Electrical wiring &types, Earthing , fuses and its types

Electrical wiring &types, Earthing , fuses and its types

1. Earthing Earthing refers to connecting the electrically conductive part of an electrical equipment to the ground through earth plate or electrode with negligible resistance for safety. It helps in protecting the equipment and provides a return path for the leakage or fault current to pass to the ground through electrode. Galvanised iron is used as the material for earthing purpose.

2. Functions of Earthing ( i ) It ensures that electrical equipment through which the current flows does not rise to a potential greater than its designed value (ii) It provide safety to human lives. It is noted that the neutral of the supply system is also earthed.

3. Need for Earthing ( i ) Protect the human lives and electrical equipment from fault current. (ii) Maintain the voltage at a constant level even when a fault occurs in any system. (iii) Protect the electrical equipment and buildings from over voltages occurring due to lighting. (iv) Provide a return path for the fault current occurring in the system. (v) Prevent fire in electrical systems

4. Terms Related to Earthing Earth: The connection between electrical systems to the buried plate in the ground is called Earth. Earthed: The electrical system that is connected to the ground through an electrode is called earthed device or earthed. Solidly Earthed: If there is no protection device like fuse, circuit breaker, etc., between an electrical system and the ground, then it is called solidly earthed. Earth Electrode: The conductor that is buried inside the ground to provide safety for the electrical system is called Earth Electrode and it is available in different shapes. Earthing Lead or Earth conductor: The connection between the electrical system and the earth electrode is done with the help of a conductor wire or strip called Earthing lead. Earth Continuity Conductor: The conductor wire or strip that is used to connect different electrical devices is called earth continuity conductor. It is also defined as the conductor wire used in connecting earthing lead and electrical devices and it is available in different shapes. Sub Main Earthing Conductor: The conductor that helps in connecting switch board and distribution board is called sub main earthing conductor. Earth Resistance: It is defined as the total resistance between earth electrode and earth. Also, it is defined as the algebraic sum of the resistances of earth continuity conductor, earthing lead, earth electrode and earth.

5. components of earthing system Earthing continuity conductor Earthing lead or earth conductor Earth electrode

6. Methods of Earthing ( i ) Pipe earthing and (ii) Plate earthing.

7. Pipe Earthing In this method, a galvanised iron (GI) pipe is used as an earth electrode. The length and diameter of pipe depend upon the current to be carried and soil type to which earth electrode is buried. According to I.S.I standard, the diameter of the pipe used for earthing should be greater the 3.81 cm and length of the pipe used should be 2 m, 2.75 m and 1.75 m for ordinary, dry and rocky soils respectively.

8. Pipe Earthing The GI pipe is vertically placed and buried in the wet ground. The depth at which the GI pipe is to be buried depends on moisture level of the ground. According to I.S.I standard, the depth should be 4.75 m and it can be less if the moisture content of the ground is sufficient. In the underground, broken coke or charcoal piece is used to surround the GI pipe for a distance of 15 cm. It is necessary since coke along with salt helps in decreasing the earth resistance. Generally, alternate layers of coke and salt are used. In summer, to prevent the increase in earth resistance due to decrease in moisture level, buckets of water is poured to the funnel connected to GI pipe.

9. Plate Earthing In this type of earthing, a GI or copper plate is used as an earth electrode. The size of GI plate should be greater than 60 cm * 60 cm * 6.35 mm and the size of copper plate should be 60 cm * 60 cm * 3.18 mm. These plates with their face vertical are buried inside the ground so that the distance between the ground level and plate is greater than 2 m. If the moisture content of the place is high and it should be 0.6 m away from building foundations. The plate shall be completely covered by 15 cm of coke and salt.

10. Factors to be Remembered in Providing Earthing The factors to be considered while providing earthing are: Distance: The distance between earth electrode and electrical system should be greater than 1.5 m. (ii) Cross Section of the Earthing Lead : The cross section of the earthing lead should be greater than half the cross section of the main wire or conductor. Its minimum size should not be less than 12.97 sq. mm. (iii) Cross Section of the Earth Continuity Conductor : The size of the earth continuity conductor should be greater than 2.894 sq. mm. (iv) Electrode: The material used for earth electrode and earth lead should be same and it should always be placed in vertical position. The size of the electrode varies with respect to load and insulation material. (v) Earth Resistance: The maximum value of the earth resistance should be 5 W. The earth resistance depends on the electrode area in contact with the ground, coal, salt and quantity of earth.

11. Circuit breaker A switching device, which can be used to make or break a circuit manually, automatically or with the help of remote control, under different conditions i.e., normal and under faulty conditions, is known as a circuit breaker. Special attention must be given while designing a circuit breaker, to safely interrupt the arc produced during its operation.

12. The fixed and moving contacts, which are called electrodes, exist in a normal circuit breaker. The medium in which these contacts are placed could either be oil or air. When the power system is operating normally, these contacts will remain closed and will not open automatically until a faulty condition occurs in the system. Whenever a fault occurs in the system, these contacts can be opened either manually or automatically or by using a remote control. During a faulty condition, due to energization of trip coils of the circuit breaker, the moving contact is pulled apart, which opens the circuit and an arc is formed between these contacts.

13. An arc develops between the fixed and moving contacts, when a fault occurs in the power system. The faulty current in the power system will continue to flow until this arc is extinguished or stopped. Therefore, the formation of the arc not only delays the interruption of faulty current, but also generates huge amounts of heat, which might cause damage to the power system or the circuit breaker itself. Hence, it is necessary to extinguish the arc developed in a short interval of time, so that the magnitude of heat generated will not exceed a maximum value. (High-resistance method and Low-resistance method or current zero method)

14. Classification of Circuit Breakers Circuit breakers are classified based on: medium in which the circuit breaker operates ● Oil circuit breaker ● Air Blast circuit breaker ● SF6 circuit breaker ● Vacuum circuit breaker actuating signal in which it works ● Spring-operated circuit breaker ● Pneumatic circuit breaker ● Hydraulic circuit breaker

15. Construction type ● Low-voltage circuit breakers (< 1 kV) ● Medium-voltage circuit breakers (1-72 kV) ● High-voltage circuit breakers (> 72 kV Based on voltage level: ● Outdoor circuit breaker ● Indoor circuit breaker

16. Fuse A fuse is a short piece of wire or a thin strip of metal, which is inserted in series to the circuit. When the fault current flows through the fuse for a sufficient time, it melts the fuse, thus isolating the circuit. Under normal operation, the fuse is kept at a temperature below the melting point of the material used, which helps in carrying the normal current without any rise in temperature. But when fault occurs in the power system due to a short circuit or when an overload current, which is greater than the normal current, flows through the fuse, this fault current will increase the temperature above the melting point of the material used for the fuse. Hence, the material melts or blows, thereby isolating the healthy part and protects the circuit. The magnitude of excessive current flowing in the circuit is an important factor in deciding the time taken for melting or blowing out the fuse. Greater the fault current, lesser the time required to melt or blow out the fuse.

17. Advantages and Disadvantages Advantages : 1. Cheapest form of protection device. 2. Requires no maintenance. 3. Operation of fuse is completely automatic. 4. Easily breaks a large amount of fault current. 5. Pollution-free protection device i.e., does not create any smoke or noise. 6. Suitable for over-current conditions due to its inverse current–time characteristics. 7. Requires less time for isolating the faulty part of the circuit. Disadvantages 1. Rewiring or replacing a fuse takes a considerable time. 2. Discrimination between fuses connected in series is not possible. 3. Correlation of the characteristics of fuse with the protected device is not always possible.

18. Characteristics of Fuse Element Materials Low melting point, e.g., tin, lead High conductivity, e.g., silver, copper Least reactive to oxidation, e.g., silver Affordable, e.g., lead, tin, copper

19. Materials used as fuse Lead, tin, copper, zinc and silver are the most commonly used fuse materials. Tin, or an alloy of lead and tin (0.37 and 0.63 respectively) is used as a fuse element material, where the rating of current is up to 10 A. For larger currents, copper or silver is used as fuse element material. Usually, the copper is tinned to prevent oxidation effect. Zinc, in strip form, is used where a considerable time-delay is required

20. Classification of Fuses 1. Low-voltage fuses Semi-enclosed re- wireable fuse High rupturing capacity (HRC) cartridge fuse with and without tripping device 2. High-voltage fuses Cartridge type Liquid type Metal-clad fuses

21. Comparison Between Circuit Breaker and Fuse