage of current to earth continues, the potential difference between bare conductor and consumer’s earth terminal shall not exceed 40 volts (R.M.S.) (5) Requirements for Earthing A complete protective system consists of two parts-a protective device connected in the supply conductor (such as re-wireable fuse, H.R.C. fuse, or overload circuit breaker). And a low resistance earth return path bonded to all exposed metal work and earth. A protective system must e able to deal efficiently with either one or all of the possible circuit faults shown in figure. Fire can be caused by any of these faults. Shock is usually caused by d) or e).
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