Household Circuits Flashcards
transmission of power from generating station to consumer
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)
why is power generated at generating station not directly transmitted to consumers at 11 kV
voltage is raised to reduce the loss of energy in the form of heat in transmission line wires
why is ac used instead of dc
it is impossible to step up and step down with dc, so ac is used where there is an alternating voltage
how are live and neutral wires connected
the neutral and earth wires are connected at the local substation so that they have the same potential 0V
company fuse
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
rating of company fuse depends on
load for which the connection is taken from the company
main switch also known as
earth leakage circuit breaker/ ELCB
ELCB
connected to both live and neutral wires
MCB
connected only to live wire
what type is main switch
double pole switch
advantage of main switch
breaks the connections of live and neutral wire simultaneously from main supply
consumer unit
contains a main switch/ELCB and an MCB for each circuit of the house
rating of MCB in ring circuit
30 A
advantages of ring system
- 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
- each appliance has a separate fuse
- all plugs and sockets used are same size, but each socket should have its own fuse of rating suitable for appliance connected
- 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)
advantages of parallel connection
- each appliance gets 220 V
- operates independently and is not affected by the working of other appliances
disadvantages of connecting in series
- voltage gets divided, so each appliance does not operate at its rated voltage
- on connecting more appliances, the resistance increases
- affected by the working of other appliances
define fuse
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
principle of fuse
heating effect of current
dependence of heating effect on current and radius of wire
directly proportional to square of current
inversely proportional to cube of radius
material of fuse wire
- short and thin
- uniform area of cross section
- low melting point
- high resistance to produce sufficient heat to melt it
- alloy of tin and lead (melting point is 250), and specific resistance is high
why can’t fuse be connected to neutral wire
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
rating for fuse wire in houses
5 A, because live wire has current carrying capacity of 5A
where is thick fuse wire of higher rating used
thick fuse wire of 15 A is used in the line of power circuit for heavy current
condition for fuse wire connected to an appliance
current rating of fuse must be a little more than the maximum current that can be drawn by the appliance before being overheated
advantages of MCB
- avoids inconvenience of connecting a new fuse wire
- much safer due to its quick response
define switch
an on-off device for current in a circuit (or in an appliance). it is connected to the live wire si
single pole switch
used with an appliance to start or stop the flow of current within it
only disconnects the live wire from the appliance
double pole switch
main switch at distribution board, used to switch on or off the mains
disconnects both live and neutral wires simultaneously
ex: staircase
how is local earthing done
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
why is metallic case of appliance painted
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
safety by earthing of an appliance
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.
why are the pins of a plug splitted at the end
to provide a spring action so that they fit in the socket holes tightly
why is earth pin made longer and thicker
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
safety precautions for plug and socket
- hands must be completely dry
- check that plug pins fit into the socket tightly
high tension wire
low resistance- so that heavy current can pass with less heat
large surface area- so that heat produced is radiated more quickly to surroundings
precaution while using electricity
- one must use wires of higher current carrying capacity than total current that flows through the circuit while using all appliances simultaneously
- insulation must be of good quality
- no wet hands
- metallic case to be earthed
- proper fuse
- proper local earthing at kWh meter
