Chapter 9: Household Circuits
On the afternoon of 4 September 1882, in a narrow building on Pearl Street in lower Manhattan, Thomas Edison’s engineers closed a switch. Steam engines turned generators, and in a few dozen offices and shops nearby, electric lamps came on together for the first time from a single central station. Before that day, anyone who wanted electric light had to keep their own generator in the basement. Now the electricity was made in one place and delivered to many.
There was a catch, and Edison knew it. His station sent out direct current at a little over one hundred volts, and at that low voltage the current needed was large. Large current in a wire means heat, and heat means energy wasted on the way. The thicker the copper he laid, the more it cost. In practice the supply could not usefully be carried much more than a mile from Pearl Street. A city could be lit street by street, but a whole region could not.
The way out was not thicker copper. It was higher voltage — and a kind of current whose voltage could be raised and lowered at will. That idea is why the electricity reaching your switchboard today began its journey hundreds of kilometres away, at a voltage that would be lethal to stand near, and arrives at a safe 220 V. Fig. 9.0-1 shows that journey in outline.
Diagram
Fig. 9.0-1 — The Journey Of Electricity From Power Station To Home A left-to-right landscape strip. At the far left, a power station building with a turbine and generator, labelled ‘generating station, 11 kV’. Next, a small sub-station with a transformer, labelled ‘stepped up to 132 kV’. Then a run of tall lattice transmission towers carrying overhead lines across open country, labelled ‘transmission at high voltage, low current’. Then two more sub-stations in sequence, labelled ‘stepped down to 33 kV’ and ‘stepped down to 11 kV’. Then a city sub-station labelled ‘stepped down to 220 V’. At the far right, an ordinary house with an electric pole outside it, a meter on the wall and a lit lamp inside, labelled ‘consumer, 220 V’. Draw a thin arrow along the whole strip showing the direction of energy flow.
9.1 Transmission Of Power From The Generating Station To The Consumer
Electric power is produced at generating stations, which are the places where some other form of energy is converted into electrical energy. These stations are usually built far from the towns and cities that use the power, close to a river, a coalfield or a coast. The electricity therefore has a long journey to make before it reaches a switchboard in a house.
That journey is the subject of this section. Two questions decide how it is made: at what voltage the power should be generated, and at what voltage it should be carried.
9.1.1 Why Power Is Generated At 11 kV
At the generating station, the electric power is generated at 11 kV. The symbol kV stands for kilovolt, and \(1\\ \\text{kV} = 1000\\ \\text{V}\), so \(11\\ \\text{kV} = 11,000\\ \\text{V}\).
This particular value is a compromise between two difficulties. Generation at a voltage higher than 11 kV causes insulation difficulties, because the higher the voltage between two conductors, the harder it is to stop electricity leaking across the gap between them. Generation at a voltage lower than 11 kV involves a very high current, because for the same power a lower voltage must be paired with a larger current.
A very large current inside the machines of a station is unwelcome for the reason given in the next subsection: it heats the conductors carrying it.
9.1.2 Why The Voltage Is Raised Before Transmission
The power generated at the generating station is not directly transmitted to the consumers at 11 kV. Before its transmission, its voltage is raised, and the purpose of raising it is to reduce the loss of energy in the form of heat in the transmission line wires.
To see why raising the voltage helps, start from the relation between electric power, voltage and current that you have already met. The power supplied is the product of the voltage and the current:
\[P = VI\]
where \(P\) is the electric power in watt (W), \(V\) is the voltage in volt (V) and \(I\) is the current in ampere (A).
The Same Power, A Smaller Current
Rearranging this relation to make the current the subject gives
\[I = \\frac{P}{V}\]
For a given power \(P\), a higher voltage \(V\) therefore means a lower current \(I\). The power to be delivered is fixed by the consumers; the voltage is ours to choose, and choosing it high forces the current down.
Now the transmission wires themselves have some resistance, and a current passing through a resistance always produces heat. The power wasted as heat in a line of resistance \(R\) is
\[\\text{power lost} = I^2R\]
and over a time \(t\) the energy wasted is \(I^2Rt\), where \(I\) is the current in the line in A, \(R\) is the resistance of the line in ohm (Ω), \(t\) is the time in s and the energy is in joule (J).
The current appears here as a square. Halving the current cuts the wasted power to a quarter, not to a half. Thus, by supplying a given electric power at a high voltage, the current becomes low and the loss of energy due to heating in the line wires becomes much less.
9.1.3 Why The Supply Is Alternating
Raising and lowering a voltage is the work of a transformer, and a transformer responds only to a current that is changing. A steady direct current produces no such change, so it is not possible to step up or step down the d.c. voltage.
For this reason an alternating voltage is generated at the generating station — a voltage whose direction reverses again and again, so that the current it drives flows first one way and then the other. Its frequency, the number of complete cycles it goes through each second, is 50 Hz in India. The unit hertz (Hz) is simply one cycle per second.
In each cycle the terminals are positive once and negative once. At 50 Hz, therefore, the polarity at the terminals changes 100 times a second — 50 times positive and 50 times negative.
9.1.4 The Chain Of Sub-Stations From Station To Consumer
The voltage is not changed once, but several times along the route, each change made at a sub-station by a transformer. A step-up transformer raises the voltage; a step-down transformer lowers it. The whole route is drawn in Fig. 9.1-1.
Diagram
Fig. 9.1-1 — Transmission Of Electricity From The Generating Station To The Consumer A left-to-right schematic line diagram. At the far left, three linked boxes labelled ‘boiler’, ‘turbine’ and ‘generators’, with the output marked ‘11 kV, 50 Hz’. This feeds a box labelled ‘step up, 11 kV to 132 kV’ inside a larger block labelled ‘grid sub-station’; mark the whole left group ‘power station’. From it a line marked ‘132 kV’ runs to a tall lattice transmission tower and on to a box labelled ‘step down, 132 kV to 33 kV’ inside a block labelled ‘main sub-station’. From this, one branch marked ‘33 kV’ goes up and away to a label ‘to heavy industry’, and another line marked ‘33 kV’ goes to a box labelled ‘step down, 33 kV to 11 kV’ inside a block labelled ‘intermediate sub-station’. From this, one branch marked ‘11 kV’ goes up to a label ‘to light industry’, and another line marked ‘11 kV’ goes to a box labelled ‘step down, 11 kV to 220 V’ inside a block labelled ‘city sub-station’. Its output line is marked ‘220 V’ and ends at a label ‘city consumers’. Label every voltage value on its own line.
Stage By Stage Along The Route
The alternating voltage generated is first stepped up from 11 kV to 132 kV at the generating station, using the step-up transformer. The place where this is done is also called the grid sub-station. From here the power is transmitted to the main sub-station.
At the main sub-station, the voltage is stepped down from 132 kV to 33 kV using the step-down transformer. It is then transmitted to the heavy industries and to the intermediate sub-stations.
At the intermediate sub-station, the voltage is again stepped down from 33 kV to 11 kV using the step-down transformer. From here it is transmitted to light industries and to the city sub-station.
At the city sub-station, it is further stepped down from 11 kV to 220 V using the step-down transformer, to supply it to the domestic consumers. The same chain is set out node by node in FC 9.1.
Flowchart
FC 9.1 — Voltage Along The Route From Generating Station To Consumer Generating station at 11 kV → grid sub-station, stepped up to 132 kV → main sub-station, stepped down to 33 kV → [branch: to heavy industry at 33 kV] → intermediate sub-station, stepped down to 11 kV → [branch: to light industry at 11 kV] → city sub-station, stepped down to 220 V → domestic consumer
Notice that each class of user is served at the voltage that suits it. Heavy industry takes its supply at 33 kV, light industry at 11 kV, and homes at 220 V, the voltage at which household appliances are made to work.
9.2 Power Distribution To A House
The last stage of the journey is the short one, from the city sub-station to the meter on the wall of a house. To supply the electric power to a house from the city sub-station, either the overhead wires carried on poles, or a cable on poles, or an underground cable is used.
Whichever route is taken, what arrives at the house is a cable containing three wires. Knowing what each of the three does is the foundation of everything else in this chapter.
9.2.1 The Three Wires Of The Supply Cable
The cable has three wires: the live wire (L), also called the phase wire, the neutral wire (N), and the earth wire (E).
The live wire carries current from the source to the distribution board, while the neutral wire is for the return of current back to the source. Together they form the going and returning halves of one circuit; a current that flows in through the live wire must flow back out through the neutral wire.
The neutral and the earth wires are connected together at the local sub-station. Because of this connection, the neutral and earth wires are at the same potential, that is, at 0 V. The live wire is the only one of the three at a high potential, and it is the only one that is dangerous to touch.
9.2.2 From The Electric Pole To The Distribution Box
Before connecting the cable from the pole to the meter in a house, a fuse of high rating is first connected in the live wire at the pole, or just before the meter. This fuse is called the company fuse, or pole fuse. Only persons of the electric supply company are authorised to handle it.
The rating of this fuse depends on the load for which the connection has been taken from the company. For a connection of load 10 kW, for example, the rating is 50 A.
After the company fuse, the cable is connected to a kWh meter, the instrument that records the electrical energy consumed in the house. The kWh meter is usually mounted on the front or outside wall of the house, where the company’s staff can read it without entering.
The order in which these components are met, from the pole to the rooms, is set out in FC 9.2, and the same connections are drawn in Fig. 9.2-1.
Flowchart
FC 9.2 — Order Of Connections From The Electric Pole To The Rooms Electric pole → company fuse in the live wire → kWh meter → main switch in the live and neutral wires → main fuse or MCB in the live wire → distribution box → separate circuits going to the different parts of the house
9.2.3 The Main Switch And The Main Fuse
From the meter, connections are made to a main switch and to a main fuse, both housed in the distribution box inside the house. In place of the main switch an earth leakage circuit breaker (ELCB) may be used — a switch that trips by itself if current begins to leak out of the circuit — and in place of the main fuse a miniature circuit breaker (MCB), which is described in 9.4.9.
Where these two are connected is not the same, and the difference matters. The main switch, or ELCB, is connected in both the live and the neutral wires. The main fuse, or MCB, is connected only in the live wire.
Diagram
Fig. 9.2-1 — Connections By Cable From The Electric Pole To The Distribution Box A left-to-right circuit-style diagram. At the left, an electric pole with three lines coming from a label ‘from city sub-station’, marked L, N and E from top to bottom. The L line passes through a small component labelled ‘company fuse’ mounted on the pole. All three lines then run as a labelled ‘cable’ into a rectangular box labelled ‘kWh meter’ with a reading window showing a number. Out of the meter, the L and N lines go to a box labelled ‘main switch’ drawn as two switch contacts operated together by a single linkage, showing that both L and N are broken at once. The E line from the meter branches downward to an earth symbol labelled ‘local earthing’. After the main switch, the L, N and E lines enter a tall rectangle labelled ‘distribution box’, inside which are four small blocks each labelled ‘MCB’, each with L, E and N terminals marked. Label the live line L, the neutral line N and the earth line E at every stage.
Why The Main Switch Is A Double Pole Switch
The main switch is a double pole switch, meaning a switch that breaks two wires at the same instant rather than one. It has an iron covering, and this covering is earthed, that is, connected to the earth wire E, so that a person operating the switch cannot receive a shock from its casing.
The advantage of using the main switch is that it breaks the connections of the live as well as the neutral wire simultaneously from the main supply. With both wires broken, no part of the house wiring beyond the switch is left joined to the supply, and the whole installation can be worked on safely.
The earth wire from the meter is locally earthed, in the compound of the house itself. How this local earthing is made, and what it protects against, is the subject of 9.7.
9.2.4 The Consumer Unit
From the distribution board, the wires go out to the different parts of the house. Nowadays a consumer unit is connected after the kWh meter to hold all of this protective equipment together in one place.
This unit contains a double pole switch or an earth leakage circuit breaker, and one MCB for each circuit in the house. Because every circuit has its own breaker, a fault in one part of the house can be cut off on its own, without switching off the rest.
9.3 House Wiring By The Ring System
Inside the house, the wiring has to reach every room without using an impossible amount of copper, and it has to let each lamp, fan and socket be controlled on its own. In a house, the wiring is commonly done by the ring system, which meets both of these needs.
9.3.1 How A Ring Circuit Is Laid Out
In this system, we have a separate ring for each portion of the house. In a portion, the wires starting from the distribution box run around all the rooms of that portion and then come back to the distribution box, thus forming a ring.
The distribution box contains an MCB of rating about 30 A for each ring circuit. This single breaker protects the whole ring. A ring system of wiring connecting a lamp, a socket with a switch, and a fan with a regulator for one portion of a house is shown in Fig. 9.3-1.
Diagram
Fig. 9.3-1 — The Ring System Of House Wiring A circuit diagram. At the left, a rectangle labelled ‘distribution box’ with three horizontal lines leaving it, marked L (top), N (middle) and E (bottom); the L line passes through a component labelled ‘30 A MCB’. The three lines run to the right, loop around a wide oval path and return to the distribution box, so that the ring is closed; mark the returning ends ‘to other appliances’. Three branches are tapped off the ring. Branch 1, at the left, feeds a lamp: from L through a fuse marked F, then a switch marked S, to the lamp; the other lamp terminal goes to N. Branch 2, in the middle, feeds a three-pin socket outlet: from L through a fuse F and a switch S to the socket’s L terminal, with the socket’s N terminal to the neutral line and its E terminal to the earth line. Branch 3, at the right, feeds a ceiling fan: from L through a fuse F, a switch S and a regulator marked R, to the fan; the other fan terminal goes to N, and the fan’s metal body to E. Label the lamp, socket outlet, fan, regulator, and every fuse and switch, and show clearly that each fuse and switch lies in the live wire.
9.3.2 Connecting An Appliance To The Ring
A separate connection is taken for each appliance from the live wire of the ring. One terminal of the appliance is connected to the live wire through a separate fuse and a separate switch, while the other terminal of the appliance is directly connected to the neutral wire.
The earth terminal, or metal covering, of the appliance is connected to the earth wire. For each appliance, the wires used for connection should be of proper current carrying capacity, that is, thick enough for the current that appliance will draw.
Notice the order along the live wire: the fuse first, then the switch, then the appliance. Both the fuse and the switch are in series with the appliance, so either of them can stop the current reaching it.
9.3.3 Advantages Of The Ring System
The ring system has the following four advantages.
- In the ring system, the current from the mains can reach an individual appliance through two separate paths, one round each side of the ring. Thus each appliance gets connected to the mains effectively through a thick wire. Therefore the wire required for the main ring is of a lower current carrying capacity than that which would be required for a direct connection to the mains, and this reduces the cost of wiring considerably.
- Each appliance has a separate fuse. Therefore if, due to some fault, the fuse of one appliance burns, it does not affect the operation of the other appliances.
- In this system, all the plugs and sockets used can be of the same size, but each socket should have its own fuse of rating suitable for the appliances to be connected with it.
- While installing a new appliance in a room, a new line up to the distribution box is not required; it can be directly connected to the ring circuit in that room. Care is taken that the total current drawn from the mains in the ring circuit does not exceed the rating of the main fuse, that is, 30 A.
9.3.4 Connection Of Appliances With The Mains
We note that all the electrical appliances — bulbs, fans, sockets and the rest — are connected in parallel with the mains. In the live wire before each appliance, a separate switch and a separate fuse are connected in series with it. Fig. 9.3-2 shows this arrangement for two appliances.
Diagram
Fig. 9.3-2 — Appliances Connected In Parallel With The Mains Three long horizontal lines across the width of the figure, labelled from top to bottom ‘live’, ‘neutral’ and ‘earth’. Two vertical branches are dropped from the live line. In the first branch, going down: a fuse, then a switch, then a bulb, whose lower terminal joins the neutral line. In the second branch, going down: a fuse, then a switch, then a three-pin socket, whose neutral terminal joins the neutral line and whose earth terminal joins the earth line. Label the fuses, switches, bulb and socket, and show that the two branches are independent of each other.
The two arrangements are compared feature by feature in Table 9.1, and the reasons behind the entries are given in the two subsections that follow.
9.3.4.1 Advantages Of Connecting Appliances In Parallel
Connecting the appliances in parallel with the mains has the following two main advantages.
- Each appliance gets connected to the 220 V supply, which is equal to its voltage rating, and so works normally. An appliance is designed for a stated voltage, and it delivers its rated power only when that voltage appears across it.
- Each appliance operates independently, without being affected by the presence or working of the other appliances.
9.3.4.2 Why Appliances Are Not Connected In Series
Different appliances are not connected in series to the mains for the following three reasons.
- The voltage of the source gets divided among all the appliances connected in series, in the ratio of their resistances. So each appliance does not operate at its rated voltage.
- On connecting one more appliance in the same circuit, the resistance of the circuit will increase. Hence it will reduce the current in the circuit, so each appliance will get less power.
- All appliances connected in series operate simultaneously. None of the appliances can be operated independently. If one appliance is switched off, or is not operating, all the other appliances connected with it in series stop operating.
The difference is not only a matter of convenience; it can be put in numbers, as Example 9.4 shows for two identical bulbs joined the two ways. The two arrangements are drawn in Fig. 9.3-3.
Diagram
Fig. 9.3-3 — Two Identical Bulbs Joined To The Mains In Series And In Parallel Two small circuit diagrams side by side, each supplied from an alternating mains source symbol labelled ‘220 V’. Left diagram, headed ‘in series’: the two bulbs, labelled ‘bulb 1’ and ‘bulb 2’, drawn one after the other in a single loop with the mains; mark ‘110 V’ across each bulb. Right diagram, headed ‘in parallel’: the two bulbs drawn on two separate branches across the same pair of mains lines; mark ‘220 V’ across each bulb.
9.4 The Fuse
We have read that a main fuse, or MCB, is connected in the live wire after the kWh meter and before the main circuit. A fuse is also connected with each ring circuit and with each electrical appliance. It is the simplest safety device in the whole house, and the one every student of this chapter should be able to explain.
9.4.1 What A Fuse Does And How It Works
The fuse protects the circuit by blowing off when the current in the circuit exceeds the current carrying capacity of the fuse. This can happen for either of two reasons.
The first is over loading, which means that the total load in the house exceeds the load limit for which the connection has been taken from the company. The second is short circuiting, which occurs when the live wire comes into direct contact with the neutral or the earth wire. A path of almost zero resistance is then provided to the current, and a heavy current passes through the live wire of the circuit.
A fuse wire permits the flow of current through it only up to a certain fixed limit, which is called the current rating of the fuse. As the current flows in the circuit, the fuse, while passing that current, gets heated up. When the current tends to exceed the safe limit, the temperature of the fuse wire reaches its melting point and the wire melts. As a result, the circuit gets broken.
The whole action, from the fault to the appliance being saved, is a single chain of causes, set out in FC 9.3.
Flowchart
FC 9.3 — How A Fuse Protects A Circuit Over loading or short circuiting → current in the circuit rises above the current rating → heating effect raises the temperature of the fuse wire → temperature reaches the melting point of the fuse wire → fuse wire melts and a gap appears → circuit is broken and current stops → appliance and wiring are saved from damage
9.4.2 Current Rating And Thickness Of A Fuse Wire
Whether a fuse melts depends on how hot it gets, and how hot it gets depends on two things: the current rating \(I\) for which it is meant, and its radius \(r\). It is found that the rise in temperature of a fuse wire is directly proportional to the square of the current rating and inversely proportional to the cube of its radius:
\[\\Delta T \\propto \\frac{I^2}{r^3}\]
where \(\\Delta T\) is the rise in temperature in \(^\\circ\\text{C}\) (or K), \(I\) is the current rating in A, and \(r\) is the radius of the wire in m. Note carefully what is absent: the rise in temperature does not depend on the length of the wire.
Since every fuse wire of a given material melts at the same temperature, a wire meant for a larger current must be given a larger radius to keep \(\\Delta T\) down to that same value. Thus the thickness of a fuse wire depends on its current rating: the higher the current rating, the thicker is the fuse wire.
9.4.3 Material Of A Fuse Wire
A fuse is a short and thin piece of wire of uniform area of cross-section. Two properties are demanded of the material it is made from.
It is made of a material of low melting point, so that it may easily melt due to overheating when a current in excess of the prescribed limit passes through it. Further, it should have a high resistance, so as to produce sufficient heat to melt it.
Generally an alloy of lead and tin is used as the material of the fuse wire, because its melting point is low, about \(250\\ ^\\circ\\text{C}\), and its specific resistance is high — higher than that of copper, aluminium and similar metals.
A copper or aluminium wire is unsuitable for use as a fuse wire because these metals have a high melting point, about \(1080\\ ^\\circ\\text{C}\). Moreover, the use of an ordinary copper or aluminium wire as a fuse must be avoided, as the wire normally available is a thick one which will not melt even if the current exceeds its safe limit.
9.4.4 Construction Of A Fuse
One of the most common fuse arrangements is drawn in Fig. 9.4-1. The fuse wire F is stretched between the two metallic terminals \(\\text{T}\_1\) and \(\\text{T}\_2\) of a porcelain holder, porcelain being used because it is an insulator of electricity.
This holder fits into a porcelain socket having two metallic terminals, to each of which the live wire of the circuit is connected. In this way the fuse wire gets connected in the live wire.
Diagram
Fig. 9.4-1 — Fuse Arrangement Two parts drawn one above the other. Upper part: a removable porcelain holder shown in a three-quarter view, with two metal terminals marked T₁ and T₂ at its two ends and a thin wire stretched between them, labelled ‘fuse wire F’; label the body ‘porcelain holder’. Lower part: a porcelain socket fixed on a board, with two matching metal terminals also marked T₁ and T₂, and a live wire entering each terminal from opposite sides; label these incoming wires ‘live wire’ and label the body ‘porcelain socket (fixed on board)’. Draw a downward arrow showing the holder being pushed into the socket.
9.4.5 Working Of A Fuse
When the current in the circuit exceeds the specified value — due to any reason such as high voltage, short circuiting or over loading — the temperature of the fuse wire rises to the extent that it melts. As a result, a gap is produced and the circuit breaks, as shown in Fig. 9.4-2.
Now the current does not flow through the live wire, and the appliance, or the circuit, is saved. After removing the fault in the circuit, a new fuse wire of the same current rating is fixed in the holder to complete the circuit again.
Fitting a wire of higher rating instead is dangerous, because the circuit would then be left unprotected against exactly the current the original fuse was chosen to stop.
Diagram
Fig. 9.4-2 — A Blown Fuse Breaks The Circuit A side view of the porcelain holder seated in its socket, with the two terminals marked T₁ and T₂. Between them the fuse wire is drawn with a clear gap in the middle and its two melted ends drawn back from the gap; label it ‘broken fuse wire’. Label the holder and the socket. Show the live wire entering the socket at the bottom and leaving at the top, labelled ‘live wire’ at both ends, and mark the gap with the note ‘circuit broken here’.
9.4.6 Why The Fuse Is Connected In The Live Wire
The fuse is always connected in the live wire before the appliance. Then, as soon as the current in the circuit exceeds the rating of the fuse, it melts, breaks the circuit and prevents the heavy current from reaching the appliance, so no current flows in the appliance. The two states of such a circuit are drawn in Fig. 9.4-3.
Diagram
Fig. 9.4-3 — Function Of A Fuse Placed In The Live Wire Two circuit diagrams side by side. In each, a box labelled ‘appliance’ sits at the top, a vertical line on the left labelled ‘live’ and a vertical line on the right labelled ‘neutral’, both meeting a block labelled ‘mains’ at the bottom. In diagram (a), captioned ‘current flowing in the appliance through the fuse’, a small intact fuse marked F is drawn in the live line between the mains and the appliance, and arrows along the wires show current flowing round the complete circuit. In diagram (b), captioned ‘no current flowing in the appliance when the fuse blows off’, the fuse F in the live line is drawn with a visible break, no current arrows are shown, and the appliance is marked ‘no current’.
The Danger Of A Fuse In The Neutral Wire
It is quite unsafe to connect the fuse in the neutral wire. Fig. 9.4-4 shows what happens when it is done. If, due to some defect in the appliance, an excessive current flows in the circuit, the fuse blows off and the current stops flowing in the circuit.
But the appliance still remains connected to the high potential point of the supply through the live wire. Now if a person touches the faulty appliance, he gets an electric shock, because the person comes into direct contact with the mains through the live wire. Thus it is highly unsafe and deceptive to use a fuse in the neutral wire — deceptive because the appliance appears dead while it is in fact still live.
Diagram
Fig. 9.4-4 — Fuse Connected In The Neutral Wire Two circuit diagrams side by side, drawn like Fig. 9.4-3 but with the fuse F now placed in the right-hand line, which is labelled ‘neutral’; the left-hand line is labelled ‘live’ and carries no fuse. In diagram (a), captioned ‘current flowing in the circuit’, the fuse F is intact and arrows show current flowing round the circuit through the appliance. In diagram (b), captioned ‘appliance connected to the high potential even when the fuse blows off’, the fuse F is drawn broken, no current arrows are shown, but the unbroken live line from the mains to the appliance is marked in bold with the label ‘still at high potential — dangerous to touch’.
9.4.7 Cartridge Fuses
Nowadays costly appliances such as a refrigerator, an air conditioner, a geyser or a television are provided with a cartridge fuse, shown in Fig. 9.4-5. These fuses are available in various current ratings such as 1 A, 2 A, 5 A, 10 A and 13 A, and their ends get fitted in the socket. When the fuse blows off, the cartridge is replaced by another one.
Diagram
Fig. 9.4-5 — Cartridge Type Fuse A horizontal cylinder drawn in outline, labelled ‘glass case’, with a metal cap at each end labelled ‘metal caps’. Inside the glass case, a straight thin wire runs from one cap to the other, labelled ‘fuse wire (soldered to the ends)’. Show the soldered joints where the wire meets each cap.
9.4.8 Choosing The Current Rating Of A Fuse
In our houses, the electric wiring for light and fan circuits uses a thin fuse wire of low current rating, about 5 A, because the live wire of these circuits has a current carrying capacity of 5 A. A thick fuse wire of higher current rating, about 15 A, is used in the line of a power circuit, because of the heavy current consumed by appliances such as an air conditioner, a geyser or a washing machine, for which the live wire has a current carrying capacity of 15 A.
The fuse is essential in the power circuit because, if somehow the circuit is over loaded — that is, if the total power of the appliances connected in the circuit exceeds the power limit of the circuit — an excessive current flows in the circuit and the live wire or the appliances may get damaged. The use of a fuse in the circuit safeguards them from this.
The current rating of the fuse at the distribution board for a ring circuit is 30 A, while for the company fuse it is 50 A.
The Rating Required For A Given Circuit
The required current rating of the fuse in a circuit can be obtained from the following relation:
\[\\text{current rating of fuse in a circuit} = \\frac{\\text{total power of appliances in the circuit}}{\\text{voltage of the supply}}\]
where the total power is in W, the supply voltage is in V and the current rating comes out in A. This gives the current the circuit will draw when everything on it is working; the fuse actually fitted is the next standard rating above this value.
Table 9.2 gives the current rating of the fuse needed for various appliances.
Table 9.2 — Current rating of fuse for various appliances
| Appliance | Power rating at 220 V (in W) | Current drawn in running condition (in A) | Current rating for fuse |
|---|---|---|---|
| Electric bulb | 60 | 0.27 | 5 A |
| Television set | 120 | 0.54 | 5 A |
| Refrigerator | 150 | 0.68 | 5 A |
| Electric mixer | 750 | 3.4 | 5 A |
| Room heater | 1000 | 4.5 | 5 A |
| Electric iron | 1000 | 4.5 | 5 A |
| Geyser | 1500 | 6.8 | 8 A |
| Electric kettle | 2000 | 9.1 | 10 A |
| Electric oven | 3000 | 13.6 | 15 A |
Read down the third column and the pattern of the fourth becomes clear: in every row the fuse rating is a little above the running current, never below it and never far above it.
9.4.9 The Miniature Circuit Breaker
Nowadays, instead of a fuse, a miniature circuit breaker (MCB) is used for each individual circuit. If, due to short circuiting or some other fault, a heavy current flows in the line, the MCB trips — its switch falls down — and switches off the circuit in a very short time, about 25 milliseconds. After the fault in the circuit has been repaired, the MCB is raised up, or brought back into the circuit.
The use of an MCB is more convenient than a fuse for two reasons: it avoids the inconvenience of connecting a new fuse wire, and it is much safer because of its quick response. Table 9.3 sets the two devices side by side.
9.5 Switches
Every appliance in a house needs some way of being started and stopped without being unplugged. That is the work of the switch, and where it is placed in the circuit turns out to matter as much as what it does.
Switches are of various kinds and are made in many designs, but for the physics of house wiring we classify them into two groups: the single pole switch and the double pole switch.
9.5.1 Single Pole Switch
The switch used with an appliance to start or stop the flow of current in it is the single pole switch. It is the ordinary switch on a board, controlling one lamp or one fan.
A single pole switch disconnects only the live wire from the appliance. The neutral wire runs on to the appliance whether the switch is on or off, and this is enough, because breaking the circuit at any one point stops the current everywhere in it.
9.5.2 Double Pole Switch
The main switch at the distribution board, used to switch the mains on or off, is the double pole switch. A double pole switch disconnects both the live and the neutral wires simultaneously.
The dual control switch used in a staircase, which is taken up in 9.6, is also a double pole switch of a special kind. The two types of switch are compared in Table 9.4.
9.5.3 Why The Switch Is Connected In The Live Wire
The switch should always be connected in the live wire. Fig. 9.5-1(a) shows a switch S connected in the live wire and turned to the ‘on’ position. The appliance, say a bulb, gets connected to the high potential point through the live wire, and current flows in the appliance because the circuit is complete, the neutral wire providing the return path for the current.
In Fig. 9.5-1(b), the switch S is in the ‘off’ position. The circuit is incomplete and no current reaches the appliance through the live wire, so the appliance does not operate and the bulb does not glow.
It is now safe to carry out repairs in the appliance, if required. Even the live wire connecting the terminal of the appliance can be touched if the switch is in the ‘off’ position, because both the live terminal wire and the neutral wire connected with the appliance are then at zero potential.
Diagram
Fig. 9.5-1 — Switch Connected In The Live Wire Two circuit diagrams one below the other. In each, a bulb is drawn at the top left in a holder, the supply is shown at the right as two terminals marked L (upper) and N (lower) with the label ‘supply’, and the circuit is completed by lines running from the bulb to these terminals. A switch marked S is drawn in the upper line, that is, in the line running to the L terminal. In diagram (a), captioned ‘switch in on position’, the switch contacts are closed, the bulb is drawn glowing with short rays around it, and arrows on the wires show the direction of current. In diagram (b), captioned ‘switch in off position’, the switch contacts are open, the bulb is drawn not glowing and with no rays, no current arrows are shown, and the whole length of wire from the switch to the bulb and back to N is marked ‘at zero potential’.
The Danger Of A Switch In The Neutral Wire
It is deceptive and dangerous to connect a switch in the neutral wire. In Fig. 9.5-2 the switch is connected in the neutral wire. In the ‘on’ position, Fig. 9.5-2(a), it allows current to flow in the bulb, and in the ‘off’ position, Fig. 9.5-2(b), no current passes through the bulb. Thus the switch appears to serve its purpose.
But in the ‘off’ position of the switch, the appliance remains connected to the high potential terminal through the live wire, although no current flows through the appliance because the return path is incomplete. In this condition, it is not safe to carry out any repairs in the appliance, because on touching the live wire terminal of the appliance the current will pass through the body of the person and he may get a fatal shock.
Further, due to some fault, if the live wire touches the metallic body of the appliance, then it is not safe to touch the appliance even from outside. Thus, if the switch is connected in the neutral wire, it can be quite deceptive and dangerous for the user.
Diagram
Fig. 9.5-2 — Switch Connected In The Neutral Wire Two circuit diagrams one below the other, drawn like Fig. 9.5-1 but with the switch S now placed in the lower line, the one running to the N terminal; the upper line running to the L terminal carries no switch. In diagram (a), captioned ‘switch in on position’, the contacts are closed and the bulb is drawn glowing with arrows showing the current. In diagram (b), captioned ‘appliance at high potential even when the switch is in the off position’, the contacts are open and the bulb is not glowing, but the unbroken line from the L terminal to the bulb is drawn in bold and labelled ‘still at high potential — dangerous to touch’.
9.5.4 Safety Precaution While Using A Switch
A switch should never be touched with wet hands. The water from the hand reaches the live wire and forms a conducting layer between the hand and the live wire of the switch. As a result, current reaches the hand from the live wire, and the person may get a fatal shock.
Pure water is a poor conductor, but the water on a hand is never pure; it carries dissolved salts from the skin, and this makes it conducting. Hands should therefore be dried before a switch is operated — which is why switches are never fitted inside bathrooms in the ordinary way.
9.6 Circuits With Dual Control Switches (Staircase Wiring)
We have read that the switch used in the live wire with an appliance, to put it ‘on’ or ‘off’, is the single pole switch. A staircase, however, needs something more: a light that can be turned on at the bottom and turned off again at the top, by someone who has no intention of walking back down.
The dual control switch meets this need. It is a double pole switch of a special kind, generally used one at the top and the other at the bottom of a staircase, or one at each of the opposite ends of a long corridor. With such switches, the appliance, say a bulb, can be switched on or off independently from two different places.
9.6.1 Construction Of A Dual Control Switch
The connections of a dual control switch are illustrated in Fig. 9.6-1. It has three terminals, marked a, b and c. The live wire L is connected at the middle terminal b, and the terminals a and c are joined to the wires going to the other switch.
The switch is operated in two ways. By shifting the knob of the switch to the right, or up, the live wire L connected at terminal b makes contact with terminal a, so as to make the connection ‘ba’ live through the springy metal strip. By shifting the knob to the left, or down, the terminal b makes contact with the terminal c, so as to make the connection ‘bc’ live.
The essential point is that the contact at b is never broken and never doubled. The knob does not switch the current off; it only sends it out by one of two paths.
Diagram
Fig. 9.6-1 — Dual Control Switch A single switch drawn inside a square outline, seen from the front with its cover removed. Three terminals are drawn: terminal a at the upper left, terminal c at the lower left, and terminal b at the right; label each with its letter. From terminal b a springy metal strip is drawn pivoting so that its free end can rest either on terminal a or on terminal c; draw it resting on terminal a as a solid line and show its alternative position on terminal c as a dashed line, with a curved arrow between the two positions. Label the strip ‘springy metal strip’, label the operating knob below it ‘knob’, and label the wire entering terminal b as ‘L’. From terminals a and c, draw two wires leaving to the left, both labelled ‘to other switch’.
9.6.2 Working Of Two Dual Control Switches
Fig. 9.6-2 illustrates the working of dual control switches in an electric circuit. Two such switches, \(\\text{S}\_1\) and \(\\text{S}\_2\), are used. The switch \(\\text{S}\_1\) is fitted at the bottom and the switch \(\\text{S}\_2\) at the top of the staircase. Fig. 9.6-2(a) shows the ‘off’ position of the bulb.
The bulb can now be switched ‘on’ independently by either the switch \(\\text{S}\_1\) or the switch \(\\text{S}\_2\). If the switch \(\\text{S}\_1\) is operated, the connection ‘ba’ is changed to ‘bc’, which completes the circuit and the bulb lights up, as in Fig. 9.6-2(b). Similarly, on operating the switch \(\\text{S}\_2\) from the position shown in Fig. 9.6-2(a), the connection ‘bc’ changes to ‘ba’, which again completes the circuit, as in Fig. 9.6-2(c), and the bulb lights up.
Diagram
Fig. 9.6-2 — Working Of A Pair Of Dual Control Switches Three circuit diagrams one below the other, all drawn alike. In each, a bulb is drawn at the top centre; at the right the supply is shown as two terminals marked N (upper) and L (lower) with the label ‘supply’. Two dual control switches are drawn side by side in the middle of the circuit, the left one labelled S₁ and the right one labelled S₂; each has three terminals labelled a, b and c, with a from the left switch joined by a wire to a of the right switch, and c likewise joined to c. The terminal b of S₁ is connected through the circuit to the bulb, and the terminal b of S₂ is connected to the L terminal of the supply. In diagram (a), captioned ‘bulb off’, S₁ rests on a while S₂ rests on c, so the two connecting wires do not form a complete path; draw the bulb unlit. In diagram (b), captioned ‘bulb on through switch S₁’, S₁ has moved to c and S₂ still rests on c; draw the completed path along the c wire in bold with current arrows, and draw the bulb glowing. In diagram (c), captioned ‘bulb on through switch S₂’, S₁ rests on a and S₂ has moved to a; draw the completed path along the a wire in bold with current arrows, and draw the bulb glowing.
The Rule Behind The Three Figures
Comparing the three diagrams shows the whole idea in one sentence: the bulb glows when both switches are resting on the same connecting wire, and stays dark when they are resting on different ones. The four possible settings are therefore:
- both on a — the circuit is complete through the a wire, and the bulb glows; - both on c — the circuit is complete through the c wire, and the bulb glows; - one on a and the other on c — no complete path exists, and the bulb is dark; - one on c and the other on a — again no complete path, and the bulb is dark.
Moving either knob changes the setting from a matching pair to a mismatched one, or the other way round. That is why either switch, on its own, always reverses the state of the bulb.
Now, if the bulb is in the ‘on’ position as shown in Fig. 9.6-2(b), one can switch the bulb ‘off’ by changing the connection ‘bc’ to ‘ba’, either by operating the switch \(\\text{S}\_1\) or the switch \(\\text{S}\_2\). But if the bulb is in the ‘on’ position as shown in Fig. 9.6-2(c), it can be switched ‘off’ by changing the connection ‘ba’ to ‘bc’, either by the switch \(\\text{S}\_1\) or the switch \(\\text{S}\_2\).
9.6.3 Staircase Wiring In Use
In a staircase, while going up, a person puts ‘on’ the light by operating the switch \(\\text{S}\_1\), so that the connection ‘ba’ changes to ‘bc’ and makes the current flow in the circuit. On reaching the top, he operates the switch \(\\text{S}\_2\) to put ‘off’ the light, so that the connection ‘bc’ changes to ‘ba’ and the flow of current stops.
The same system is operative when a person coming down puts ‘on’ the light by operating the switch \(\\text{S}\_2\), changing the connection ‘bc’ to ‘ba’, and switches ‘off’ the light by operating the switch \(\\text{S}\_1\), changing the connection ‘ba’ to ‘bc’.
Notice that nobody has to remember which switch was used last. Whatever the state of the lamp, either switch will reverse it, so the light is always controlled from wherever the person happens to be standing.
9.7 Earthing
The third wire of the supply cable has been named but not yet explained. It carries no current when everything is working, and for that reason it is easy to think of it as spare. It is not. It is the wire that decides whether a fault in a metal-bodied appliance ends in a blown fuse or in a fatal shock.
9.7.1 Local Earthing
We have read that the local earthing is done in the house near the kWh meter. For this purpose, a hole nearly 2–3 metre deep is dug in the ground. A copper rod, or a thick copper wire, covered by a hollow insulating pipe, is inserted in the hole.
A thick copper plate of dimensions \(50\\ \\text{cm} \\times 50\\ \\text{cm}\) is welded at the lower end of the copper rod, and it is buried inside the ground as shown in Fig. 9.7-1. The plate is surrounded by a mixture of charcoal and salt to make a good contact between the plate and the earth.
To keep the ground damp, water is poured through the pipe from time to time. This forms a conducting layer between the plate and the ground. Dry soil is a poor conductor, so without this dampness the earth connection would not be able to carry away the current it is meant to carry. The upper end of the copper rod is joined to the earth connection at the kWh meter.
Diagram
Fig. 9.7-1 — Local Earthing A vertical section through the ground, drawn as a tall narrow figure. At the top, a wavy horizontal line marks the ground surface, labelled ‘ground’, with a labelled ‘hole’ going down from it. A vertical line from the top of the figure is labelled ‘to earth connection at kWh meter’, and continues down the hole as a labelled ‘thick copper wire’, drawn inside an outer tube labelled ‘hollow pipe’ that runs the whole depth. At the bottom of the hole, a rectangular plate, drawn in section and labelled ‘thick copper plate’, is welded to the lower end of the wire. Around the plate, draw a shaded irregular mass labelled ‘mixture of charcoal and salt’. Mark the depth of the hole as ‘2–3 m’.
9.7.2 Safety Given By Local Earthing
If, due to some reason such as short circuiting, an excessive current flows through the live wires, it passes to the earth through the earth wire if there is local earthing. Otherwise the live wires may get over heated, and this may cause a fire.
The earth connection is thus an escape route of very low resistance. A current offered such a route takes it, in preference to heating up the house wiring.
9.7.3 Earthing Of An Appliance
It is essential to provide a good earthing connection for an electrical appliance such as a refrigerator, a toaster, a geyser, an electric iron or an electric cooler, which we handle physically. The outer metallic case of these appliances is provided with a terminal on the case, and the earth wire of the cable is connected to this terminal.
Fig. 9.7-2 shows the live and neutral wires of the cable from the electric supply passing through the insulating plugs on the metallic case and connected to the element of the appliance. The switch and the fuse are connected in the live wire, and the earth wire of the cable is connected to the metallic case of the appliance.
Diagram
Fig. 9.7-2 — Earthing Of An Appliance A circuit diagram. On the left, three horizontal lines come in from a label ‘source’, marked L (top), N (middle) and E (bottom). The L line passes first through a component labelled ‘fuse’ and then a component labelled ‘switch’. A large rectangle on the right represents the appliance; label its outline ‘metallic case of the appliance’. The L and N lines enter this rectangle through two small blocks on its wall labelled ‘insulator’, and inside the rectangle they connect to the two ends of a coil labelled ‘element of appliance’. The E line runs to a point on the wall of the rectangle, marked with a small dot and labelled ‘terminal’. Label the whole earth line ‘earth wire’.
9.7.4 Safety Given By Earthing An Appliance
When the live wire of a faulty appliance comes into direct contact with its metallic case, due to a break of insulation after constant use or otherwise, the appliance acquires the high potential of the live wire. A person touching the appliance will get a fatal shock, because current flows through his body to the earth.
But if the metallic case of the appliance is properly earthed, then as soon as the live wire comes into contact with the metallic case, a heavy current flows to the earth through the case of the appliance, since the metallic case has almost zero resistance. The fuse connected in the circuit of the appliance blows off, and the appliance gets disconnected from the mains supply.
Thus the person touching the defective appliance does not get any shock, and the appliance is also saved from being damaged. The chain of events is set out in FC 9.4.
Flowchart
FC 9.4 — What Happens When The Live Wire Touches An Earthed Metal Case Insulation inside the appliance breaks → live wire touches the metallic case → case is joined to earth by the earth wire of almost zero resistance → a heavy current flows through the case to the earth → the fuse in the live wire of the appliance blows off → appliance is disconnected from the mains → the case falls to zero potential and is safe to touch
Notice how the fuse and the earth wire work as a pair. The earth wire turns a dangerous leak into a large current; the fuse answers that large current by cutting off the supply. Neither would do the whole job alone, which is why an appliance must have both.
9.8 Three-Pin Plug And Socket
All electric appliances are provided with a cable having a plug at one end, to connect the appliance to the electric supply. The plug and the socket it enters are the last link in the chain that began at the generating station, and both are designed so that the three wires can never be joined up wrongly.
9.8.1 The Three-Pin Plug
The three-pin plug is a fixture provided with three metallic pins, usually of brass, held in an ebonite case, as shown in Fig. 9.8-1.
In a three-pin plug, the top pin is for earthing, the pin on the left is for live and the pin on the right is for neutral. In good quality plugs these are marked E, L and N respectively.
The pins are split at their ends to provide a spring action, so that they fit in the socket holes tightly. A tight fit matters for a reason given in 9.8.4.
Diagram
Fig. 9.8-1 — Three-Pin Plug A three-pin plug drawn in a three-quarter view, so that the oval ebonite body and all three pins are visible. The three pins project from the flat face: one at the top, larger than the others, and two lower down, one at the left and one at the right. Mark the top pin E, the left pin L and the right pin N. Draw the top pin visibly thicker and longer than the other two. Show the ends of all three pins as split lengthwise into two halves. Label the body ‘ebonite case’.
9.8.2 Why The Earth Pin Is Thicker And Longer
The earth pin is thicker and longer than the other two, and each of these two differences does a separate job.
The earth pin is made long so that the earth connection is made before the live connection. This ensures the safety of the user in case of a defective appliance. On inserting the plug in the socket, as soon as the live pin gets connected, the current through the body of the defective appliance passes to the earth, since that connection is already there, and the fuse blows off.
Further, the earth pin is made thicker so that, even by mistake, it cannot be inserted into the hole of the live or the neutral connection. A thick pin simply will not enter a narrow hole, and a wrong connection is thus made impossible rather than merely unlikely.
9.8.3 The Three-Pin Socket
Fig. 9.8-2 shows a socket. It is a fixture in an electric circuit in which the plug is inserted. The socket has three holes, and the inner wall of each hole is made of a hollow metallic tube, usually of brass, forming the terminal at its back end. These terminals are connected to the live, neutral and earth wires of the supply line.
The upper bigger hole in the socket is for the earth connection, while the hole on the right side is for connection to the live wire and the hole on the left side is for connection to the neutral wire.
Diagram
Fig. 9.8-2 — Three-Pin Socket A socket drawn from the front as a rectangular plate. On the left of the plate, a rocker switch. On the right, three holes arranged in a triangle: one larger hole at the top, and two smaller holes below it, one on the left and one on the right. Label the top hole E, the left hole N and the right hole L, with the labels placed outside the plate and short lines pointing to each hole. Show each hole as the mouth of a hollow metallic tube by drawing a small circle inside a slightly larger one.
9.8.4 Safety Precautions For Plug And Socket
While inserting the plug into a socket, the following two main precautions are taken.
The hands must be completely dry, since damp skin conducts and can carry current from a pin to the hand.
Check that the plug pins fit in the socket tightly. Any loose connection gives rise to sparking and burning of either the socket or the plug.
A loose pin touches its terminal only over a very small area, and a poor contact means a high resistance at that point. By the heating effect of current, the heat produced there is large, and this is what burns the plug or the socket and can start a fire.
9.9 Colour Coding Of Wires In A Cable
Each electric appliance is provided with a three-core flexible cable — a single cable carrying three separately insulated wires inside one outer covering. Once the cable is cut, there is nothing about the wires themselves to say which is which, so the insulation on the three wires is made of different colours.
The colours used are red, or brown, for the live wire; black, or light blue, for the neutral wire; and green for the earth wire. The first colour in each pair belongs to the older convention and the second to the newer international one, as set out in Table 9.5. A cross-section of such a cable is drawn in Fig. 9.9-1.
Table 9.5 — Colour coding of wires in a cable
| Wire | Colour of insulation, old convention | Colour of insulation, new international convention |
|---|---|---|
| Live | Red | Brown |
| Neutral | Black | Light blue |
| Earth | Green | Green or yellow |
The earth wire of the new convention is usually seen as green with a yellow stripe running along it, so that it can be told apart at a glance from every other wire in the cable.
Diagram
Fig. 9.9-1 — Cross-Section Of A Three-Core Flexible Cable A short length of cable drawn with its outer sheath cut back at one end, so that three separate insulated wires are seen emerging. Draw the three wires side by side and label them, from left to right, ‘live — brown (red in the old convention)’, ‘neutral — light blue (black in the old convention)’ and ‘earth — green with yellow stripe (green in the old convention)’. Show a small bare copper conductor at the tip of each wire where its insulation has been stripped, and label the outer covering ‘outer sheath’.
The colour coding of wires helps us to connect the switch, the fuse, the socket and the rest into the proper wire of the circuit of the house wiring. Everything this chapter has established depends on it: the fuse and the switch must go into the live wire, the appliance case must go to the earth wire, and the return path must be the neutral wire. Without a colour code, none of these could be identified inside a wall.
9.10 High Tension Wires
Each wire in a cable is capable of withstanding up to a specific limit of current. If the current exceeds this limit, due to short circuiting or high voltage fluctuations, the wire may burn on account of excessive heating, and it may cause a fire.
To avoid this, for high voltage and heavy current a special type of wire, called the high tension wire, is used. A high tension wire has a low resistance and a large surface area.
Each of these two properties answers a separate part of the problem. The resistance is kept low so as to allow a heavy current to pass through it with the least heating, since the heat produced in a wire depends on its resistance as well as on the current. The surface area is made large so that whatever heat is produced is radiated more quickly to the surroundings, and the wire does not go on getting hotter.
Therefore, instead of taking a single thick wire of low resistance as the high tension wire, it is made by twisting together a number of thin wires insulated from each other. This provides a large surface area as compared with a single thick wire of the same material, as Fig. 9.10-1 shows.
Diagram
Fig. 9.10-1 — A Single Thick Wire Compared With A Twisted High Tension Wire Two drawings side by side. On the left, a short length of one thick cylindrical wire, with its circular cross-section shown at the cut end; caption below it ‘single thick wire — small surface area’. On the right, a short length of a wire of the same overall thickness made of many thin strands twisted together in a spiral, with its cut end showing the many small circles packed inside; caption below it ‘many thin wires twisted together — large surface area’. Between the two drawings, an arrow labelled ‘same overall thickness’.
9.11 Safety Precautions While Using Electricity
While using electricity, precautions are taken to avoid the two major dangers: a fire, and an electric shock. Each has its own causes and its own remedies, and both follow directly from the physics of the earlier sections.
9.11.1 Guarding Against Fire
A fire is caused when there is over heating of the live wires, or the cable, for various reasons such as a break of insulation or short circuiting.
To avoid it, one must use wires, or cable, of a current carrying capacity higher than the total current which can flow through the circuit while using all the appliances simultaneously. The test is not the current drawn on an ordinary day, but the largest current the circuit could ever be asked to carry.
9.11.2 Guarding Against Electric Shock
An electric shock may be caused from an appliance due to any of the following: damaged or poor insulation of wires; touching the appliance with wet hands; no earthing of the appliance; no fuse provided with the appliance; and no local earthing.
To avoid it, five precautions are taken.
The insulation of wires must be of good quality, and it should be checked from time to time. The insulation over a wire becomes brittle with the passage of time, and it cracks off, leaving the wire naked.
The appliance should never be operated, or touched, with wet hands. The hands should always be kept dry.
Each appliance must have its metallic case earthed.
Each appliance must be provided with a proper fuse in its live wire, before the appliance.
There must be proper local earthing at the kWh meter.
Read as a set, these five are simply the safety conclusions of the whole chapter gathered in one place. Two of them keep the current inside the wire where it belongs; two of them give a leaking current a safe path to the earth; and the fifth makes sure that path reaches the earth at all.
9.12 Summary
Electric power is generated at generating stations, usually far from the places where it is used, and must be carried a long distance to the consumer.
Power is generated at 11 kV. A higher voltage causes insulation difficulties, and a lower voltage involves a very high current.
The voltage is raised before transmission to reduce the loss of energy as heat in the transmission line wires.
For a given power, voltage and current trade against each other: \[P = VI, \\qquad I = \\frac{P}{V}\] where \(P\) is the power in watt (W), \(V\) the voltage in volt (V) and \(I\) the current in ampere (A).
The power wasted as heat in a line of resistance \(R\) is \[\\text{power lost} = I^2R\] and the energy wasted in a time \(t\) is \(I^2Rt\), where \(R\) is in ohm (Ω), \(t\) in second (s) and the energy in joule (J). Because the current is squared, a small current makes the loss very much smaller.
A d.c. voltage cannot be stepped up or stepped down, so an alternating voltage is generated. Its frequency is 50 Hz, so the polarity at the terminals changes 100 times a second, 50 times positive and 50 times negative.
Along the route the voltage is changed at sub-stations by transformers: stepped up from 11 kV to 132 kV at the grid sub-station; stepped down to 33 kV at the main sub-station; to 11 kV at the intermediate sub-station; and to 220 V at the city sub-station, for the domestic consumer.
The supply cable to a house has three wires: the live or phase wire (L), the neutral wire (N) and the earth wire (E). The live wire carries current from the source; the neutral wire is the return path.
The neutral and earth wires are joined together at the local sub-station, so both are at the same potential, 0 V.
A company fuse, or pole fuse, of high rating is connected in the live wire at the pole or just before the meter, and only the supply company’s staff may handle it. After it comes the kWh meter, which records the energy consumed.
The main switch, or ELCB, is connected in both the live and the neutral wires; the main fuse, or MCB, is connected only in the live wire.
The main switch is a double pole switch with an earthed iron covering. Its advantage is that it breaks the live and the neutral connections from the supply simultaneously.
In the ring system, the wires of each portion of the house leave the distribution box, run round all its rooms and return to the box, forming a ring protected by an MCB of about 30 A.
Each appliance is tapped off the live wire of the ring through its own fuse and its own switch; its other terminal goes to the neutral wire, and its metal case to the earth wire.
Advantages of the ring system: current reaches an appliance by two paths, so thinner and cheaper wire may be used; each appliance has its own fuse; sockets may all be of one size, each with a suitable fuse; and a new appliance may be joined to the ring in the room, provided the total current stays within 30 A.
All appliances are connected in parallel with the mains, while the switch and the fuse of each appliance are in series with it in the live wire.
Parallel connection gives each appliance the full 220 V, which is its rated voltage, and lets each work independently of the others.
Appliances are not joined in series because the voltage would divide among them, adding one would reduce the current to all, and none could be operated on its own.
A fuse is a safety device used to limit the current in a circuit, safeguarding the circuit and its appliances from damage. It works on the heating effect of current.
A fuse blows when the current exceeds its current rating, either through over loading or through short circuiting, which is the direct contact of the live wire with the neutral or earth wire.
The rise in temperature of a fuse wire is \[\\Delta T \\propto \\frac{I^2}{r^3}\] where \(\\Delta T\) is the temperature rise in \(^\\circ\\text{C}\), \(I\) the current rating in A and \(r\) the radius of the wire in m. It does not depend on the length of the wire. Hence the higher the current rating, the thicker the fuse wire.
A fuse wire is short, thin and of uniform cross-section, and is made of a material of low melting point and high resistance — usually an alloy of lead and tin, melting at about \(250\\ ^\\circ\\text{C}\). Copper and aluminium are unsuitable, their melting point being about \(1080\\ ^\\circ\\text{C}\).
The fuse wire is stretched between two terminals in a porcelain holder, porcelain being an insulator, and the holder fits into a porcelain socket whose terminals are joined to the live wire.
The fuse is always connected in the live wire before the appliance. A fuse in the neutral wire is unsafe and deceptive, since after it blows the appliance remains joined to the high potential through the live wire.
Cartridge fuses, used with costly appliances, are available in ratings of 1 A, 2 A, 5 A, 10 A and 13 A, and are replaced whole when they blow.
Light and fan circuits use a fuse of about 5 A and power circuits about 15 A; a ring circuit at the distribution board takes 30 A and the company fuse 50 A. The rating needed is \[\\text{current rating of fuse} = \\frac{\\text{total power of appliances in the circuit}}{\\text{voltage of the supply}}\] with the power in W, the voltage in V and the rating in A.
An MCB is used in place of a fuse for each circuit. It trips in about 25 milliseconds and is raised again after the fault is repaired, so it avoids fitting a new fuse wire and is safer through its quick response.
A switch is an on-off device for the current in a circuit, and is connected in the live wire. A single pole switch disconnects only the live wire; a double pole switch disconnects the live and neutral wires simultaneously.
With the switch in the live wire and turned off, the appliance is left at zero potential and is safe to work on. A switch in the neutral wire leaves the appliance connected to the high potential, and is therefore deceptive and dangerous.
A switch must never be touched with wet hands, since the water forms a conducting layer between the hand and the live wire of the switch.
A dual control switch is a double pole switch of a special kind. Its middle terminal b, joined to the live wire, is made to touch either terminal a or terminal c. Two such switches, one at each end of a staircase, let a lamp be switched on or off independently from either place.
For local earthing, a copper rod inside a hollow pipe is sunk 2–3 m into the ground, with a copper plate of \(50\\ \\text{cm} \\times 50\\ \\text{cm}\) welded at its lower end, surrounded by a mixture of charcoal and salt and kept damp with water. Its upper end goes to the earth connection at the kWh meter.
Local earthing carries an excessive current safely to the earth; without it the live wires may over heat and cause a fire.
The metal case of an appliance is joined to the earth wire. If the live wire then touches the case, a heavy current flows to earth through the case, the fuse blows, and the appliance is disconnected, so the user gets no shock.
A three-pin plug has the earth pin at the top, the live pin on the left and the neutral pin on the right, marked E, L and N, with split ends for spring action. The earth pin is longer, so the earth connection is made first, and thicker, so that it cannot enter the live or neutral hole.
In a three-pin socket the bigger upper hole is for the earth, the right-hand hole for the live wire and the left-hand hole for the neutral wire. The plug must be inserted with dry hands and must fit tightly, since a loose contact causes sparking and burning.
Colour coding of the three-core cable: live red, or brown in the new international convention; neutral black, or light blue; earth green, or green or yellow. It allows each wire to be identified when connections are made.
A high tension wire has low resistance, so a heavy current passes with least heating, and a large surface area, so the heat produced is radiated quickly. It is therefore made of many thin wires twisted together rather than one thick wire.
Fire is avoided by using wires of a current carrying capacity higher than the largest total current the circuit can carry.
Electric shock is avoided by five precautions: good insulation, checked from time to time; never touching an appliance with wet hands; earthing the metallic case of every appliance; a proper fuse in the live wire before each appliance; and proper local earthing at the kWh meter.
Flowchart
FC 9.5 — The Chapter In One Chain, From Generating Station To Appliance Generating station at 11 kV → stepped up to 132 kV → transmission line at high voltage and low current → stepped down at the main, intermediate and city sub-stations → 220 V supply on three wires, live, neutral and earth → company fuse in the live wire → kWh meter → main switch in the live and neutral wires → main fuse or MCB in the live wire → distribution box → ring circuit of a portion of the house → separate fuse and switch in the live wire of each appliance → appliance, with its metal case joined to the earth wire
9.13 Key Terms
Alternating voltage — a voltage whose direction reverses again and again, so that the current it drives flows first one way and then the other; measured in volt (V).
Cartridge fuse — a fuse in which the fuse wire is sealed inside a small glass case with metal caps at its ends, replaced whole when it blows; its current rating is in ampere (A).
Company fuse — the fuse of high rating fitted in the live wire at the pole, or just before the meter, and handled only by the supply company’s staff; also called the pole fuse.
Consumer unit — the unit fitted after the kWh meter, containing a double pole switch or ELCB together with one MCB for each circuit in the house.
Current rating — the largest current a fuse will carry without melting; measured in ampere (A).
Distribution box — the box inside the house from which the separate circuits leave, holding the main fuse or the MCBs.
Double pole switch — a switch that disconnects both the live and the neutral wires at the same instant.
Dual control switch — a double pole switch of a special kind, used in pairs so that one lamp can be switched on or off from two different places.
Earth wire — the third wire of the supply cable, at 0 V, which carries no current in normal working and provides a safe path to the earth for a leaking current.
Earthing — the joining of the metallic body of an appliance, and of the earth wire of the supply, to the earth through a conductor of very low resistance.
Frequency — the number of complete cycles an alternating voltage goes through each second; measured in hertz (Hz).
Fuse — a safety device that limits the current in a circuit by melting when the current exceeds its rating, so safeguarding the circuit and its appliances.
Generating station — the place where some other form of energy is converted into electrical energy, at 11 kV.
High tension wire — a wire of low resistance and large surface area, made of many thin wires twisted together, used for high voltage and heavy current.
kWh meter — the instrument that records the electrical energy consumed in a house; its unit is the kilowatt hour (kWh), where \(1\\ \\text{kWh} = 3.6 \\times 10^{6}\\ \\text{J}\).
Live wire — the wire that carries current from the source to the distribution board, and the only one of the three at a high potential; also called the phase wire.
Main fuse — the fuse fitted in the live wire only, after the kWh meter and before the main circuit.
Main switch — the double pole switch at the distribution board, connected in both the live and the neutral wires, used to switch the whole supply on or off.
Miniature circuit breaker (MCB) — a switch fitted in the live wire of a circuit that trips by itself in about 25 milliseconds on a heavy current, and is raised again after the fault is repaired.
Neutral wire — the wire at 0 V that provides the return path for the current back to the source.
Ring system — the method of house wiring in which the wires of each portion leave the distribution box, run round all its rooms and return to the box, forming a closed ring.
Short circuiting — the direct contact of the live wire with the neutral or the earth wire, giving the current a path of almost zero resistance.
Single pole switch — a switch that disconnects only the live wire from an appliance.
Step-down transformer — the device at a sub-station that lowers the voltage of an alternating supply.
Step-up transformer — the device at a sub-station that raises the voltage of an alternating supply.
Sub-station — a station along the transmission route at which the voltage is changed by a transformer.
Switch — an on-off device for the current in a circuit or in an appliance, connected in the live wire.
Three-core flexible cable — a cable carrying three separately insulated wires, live, neutral and earth, within one outer sheath.
Three-pin plug — the fixture with three brass pins, earth at the top, live on the left and neutral on the right, by which an appliance is joined to the supply.
9.14 Points To Ponder
The earth wire carries no current at all while everything in the house is working properly. Why, then, is running it to every room and every metal-bodied appliance not a waste of copper? Try to describe what a house would be like without it, fault by fault.
If high voltage makes transmission so much more efficient, why is the supply not brought into the house at 132 kV, leaving each appliance to step it down for itself? Set out what would have to be true of the wiring, the switches and the user for that arrangement to be workable.
A fuse protects a circuit by destroying itself. Can you think of other safety devices, electrical or otherwise, that work by failing on purpose? What does deliberate failure achieve that a device which merely warns of danger cannot?
Suppose a wire were found whose resistance was exactly zero at ordinary temperatures. Which arguments in this chapter would collapse, and which would survive? Would there still be any reason to transmit power at high voltage?
A pair of dual control switches works a lamp from two places. Imagine a staircase with a landing, where the lamp must be worked from three places. Reason out what a third switch would have to do to the two connecting wires, and why an ordinary dual control switch cannot be the one placed in the middle.
Our supply alternates 50 times a second. What might have decided that particular number? Think about what you would notice in a lamp, and in a fan, if the frequency were 5 Hz instead, and what might go wrong if it were 5000 Hz.
9.15 Objective Questions
Multiple Choice Type
- At the city sub-station, the voltage of the alternating supply is stepped down to: (1 mark)
- 132 kV
- 33 kV
- 11 kV
- 220 V
- The main fuse of a house circuit is connected in: (1 mark)
- the neutral wire only
- the live wire only
- both the live and the neutral wires
- the earth wire
- The material generally used for a fuse wire is an alloy of: (1 mark)
- copper and tin
- lead and tin
- lead and copper
- iron and aluminium
- The rise in temperature of a fuse wire does not depend on its: (1 mark)
- current rating
- radius
- length
- melting point
- In a three-pin plug, the pin on the left is meant for: (1 mark)
- the earth wire
- the neutral wire
- the live wire
- any of the three wires
- In the new international convention, the insulation of the neutral wire is coloured: (1 mark)
- brown
- black
- light blue
- green
Assertion–Reason Type
For each of the following, choose from: a. both A and R are true and R is the correct explanation of A b. both A and R are true but R is not the correct explanation of A c. A is false but R is true d. A is true but R is false
Assertion (A): The earth pin of a three-pin plug is made longer than the other two pins. Reason (R): The earth connection is then made before the live connection when the plug is inserted. (1 mark)
Assertion (A): An appliance whose switch is fitted in the neutral wire is safe to repair when that switch is off. Reason (R): No current flows through the appliance when its switch is off. (1 mark)
Match The Following
- Match each circuit in Column A with the current rating of its fuse or breaker in Column B. (2 marks)
| Column A | Column B |
|---|---|
| a. Light and fan circuit | i. 15 A |
| b. Power circuit | ii. 30 A |
| c. Ring circuit at the distribution board | iii. 50 A |
| d. Company fuse for a 10 kW connection | iv. 5 A |
Fill In The Blanks
- Complete the following. (3 marks)
- The electric power is generated at the generating station at a voltage of ………… .
- The main switch is a ………… pole switch, since it disconnects ………… wires at the same time.
- The rise in temperature of a fuse wire is inversely proportional to the ………… of its radius.
True Or False
- State whether each of the following is true or false. (3 marks)
- The neutral wire and the earth wire are at the same potential.
- An MCB has to be replaced by a new one every time it trips.
- All the appliances in a house are connected in series with the mains.
Name The Following
- Name the following. (3 marks)
- The device used at the generating station to raise the voltage of the alternating supply.
- The wire of the supply cable that provides the return path for the current.
- The safety device that switches off a circuit in about 25 milliseconds and can be raised again after the fault is repaired.
Give One Word Or Phrase For
- Give one word or phrase for each of the following. (2 marks)
- The direct contact of the live wire with the neutral or the earth wire.
- The method of house wiring in which the wires of a portion leave the distribution box, run round its rooms and return to it.
State The SI Unit Of
- State the SI unit of each of the following. (2 marks)
- The current rating of a fuse.
- The frequency of the alternating supply.
Odd One Out, And Give A Reason
- Fuse, MCB, ELCB, kWh meter. (1 mark)
9.16 Check Your Concepts
Define the current rating of a fuse and state its SI unit. (2 marks)
State two advantages of wiring a house by the ring system. (2 marks)
Give a reason why an alternating voltage, and not a direct voltage, is generated at the power station. (2 marks)
Differentiate between a single pole switch and a double pole switch, stating clearly the basis of the difference. (3 marks)
Why is a piece of copper wire unsuitable for use as a fuse wire? Give two reasons. (3 marks)
A fuse is always connected in the live wire and never in the neutral wire. Give the reason, and state why the wrong arrangement is described as deceptive. (3 marks)
Explain why the voltage of the power generated at a station is stepped up before the power is transmitted. Support your answer with the relation between the current and the power lost in the line. (4 marks)
Different appliances in a house are never connected in series with the mains. State three reasons. (4 marks)
Explain how the earthing of the metal case of an appliance protects a person who touches it after the insulation inside has broken. Name the other safety device that must act for this protection to be completed. (4 marks)
What do you mean by local earthing? Describe how it is done, and state the danger it guards against. (5 marks)
Explain, with reference to the connections ‘ba’ and ‘bc’, how two dual control switches allow a staircase lamp to be switched on at the bottom of the stairs and off again at the top. (5 marks)
9.17 Diagrams, Graphs & Constructions
Draw a labelled circuit diagram showing how a lamp and a three-pin socket in one room are connected to the mains, using a fuse and a switch wherever necessary. Mark the live, neutral and earth wires. (4 marks)
Fig. 9.4-4 shows a fuse connected in the neutral wire of a circuit. State what happens to the appliance when this fuse blows off, and redraw the circuit with the fuse correctly placed. (3 marks)
Draw a labelled diagram of a three-pin plug seen from the front. Mark the pins E, L and N, and state which of the three is made longer and thicker, and why. (3 marks)
Draw a circuit diagram using two dual control switches to light a staircase lamp, with the lamp shown in the ‘on’ condition. Label the terminals a, b and c of each switch. (4 marks)
Fig. 9.7-1 shows the arrangement used for local earthing. Name the parts marked in the figure, and state the purpose of the mixture of charcoal and salt. (3 marks)
Draw a block diagram showing the transmission of electric power from the generating station to a domestic consumer. Write the voltage at each stage. (4 marks)
9.18 Practice With Numericals
An electric iron rated 1000 W is used on a 220 V supply. Find the current it draws. (2 marks)
The appliances in a circuit have a total power of 1540 W and the supply voltage is 220 V. Find the current rating of the fuse required for this circuit. (2 marks)
An electric bulb is rated 100 W, 220 V. Find the resistance of its filament when it is glowing normally. (3 marks)
An appliance of power 2 kW is used for 3 hours every day. Find the energy consumed in one day (i) in kilowatt hour, and (ii) in joule. (3 marks)
Two identical bulbs, each rated 100 W, 220 V, are connected in series across a 220 V supply. Find (i) the voltage across each bulb, (ii) the power spent in each bulb, and (iii) the total power consumed. Assume the resistance of each filament does not change. (4 marks)
A fuse wire of current rating 2.0 A has a radius of 0.3 mm. Find the radius of a fuse wire of the same material having a current rating of 16.0 A. (4 marks)
A power of 660 kW is to be transmitted through a line of total resistance 10 Ω. Find the current in the line and the power lost as heat in it when the transmission voltage is (i) 11 kV and (ii) 132 kV. (4 marks)
A house works at 220 V and its main fuse is rated 15 A. A geyser of 1500 W, a refrigerator of 150 W and eight lamps of 60 W each are switched on together. Find (i) the current drawn from the mains, and (ii) whether an electric oven of 3000 W can also be worked at the same time. Give a reason for your answer. (5 marks)