tailieunhanh - Industrial Control Wiring Guide 2 2010 Part 13

Tham khảo tài liệu 'industrial control wiring guide 2 2010 part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 11. EARTHING AND SCREENING . Earthing - the protective bonding circuit The earthing of electrical equipment is a protective measure designed to protect us from electric shock by preventing the exposed conductive parts of the equipment from becoming live should a fault occur. The exposed conductive parts are things like the metal cabinet housing the control circuits the metal trunking carrying cables from the cabinet to the machine and the machine itself. Technically they are all the conductive parts of the system that under normal conditions are not required to carry electric current. To see why this system works we have to look at how the electricity supply is connected into a factory building. The electricity is generated at the power station and then fed at very high voltages through the national grid. Eventually it will arrive at the local sub-station where it will be connected through a transformer to supply the factory. The voltage supplied to the input side may be 11 000 V or 33 000 V and it is the job of the transformer to reduce this to the required factory voltages of 415 V and 240 V. Notice that there are three lines marked L1 L2 and L3. These three lines are known as phases and in effect give three live connections with respect to the line marked neutral hence the term three phase supply . The voltage between each of these live lines is 415 V. The voltage between each of them and the neutral line is 240 V. 114 11. EARTHING AND SCREENING The three phase supply at 415 V is used to power most electrical machinery in the factory since it is able to supply large currents. The lower voltage between one phase and neutral is connected to the lights and 13 amp sockets as the single phase supply just as we have at home with a single live connection and a neutral connection. Look back at the sub-station transformer diagram and see that the neutral point is also connected to the earth. This is usually accomplished by burying a large copper plate under the .