Safe Use
In a city the electric energy is distributed to various house and factories from a power station, by means of two wires (usually). One wire is earthed at power station. Its electrical potential is zero. Its is called “neutral wire”. The tester shows no glow at this wire. The electric potential of the other wire changes sinusoidally . This wire is called “live wire” and tester shows glow at this wire. Human body is conducting, so electric current can pass through it.How a person may be electrocuted
When person touches the live wire, then potential difference is created between touching point of body and feet of the person (on earth). Due to which electric current flows through body of the person. This is how a person may be electrocuted; it may prove fatal for him. To avoid this the supply lines, in houses are covered with insulating material. “An insulation covered wire is called cable”.
Why a cable be dangerous
Due to heavy current the cable is heated up and its insulating cover is burnt, and it becomes dangerous. The electric wiring in houses is made such that each household appliance is connected in parallel to the supplied voltage. In figure below a typical electrical circuit is shown. Each appliance has a switch which can be made ON or OFF. When switch is made ON then current passes through the appliance and when it is made off, then no current passes through it. The switch gets heated if electric current exceeds the rated, value. Hence it should be ascertained that maximum current that would pass through the switch is within its rated value. It is dangerous to connect the electrical appliance directed between live and neutral wires. This should be avoided. The connections should be made through sockets and plug.Fuse wire
A small wire connected in series with the live wire. It is known as fuse wire of Fuse. To avoid damage to an electrical appliance or to an electric circuit a thin wire of low melting point conductor is connected in series with live wire. It is known as fuse wire(or fuse). When electric current exceeds the limit of rated value, then the fuse wire is heated up due to dissipation of electrical energy into heat. And temperature of fuse wire goes to its melting point. This results the fuse wire to fuse or melt. Due to which this wire breaks and the electric circuit automatically breaks. This avoids heavy damage of electric circuit or of electrical appliance. “Never use thick fuse wire of conductor of high melting point”. The fuse wire is useful when electric circuit becomes short due to some fault in it.Let us suppose that we want to insert a fuse along with air conditioner or heater of power 30000 W. If voltage supplied is equal to 240 V, then according to the available fuses in the market are usually rating 5A, 10A, 13A, 30A etc. Hence suitable fuse for the above is of 13A .
Circuit breaker
A safety device used in place of fuse is called “Circuit breaker”. It breaks the electric circuit if current increases beyond the given rating. Due to any fault or by switching many appliances simultaneously the value of current may increase the rating value (It may be due to sudden decrease (fluctuation) of voltages. Then the switch of circuit breaker becomes OFF automatically (moves down) due to which circuit breaks and flow of current is stopped. After removing the fault in the circuit, the switch of the circuit breaker is again made ON manually. connected to earth in the house.Such an electric circuit has three wires
(1) live wire
(2) neutral wire
(3) earth wire. This circuit requires three-pin socket, and three-pin plug. When ever metal conducting casing of electrical appliance gets connected to the live wire, then due to short circuit large current flows to the ground (earth) and causes the fuse wire to become hot and ultimately So that circuit breaks down. This avoids electric shock to the person touching the metal casing of electrical appliance. A circuit breaker has a coil in which electric current flows when circuit is ON. This coil behaves like electromagnet. The strength of this electromagnet depends upon the value of electric current passing through it. When current exceeds a limit (rating value), the magnetic force of attraction is increased so much that it becomes able to attract the iron strip “A”; and its contact with the electric circuit is broken. It makes the electric circuit OFF. If the current flowing in the circuit (and hence in the coil of circuit breaker) remains within rating value then iron strip .
A remains in position due to low attractive force of electromagnet and circuit remains ON.
Most of electrical appliances like electric iron, electric kettle, refrigerators and motor pump motors, etc are such that their outer part is made up of conducting material. This part must be separated from the electric circuit inside. So that no current can flow through the outer part.
Earth wire
Sometime between, due to some fault, this insulation between outer body and the inner circuit is damaged. Then it is quite dangerous to touch the outer body of the appliance.It may result in electric shock. In order to avoid this risk an additional wire is used, along with live wire, and neutral wire. This is known as “earth wire”. Its one end is directly connected to earth in the house. Such an electric circuit has three wires(1) live wire
(2) neutral wire
(3) earth wire.
This circuit requires three-pin socket, and three-pin plug. When ever metal conducting casing of electrical appliance gets connected to the live wire, then due to short circuit large current flows to the ground (earth) and causes the fuse wire to become hot and ultimately fuse or melt So that circuit breaks down. This avoids electric shock to the person touching the metal casing of electrical appliance.
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