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The importance of fuse wire in the domestic electric circuit
Electricity is generated at the power station. It is brought to our homes by two thick copper wires or aluminium wires fixed over tall electric poles (or by underground cables). From the electric pole situated in our street, two insulated wires L and N come to our house (see Figure).
One of these wires is called live wire (read as laa-ive wire) and it is at a high potential of 220 volts whereas the other wire is called neutral wire and it is at the ground potential of zero volt. Thus, the potential difference between the live wire and the neutral wire in India is 220 – 0 = 220 volts.
In Figure, L is the live wire and N is the neutral wire. The live wire has red insulation covering whereas neutral wire has black insulation covering. There is no harm if we touch the neutral wire but we will get an electric shock if, by chance, we touch the live wire.
The two insulated wires L and N, coming from the electric pole, enter a box fitted just inside our house. In this box, a main fuse F1 is put in the live wire. This fuse has a high rating of about 50 amperes. The two line wires then enter the electricity meter M which records the electrical energy consumed by us in the units of kilowatt-hours. The main fuse and the meter are both installed by the Electric Supply Department of our City.
The two wires coming out of the meter are connected to a main switch S. This main switch is to switch off the electricity supply when required so as to repair any faults in the internal wiring. After the main switch, there is another fuse F2 in the live wire. This is called consumer’s fuse. It is very important to note here that usually there are two separate circuits in a house, the lighting circuit with a 5 A fuse and the power circuit with a 15 A fuse.
The lighting circuit is for running low power-rating devices such as electric bulbs, tube-lights, fans, radio, and TV, etc., which draw small current. On the other hand, power circuit is for running high power-rating devices such as electric iron, room heater, geyser, electric stove, refrigerator, etc., which draw heavy current. But to make things simple, we will describe only a lighting circuit with a 5 ampere fuse.
Each distribution circuit is provided with a separate fuse so that if a fault like short-circuiting occurs in one circuit, its corresponding fuse blows off but the other circuit remains unaffected. Another point to remember is that the various distribution circuits are connected in parallel so that if a fault occurs in one circuit, its fuse will melt leaving the other circuit in operation.
For example, the lighting circuit and power circuit in our home are in parallel so that if a short-circuit occurs in, say the power circuit, then the power-fuse will blow off but our lights will not go off because our lighting circuit will keep working.
Before we describe the wiring of our rooms, it is very important to note here that alongwith the live wire and the neutral wire, a third wire called earth wire also goes into our rooms. In Figure, the earth wire E has been shown by dotted line. One end of the earth wire E is connected to a copper plate and buried deep under the earth near the house (as shown in Figure) or at the nearest electric sub-station.
The earth connection is first made to the electric meter and then to the main switch. This earth wire then goes into our room alongwith the live wire and the neutral wire. Please note that the earth wire up to the main switch of our house is usually an uncovered, thick copper wire having no plastic insulation over it. But the earth wire which goes from the main switch into our rooms is a copper wire having green insulation covering over it.
Thus, in order to distinguish between the live wire, neutral wire and earth wire, the wire having red plastic covering is made live wire, the wire having black plastic covering is made neutral wire, and the wire having green plastic covering is made earth wire.
Now, three wires, live wire, neutral wire and the earth wire enter our room where we have to use an electric bulb, a fan and a three-pin socket for radio, and TV etc. We will now describe the internal wiring of a room. In a room, all the electrical appliances like bulbs, fans and sockets, etc., are connected in parallel across the live wire and the neutral wire.
The main advantage of the parallel connection is that if one of the appliances is switched off, or gets fused, there is no effect on the other appliances and they keep on operating. Another advantage of the parallel circuits is that the same voltage of the mains line is available for all the electrical appliances. If, however, we connect the various electric bulbs in series, then if one bulb is switched off or gets fused then all other bulbs will also stop working because their electricity supply will be cut off.
On the other hand, if the bulbs are connected in parallel, then switching on or off in a room has no effect on other bulbs in the same building. Moreover, if the various electric bulbs are connected in series, they will not get the same voltage (220 V) of the mains line. The bulbs connected in series will get lower voltage (than 220 V) and hence glow less brightly. All the bulbs connected in parallel will, however, get the same voltage (220 V) and hence glow brightly.
First of all we will describe the wiring for an electric bulb. One end of the bulb-holder is connected to the live wire through a switch S1 and the other end of the bulb-holder is connected to the neutral wire (see Figure). When we press the switch the circuit for bulb gets completed and it lights up. We will now describe the wiring for a three-pin socket.
One of the lower terminals of the socket T is connected to the live wire through a switch S2 and the second lower terminal is connected to the neutral wire. The upper terminal of the socket is connected to the earth wire (see Figure). Let us now describe the wiring for a fan. The live wire is connected to one terminal of the fan through a switch and a regulator. The neutral wire is connected to the other terminal of the fan. We have not shown the wiring for a fan in Figure. Please do it yourself.
It is obvious from the circuit given in Figure that all the electrical appliances are provided with separate switches. It should be noted that all the switches are put in the live wire, so that when we switch off an electrical appliance (like an electric iron), then its connection with the live wire is cut off and there will be no danger of an electric shock if we touch the metal case of the electrical appliance. If, however, we put switches in the neutral wire, then the live wire will be in connection with the electrical appliance even when the switch is in the off position, and there is a danger of an electric shock.
Earthing of Electrical Appliances
Sometime or the other we have received an electric shock from an electric iron or a room cooler. We will now discuss why we get the electric shock and how it can be prevented. In order to work an electrical appliance like an electric iron, electric kettle or a room cooler, we need two wires of the supply line, the live wire and the neutral wire.
Sometimes, due to wear and tear or due to excessive heating, the plastic covering (or insulation) of the connecting wires gets removed or gets burnt and the live wire (which is at a high potential of 220 volts) becomes naked. This naked live wire may touch the metal case (or metal body) of the electrical appliance due to which the case becomes live and comes to the high voltage of 220 volts. If we happen to touch any part of this live appliance, a very high current flows through our body into the earth.
Due to this high current flowing through our body, we get an electric shock (see Figure). It has been found that we do not get an electric shock if we are standing on a wooden plank. This is due to the fact that wood acts as an insulator and the circuit of current with earth does not get completed through our body.
To avoid the risk of electric shocks, the metal body of an electrical appliance is “earthed”. Earthing means to connect the metal case of electrical appliance to the earth (at zero potential) by means of a metal wire called “earth wire”. In household circuits, we have three wires, the live wire, the neutral wire and the earth wire.
One end of the earth wire is buried in the earth. We connect the earth wire to the metal case of the electrical appliance by using a three-pin plug. The metal casing of the appliance will now always remain at the zero potential of the earth. We say that the appliance has been earthed or grounded. Let us make it more clear with the help of a diagram. Figure shows the earthing of an electric iron or press.
In Figure, the live wire and the neutral wire are connected to the two ends of the heating element whereas the earth wire is connected to the metal body of the electric iron. These three wires are connected to a three-pin plug P. The plug P is connected to a three-pin socket S. Let us see how the earth connection actually works.
If, by chance, the live wire touches the metal case of the electric iron (or any other appliance), which has been earthed, then the current passes directly to the earth through the earth wire. It does not need our body to pass the current and, therefore, we do not get an electric shock.
Actually, a very heavy current flows through the earth wire and the fuse of household wiring blows out or melts. And it cuts off the power supply. In this way, earthing also saves the electrical appliance from damage due to excessive current. From the above discussion we conclude that we earth the metallic body of an electrical appliance to save ourselves from electric shocks.
Thus, the earthing of electrical appliances is used as a safety measure. It should be noted that we give earth connections to only those electrical appliances which have metallic body, which draw heavy current, and which we are liable to touch.
For example, electric iron, electric heater, room cooler and refrigerator, are all provided with earth connections. We, however, do not do earthing of an electric bulb or a tube-light because we hardly touch them when they are on. The metal casings of the switches are, however, earthed.
It should be noted that the connecting cable of an electrical appliance like an electric iron, electric kettle, water heater, room cooler or refrigerator contains three insulated copper wires of three different colours: red, black and green. The red coloured wire is the live wire, the black wire is the neutral wire, whereas green wire is the earth wire (see Figure).
Electric Fuse
The electric wires used in domestic wiring are made of copper metal because copper is a good conductor of electricity having very low resistance. Now, the copper wires chosen for household wiring are of such thickness so as to allow a certain maximum current to pass through them
. If the current passing through wires exceeds this maximum value, the copper wires get over-heated and may even cause a fire. An extremely large current can flow in domestic wiring under two circumstances : short circuiting and overloading.
(i) Short Circuiting. If the plastic insulation of the live wire and neutral wire gets torn, then the two wires touch each other (see Figure). This touching of the live wire and neutral wire directly is known as short circuit. When the two wires touch each other, the resistance of the circuit so formed is very, very small. Since the resistance is very small, the current flowing through the wires becomes very large and heats the wires to a dangerously high temperature, and a fire may be started.
(ii) Overloading. The current flowing in domestic wiring at a particular time depends on the power ratings of the appliances being used. If too many electrical appliances of high power rating (like electric iron, water heater, air conditioner, etc.,) are switched on at the same time, they draw an extremely large current from the circuit. This is known as overloading the circuit.
Overloading can also occur if too many appliances are connected to a single socket (see Figure). Now, due to an extremely large current flowing through them, the copper wires of household wiring get heated to a very high temperature and a fire may be started.
It is obvious that we should have some device which may disconnect the electricity supply when a short circuit or overloading occurs so that the electric fires are prevented in our homes. To avoid this danger of electric fires we use an electric fuse in the wiring. So, when a building is wired, the wiring is protected by fuses. We will now describe what a fuse is and how it works.
A fuse is a safety device having a short length of a thin, tin-plated copper wire having low melting point, which melts and breaks the circuit if the current exceeds a safe value. The thickness and length of the fuse wire depends on the maximum current allowed through the circuit.
An electric fuse works on the heating effect of current. The fuse for protecting our domestic wiring is fitted just above our main switch on the switch board. A fuse wire is connected in series in the electric circuits.
The main fuse in domestic wiring consists of a porcelain fuse holder H having two brass terminals T1 and T2 in it [see Figure (a)]. This is connected in the live wire. The other part of the fuse is a removable fuse grip G which is also made of porcelain. The fuse grip has a fuse wire fixed in it.
When fuse grip is inserted in the fuse holder as shown in Figure (a), then the circuit of our domestic wiring is completed. So, under normal circumstances when the current is within limit, the fuse wire is intact and electric current is available in our wiring.
When a short circuit takes place, or when overloading takes place, the current becomes large and heats the fuse wire too much. Since the melting point of fuse wire is much lower than copper wires, the fuse wire melts and breaks the circuit as shown in Figure (b). When the fuse wire breaks, electricity supply is automatically switched off before any damage can be done to the rest of the wiring (or the electrical appliances being used).
We will now give some important points about the fuse wire to be used in electrical circuits. First of all we should know why we use a thin wire as a fuse wire and not a thick wire. We use a thin wire in a fuse because it has a much greater resistance than the rest of connecting wires.
Due to its high resistance, the heating effect of current will be much more in the fuse wire than anywhere else in the circuit. This will melt the fuse wire whereas other wiring will remain safe. We should not use a thick wire as a fuse wire because it will have a low resistance and hence it will not get heated to its melting point easily.
The fuse wire is usually made from tin-plated copper wire having low melting point so that it may melt easily. A pure copper wire cannot be used as a fuse wire because it has a high melting point due to which it will not melt easily when a short circuit takes place.
The fuse wire must have proper thickness which depends on the maximum current which the household wiring can safely carry. The thickness of the fuse wire should be such that it is able to withstand only a little more current than drawn by the household circuit.
Fuse wires are rated as 1 A, 2 A, 3 A, 5 A, 10 A, 13 A, 15 A, and so on. It is clear that a “10 ampere” fuse wire will be thicker than a “5 ampere” fuse wire. The fuse in the lighting and fans circuit of a small house is of 5 amperes rating which means that the fuse wire will melt if the current exceeds 5 amperes value.
The fuse used in the power circuit of a small house for running electric iron, immersion heater, geyser and toaster, etc., having power of 1000 watts or more is of 15 A capacity. A blown fuse should be replaced only after the cause of excessive current flow has been formed and removed.
These days more and more houses are using ‘Miniature Circuit Breakers’ (MCBs) to protect the household wiring from the excessive flow of electric current through it (see Figure). If the current becomes too large, the miniature circuit breaker puts off a switch cutting off the electric supply.
The MCB can be re-set when the fault has been corrected. Miniature circuit beaker (MCB) contains an electromagnet which, when the current exceeds the rated value of circuit breaker, becomes strong enough to separate a pair of contacts (by putting off a switch) and breaks the circuit. So, unlike fuses, MCBs do not work on heating effect of current. MCBs work on the magnetic effect of current.
So far we have discussed the fuses which are put on the main switch-board in our houses to protect the whole wiring of the house. Fuses are also used to protect the individual domestic electrical appliances from damage which may be caused due to excessive current flow through them (see Figure).
Costly electrical appliances like T.V. sets and refrigerators have their own fuses which protect them against damage by too much current. The fuse used for each electrical appliance should be slightly larger than the normal current drawn by it. For example, a T.V. set which normally takes less than 1 ampere of current should be fused at 2 amperes, and not, for example, at 10 amperes.
The fuse used in an electrical appliance is shown in Figure (a). It consists of a glass tube T having a thin fuse wire sealed inside it. The glass tube has two metal caps at its two ends. The two ends of the fuse wire are connected to these metal caps. The metal caps are for connecting the fuse in the circuit in a suitably made bracket. In a circuit diagram, the electric fuse is represented by the symbol shown in Figure (b). We will now solve some problems based on electric fuse.
Example Problem 1.
An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect ? Explain.
Solution:
We will first calculate the current drawn by this electric oven.
Now, Power, P – 2 kW
= 2 × 1000 W
= 2000 W
Potential difference or Voltage, V = 220 V
And, Current drawn, I = ? (To be calculated)
Now, Power, P = V × I
So, 2000 = 220 × I
And Current drawn, I = \(\frac{2000}{220}\)
= 9 A
Now, the current drawn by this electric oven is 9 amperes which is very high but the fuse in this circuit is only of 5 ampere capacity. So, when a very high current of 9 A flows through the 5 A fuse, the fuse wire will get heated too much, melt and break the circuit, cutting off the power supply. Thus, when a 2 kW power rating electric oven is operated in a circuit having 5 A fuse, the fuse will blow off cutting off the power supply in this circuit.
Example Problem 2.
A circuit has a fuse of 5 A. What is the maximum number of 100 W (220 V) bulbs that can be safely used in the circuit ?
Solution:
Suppose x bulbs can be used safely.
Now, Power of 1 bulb = 100 W
So, Power of x bulbs, P – 100 × x watts
Potential difference, V = 220 volts
Current, I = 5 amperes
Now, Power, P = V × I
So, 100 × x = 220 × 5
x = \(\frac{220 \times 5}{100}\)
x = 11
Thus, a maximum number of 11 bulbs can be used.
Example Problem 3.
What precautions should be taken to avoid overloading of domestic electric circuits ?
Answer:
- Too many high power rating electrical appliances (such as electric iron, geyser, air conditioner, etc.) should not be switched on at the same time.
- Too many electrical appliances should not be operated on a single socket.
Example Problem 4.
Name two safety measures commonly used in domestic electric circuits and appliances.
Answer:
- Provision of electric fuse.
- Earthing of metal bodies of electrical appliances.
Hazards of Electricity (or Dangers of Electricity)
Though electricity is one of the most important and convenient form of energy but its improper use is associated with the following hazards or dangers :
- If a person happens to touch a live electric wire, he gets a severe electric shock. In some cases, electric shock can even kill a person.
- Short-circuiting due to damaged wiring or overloading of the circuit can cause electrical fire in a building.
- The defects in the household wiring like loose connections and defective switches, sockets and plugs can cause sparking and lead to fires.
Precautions in the Use of Electricity
To avoid the hazards like electric shocks or electric fires, we should observe the following precautions in the use of electricity :
1. If a person accidently touches a live electric wire or if an electric fire starts in the house, the main switch should be turned off at once so as to cut off the electricity supply. This will prevent the fire from spreading.
2. The person who happens to touch the live electric wire should be provided an insulated support of wood, plastic or rubber. We should never try to pull away the person who is in contact with the live wire, otherwise we will also get a shock.
3. All the electrical appliances like electric iron, cooler, and refrigerator, etc., should be given earth connections to save ourselves from the risk of electric shocks. Even if the earth connection is there, we should avoid touching the metal body of an electric appliance when it is on.
4. All the switches should be put in the live wire of the A.C. circuit, so that when the switch is turned off, the appliance gets disconnected from the live wire and there is no risk of electric shock.
5. We should never operate switches of electrical appliances with wet hands. The plugs should also not be inserted into sockets with wet hands (see Figure). This is because water conducts electricity to some extent, so touching the switches and sockets with wet hands can lead to electric shocks.
6. The fuse should always be connected in the live wire of the circuit. The fuse wire should be of proper rating and material. We should never use a copper wire (connecting wire) as fuse wire because a copper wire has a very high current rating due to which a copper wire fuse cannot protect the wiring against short circuiting or overloading.
7. The household wiring should be done by using good quality wires having proper thickness and insulation. All the wire connections with switches, sockets and plugs should be tight, and all the wire joints should be covered with insulated adhesive tape. Defective switches, sockets and plugs should be replaced immediately.
8. We should avoid working on a live circuit for repairs, etc. If, however, it is necessary to handle a live circuit, then rubber gloves and rubber shoes must be put on, and we should stand on a dry wooden board. The electricians should wear rubber hand gloves and rubber shoes while working. The tools used for electrical repairs like testers, screw drivers, pincers, etc., should have properly insulated handles made of wood or bakelite plastic.