In this article we will discuss about:- 1. Types of Switches 2. The Essential Qualities of a Switch 3. Positions.
Types of Switches:
i. Quick Break Knife Switch:
This type of switch is generally used on the main switch board of a low voltage circuit. Fig. 76 shows a double pole quick break knife switch. The switch consists of one, two or more copper blades fixed with a handle. These are called main blades. The cross-section of the blades depends on the circuit current. Each main blade is provided with one comparatively smaller blade, called auxiliary blade.
Auxiliary blade is not fixed with the handle. It is attached to the main blade by means of a spring. The blades are hinged at one end. The arrangement is such that the main blades along with the auxiliary blades can be pressed into forked terminals or jaws at the other end. At the hinged end spring washers are used. The whole assembly is fixed either on a piece of ebonite sheet or on a marble slab or on a switch board panel.
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When the circuit is to be switched off, the handle is moved in such a way that the main blades come out of the forked terminals while the auxiliary blades still remain into the jaws and maintain connection. The handle is then moved further out, and at one point the auxiliary blades quickly come out of the terminal contacts due to the tension of the spring and remain in contact with the main blades again. Thus, the spring provides a quick break action due to which no electric arc can be formed between a blade and a forked terminal while breaking a circuit.
The difference between a quick break switch and a link switch is that the former has spring for quick action and latter has no spring attached to its blade. These switches may be single- pole, double-pole, triple-pole and double-throw type. Single-pole, triple-pole and double-throw switches are shown in fig. 78, fig. 79 and fig. 80 respectively.
In places where there is risk of fire due to spark coming out from the switch contacts or where the circuit voltage is so high that the open contacts of the switch are considered to be a source of danger to human life, the complete assembly of a link switch is enclosed in an iron box. Such a switch is called an iron-clad switch or a metal-clad switch. Inside the mines and on the main and sub-distribution boards this type of switch is widely used.
For the sake of safety the outer iron cover is connected to earth. The arrangement is such that, so long the lid of the iron box remains open, the switch cannot be placed at the on-position. On the other hand when the switch is on, the lid of the iron box cannot be opened. This is called fool-proof arrangement.
In case of low voltage circuits (circuit voltage not exceeding 250 volts) plastic cover is sometimes used in place of iron cover for the link switches. This reduces the manufacturing cost of the switch considerably. Nowadays link switch with plastic cover is widely used as main switch for house wiring.
Knife switches should be placed in such a way that due to the weight of the handle the switch blades tend to come only to the off position.
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ii. Main Switch:
According to Indian Electricity Rule No. 50(1)(a) a suitable linked switch or a circuit breaker of requisite capacity to carry and break the current is placed as near as possible to, but after the point of commencement of supply, so as to be readily accessible and capable of being easily operated to completely isolate the supply to the installation. This switch is called main switch. According to Indian Standard Specification No. 4 2 1 IS: 732-1963 such a switch must be either metal-clad or it must have a cover made of insulating material.
Usually a linked switch suitable for carrying full load current of the circuit continuously and of approved voltage grade is installed as near as possible to the supplier’s meter board. This switch may be double-poled, triple-poled (fig. 81) or triple-poled with neutral link. The base of the switch has fuse grips in all the lines excepting in the neutral line. Fuse wires of suitable size are provided in all the grips.
The metallic cover of the switch is made in such a way that its lid cannot be opened so long the circuit remains switched on. This arrangement is essential in order to prevent a person from coming in contact with the live terminals of the switch. The lid of the switch can be opened only when the circuit is switched off. Even then one cannot touch the live contacts of the switch when handling carelessly, as the contacts remain covered with porcelain or Bakelite sheet.
Up to 200 amperes of the circuit current the insulation surrounding a switch contact is nothing but air. Hence this switch is called air break switch. If the circuit current exceeds 200 amperes, the switch contacts remain immersed in oil. In that case the switch is called oil break switch.
The handle of every main switch is usually pushed up when the circuit is ‘switched on’, and it is pressed down when the circuit is ‘switched off. All the blades of a switch are attached to a single handle so that the flow of current starts and stops through them simultaneously.
iii. Voltage Grades of Switches:
Mainly there are two voltage grades of the main switches:
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a. 250-volt and
b. 500-volt grade.
If the supply pressure exceeds 500-volts, the practice is to use a circuit breaker there in place of a switch.
In a house or in a small factory or workshop where the total load does not exceed 4 kilowatt, the supply is given at low voltage (i.e. at 225 volts in case of d.c. supply and at 230 volts in case of a.c. supply). In such cases a double-pole, 250-volt grade main switch is used near the meter board.
Although the low voltage supply system consists of one live line and one neutral line, according to Indian Electricity Rule, a single-pole switch shall never be used as the main switch. For this reason a double-pole switch is to be used in the low voltage supply line. A fuse wire is provided only with that terminal of the switch with which the live line of the supply is connected. The upper and lower contacts of the terminal with which the neutral line is connected are joined together with a piece of wire.
If the total installed load is more than 4 kilowatt, the supplier gives supply usually at medium pressure (i.e. at 450 volts in case of d.c. supply and at 400 volts in case of a.c. supply). In such cases a 500-volt grade switch is used as the main switch. The switch should be double-poled or double-poled with neutral link for d.c. supply, and for a.c. supply the same should be triple-poled or triple-poled with neutral link.
iv. Current-Carrying Capacity of Switches:
As the switches have different voltage grades according to supply pressure, similarly for different circuit currents switches of different sizes are available in the market. Metal-clad switches have usually current ratings of 15, 30, 60, 100, 150 and 200 amperes. Switches of 10 and 20 amperes sizes are also manufactured usually with plastic cover. Metal-clad, air break switch may be of 300, 500 or even 750 amperes sizes, but the practice is to use an oil circuit breaker where the circuit current exceeds 400 amperes.
v. Tumbler Switch:
A tumbler switch has been shown in fig. 77. The switch has two brass contacts screwed to a porcelain or a Bakelite base. Current enters into the switch through one contact and flows out through the other contact. A copper or brass blade is fixed up between these two contacts in such a way that, when a brass lever attached with the blade and having insulated top is pressed down, the blade is inserted between the contacts and the circuit is switched on.
For switching off the circuit, the lever is pushed up and thereby the blade is detached from the switch contacts. Of course there is one type of switch available in the market in which the blade closes the contacts of the switch when the lever is pushed up.
The smallest size of tumbler switch can carry 5 amperes continuously. These switches are used in light and fan circuits. The next higher size of switches is made of 10 amperes or 15 amperes. These switches are used in the circuits supplying power to heaters, refrigerators, small pump motors etc. The switches having porcelain base have porcelain or brass covers, while the switches with Bakelite base have Bakelite cover.
For quick opening of the switch contacts, a spring is provided. When the contacts are closed through the blade, the spring is pressed. During opening of the contacts the spring pushes up the blade with a force, and the blade comes out of the contacts quickly.
Sometimes the insulation of the roller operating the lever of the switch is not perfect. In that case a person may get electric shock when he touches the knob. To avoid such accident it is advisable to test the insulation resistance between the blade and the knob by means of an insulation testing megger before the switch is used in the electric circuit.
Nowadays a new type of tumbler switch is widely used in the light, fan and heater circuits. This switch is much cheaper than other types of tumbler switch described above. The shape of the switch is like a piano key and hence it is called Piano-key Switch. A piano-key switch is shown in fig. 82(a).
The base and the cover of a piano-key switch are mode of moulded plastic. The contacts are fixed on the base and the blade is attached with the inner side of the cover. The cover is placed over the base with the help of a spring in such a way that when one end of the knob is pressed down, its other end goes up. If the bottom end is pressed, the blade is inserted between the contacts and the circuit is switched on. When the top end is pressed, the blade comes out of the contacts and the circuit is switched off.
A modification of piano-key switch, called Finger-touch Switch, is also now available in the market. Finger-touch switch is shown in fig. 82(b). This type of switch is also widely used in place of piano-key switch. The difference of this switch with the piano-key switch lies in the shape and operation of the knob. The knob of a finger-touch switch is circular in shape. When the knob is pressed first, the circuit is switched on, and pressing the same knob again, the circuit is switched off.
The Essential Qualities of a Switch:
A switch must have the following essential qualities:
(i) The blade of a switch must not be excessively heated when carrying continuously the rated current of the circuit.
(ii) Up to 30-ampere size the contacts of a switch must be able to open readily a current 30 per cent above the rated value in low voltage (voltage not exceeding 250 volts) circuits.
(iii) Electric arc of permanent nature must not be formed between the contacts when the switch is opened at a pressure 10 per cent above the rated voltage while carrying a current 1.5 times the normal rated current.
(iv) The base of a switch must be made of strong, non-inflammable, non-hygroscopic and good insulating material in which there should be no crack or break.
(v) All current-carrying parts of a switch must be well covered by a metallic covering or by a cover made of non-conducting material. In case of metallic covering there should be no spark between the switch contacts and the cover.
Positions of Switches:
It is always advantageous to follow a particular system while placing switches at different positions in the wiring system of a house. The best position of a switch controlling the light point of a room is on the wall by the side of the entrance door. While entering into the room one can easily switch on the light just after opening the door.
Similarly, while going out, one can switch off the light first, then close the door and then go away. Besides, the heights from the floor level at which different switch boards of the same house are placed must be the same. This height usually varies between 1.1 to 1.15 metres. In school buildings and in similar other places where little children roam about, the height may be increased to 1.3. to 1.5 metres for the sake of safety of the children.