Household Circuits

Question 1

transmission of power from generating station to consumer

Answer 1

power generating station (11 kV)
grid substation (132 kV)
main substation {to heavy industry} (33 kV)
intermediate substation {to light industry} (11 kV)
city substation {to consumer} (220 V)

Question 2

why is power generated at generating station not directly transmitted to consumers at 11 kV

Answer 2

voltage is raised to reduce the loss of energy in the form of heat in transmission line wires

Question 3

why is ac used instead of dc

Answer 3

it is impossible to step up and step down with dc, so ac is used where there is an alternating voltage

Question 4

how are live and neutral wires connected

Answer 4

the neutral and earth wires are connected at the local substation so that they have the same potential 0V

Question 5

company fuse

Answer 5

before connecting the cable from a pole to the meter in a house, first a fuse of high rating is connected in the live wire at the pole

Question 6

rating of company fuse depends on

Answer 6

load for which the connection is taken from the company

Question 7

main switch also known as

Answer 7

earth leakage circuit breaker/ ELCB

Question 8

ELCB

Answer 8

connected to both live and neutral wires

Question 9

MCB

Answer 9

connected only to live wire

Question 10

what type is main switch

Answer 10

double pole switch

Question 11

advantage of main switch

Answer 11

breaks the connections of live and neutral wire simultaneously from main supply

Question 12

consumer unit

Answer 12

contains a main switch/ELCB and an MCB for each circuit of the house

Question 13

rating of MCB in ring circuit

Answer 13

30 A

Question 14

advantages of ring system

Answer 14

1. current from mains to the appliance has two separate paths. each appliance gets connected to the mains through a thick wire. wire of main ring is of lower current capacity than that which would be required to connect the appliance directly to the mains, and cost of wiring reduces
2. each appliance has a separate fuse
3. all plugs and sockets used are same size, but each socket should have its own fuse of rating suitable for appliance connected
4. while installing new appliance, a new line up to the distribution box is not required, but can be directly connected to ring circuit in that room (precaution: total current drawn from mains by ring does not exceed rating on main fuse)

Question 15

advantages of parallel connection

Answer 15

1. each appliance gets 220 V
2. operates independently and is not affected by the working of other appliances

Question 16

disadvantages of connecting in series

Answer 16

1. voltage gets divided, so each appliance does not operate at its rated voltage
2. on connecting more appliances, the resistance increases
3. affected by the working of other appliances

Question 17

define fuse

Answer 17

an electric fuse is a safety device which is used to limit the current in an electric circuit
the use of a fuse safeguards the circuit and the appliances connected in that circuit from being damaged

Question 18

principle of fuse

Answer 18

heating effect of current

Question 19

dependence of heating effect on current and radius of wire

Answer 19

directly proportional to square of current
inversely proportional to cube of radius

Question 20

material of fuse wire

Answer 20

1. short and thin
2. uniform area of cross section
3. low melting point
4. high resistance to produce sufficient heat to melt it
5. alloy of tin and lead (melting point is 250), and specific resistance is high

Question 21

why can’t fuse be connected to neutral wire

Answer 21

when fuse blows off, current stops flowing in the circuit, but appliance is still connected to high potential point through live wire
if the appliance is touched, they get electrocuted

Question 22

rating for fuse wire in houses

Answer 22

5 A, because live wire has current carrying capacity of 5A

Question 23

where is thick fuse wire of higher rating used

Answer 23

thick fuse wire of 15 A is used in the line of power circuit for heavy current

Question 24

condition for fuse wire connected to an appliance

Answer 24

current rating of fuse must be a little more than the maximum current that can be drawn by the appliance before being overheated

Question 25

advantages of MCB

Answer 25

1. avoids inconvenience of connecting a new fuse wire 2. much safer due to its quick response

Question 26

define switch

Answer 26

an on-off device for current in a circuit (or in an appliance). it is connected to the live wire si

Question 27

single pole switch

Answer 27

used with an appliance to start or stop the flow of current within it only disconnects the live wire from the appliance

Question 28

double pole switch

Answer 28

main switch at distribution board, used to switch on or off the mains disconnects both live and neutral wires simultaneously ex: staircase

Question 29

how is local earthing done

Answer 29

a hole 2-3 metres deep is dug copper rod, covered by a hollow insulating pipe thick copper plate of dimension 50cm * 50 cm is welded at lower end of rod buried deep inside the ground plate is surrounded by mixture of charcoal and salt to make a good contact between plate and earth water is poured through pipe to form a conducting layer between plate and ground

Question 30

why is metallic case of appliance painted

Answer 30

it forms an insulating layer over the appliance to connect to an earthing wire, paint must be removed from the body part to which the connection is to be made

Question 31

safety by earthing of an appliance

Answer 31

when live wire of a faulty appliance comes in direct contact with its metallic case, appliance gets high potential of live wire. person gets electrocuted.

Question 32

why are the pins of a plug splitted at the end

Answer 32

to provide a spring action so that they fit in the socket holes tightly

Question 33

why is earth pin made longer and thicker

Answer 33

long- so that earth connection is made before live connection; ensures safety in case of defective plugging thicker- so that it is not inserted into live or neutral connection by mistake

Question 34

safety precautions for plug and socket

Answer 34

1. hands must be completely dry 2. check that plug pins fit into the socket tightly

Question 35

high tension wire

Answer 35

low resistance- so that heavy current can pass with less heat large surface area- so that heat produced is radiated more quickly to surroundings

Question 36

precaution while using electricity

Answer 36

1. one must use wires of higher current carrying capacity than total current that flows through the circuit while using all appliances simultaneously 2. insulation must be of good quality 3. no wet hands 4. metallic case to be earthed 5. proper fuse 6. proper local earthing at kWh meter