Electric Problems and Safety Measures

Electric wires become heated when current passes through them. If, due to some reason,excessive heating takes place, the wires may catch fire. A fault like a short circuit or an overload can cause overheating of the wires.

Short-Circuiting:

If the live wire and neutral wire come in contact either directly or via conducting wire, then the situation is called short circuiting. In such case, the resistance of the circuit is almost zero which results in the flow of a larger current. This heats up the wire dangerously and may lead to fire.

Consider the situation shown in Figure 5.26a. A bulb is connected across the live and the neutral wires of the domestic supply. The current flowing through the circuit depends on the resistance of the bulb. Now consider the situation shown in Figure 5.26b, in which the live and the neutral wires have come in contact accidentally. A very large current passes through the circuit, because the resistance between the wires is now almost zero. Such an event is called a short circuit. The large current due to the short circuit leads to overheating, which may even cause a fire. A short circuit may happen due to many reasons, including the insulations on neighbouring wires getting worn out, a conductor such as a screw falling across the live and the neutral terminals of a socket, and so on.

Overloading:

A large current flows from the circuit if many electrical appliances of high power rating are switched on at the same time. This is called overloading. The large amount of current flowing through the wire excessively heats up the wire and may lead to fire. Different types of wires can safely carry currents up to a certain limit, say 10 A or 20 A, before they start overheating. If the total current drawn through a wire by the appliances connected to it exceeds the safety limit for that wire, it gets overheated. We say that the overheating is due to overloading.

For example, suppose a live wire entering the switchboard of a room can carry a maximum current of 10 A without overheating. The board has several switches for the fans and lights in the room, and it has a few sockets too. If we switch on all these appliances, and a heater, a toaster and an iron are connected to the sockets, the total current through the live wire entering the board will exceed 10 A. The overloaded wire will then get overheated.

Safety Devices:

To prevent accidents, safety devices are used in circuits involving high voltages (normally,100 V or more). These safety devices break the circuit whenever an abnormally high current flows through it. Common safety devices include the fuse and the circuit breaker.

Fuse:

It is a safety device having short length of thin wire made of tin and lead alloy having low melting point around $200^{circ}C.$ It is used to prevent the possible damage caused by overloading and short circuiting. The fuse wire melts when the current through it exceeds its rated value. This breaks the circuit.

Circuit Breaker:

A circuit breaker has a switch and a solenoid connected in series in the live line of a circuit. A heavy soft iron core lies partially in the solenoid. When a current greater than a particular value passes through the solenoid, the core gets pulled in. The core hits the switch to open it and break the circuit. The switch can be turned on again manually once the electrical fault is rectified. Figure 5.27 shows how a circuit breaker works.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

A large current flows from the circuit if many electrical appliances of high power rating are switched on at the same time. This is called ______________________ .
A. Current uptake	B. Overloading	C. Current sport	D. None of these .
Right Option : B
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Question : 2

Which of the following are correct ? (a) A large current flows from the circuit if many electrical appliances of high power rating are switched on at the same time. This is called overloading. (b) A circuit breaker has a switch and a solenoid connected in series in the live line of a circuit.
A. Only (a) is correct .	B. Only (b) is correct .	C. Both (a) and (b) is correct .	D. None of these .
Right Option : C
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Question : 3

The large amount of current flowing _____________ the wire excessively heats up the wire .
A. Through	B. Inside	C. Outside	D. All of these
Right Option : A
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