Household Circuits

Power Station :: 11kV

Benefit of high voltage :: P = VI, for a given power P, I = P/V. At a high voltage, the current becomes low and therefore loss of energy due to heating (I^2 R t) becomes less.

a.c. voltage is used because :: it is not possible to step up or step down a d.c. current, and current is already generated as a.c.

Grid Substation :: Step up from 11kV to 132kV

Main Substation :: Step down from 132kV to 33kV

From Main Substation :: 33kV current supplied to heavy industry and intermediate substation

Intermediate Substation :: Step down from 33kV to 11kV

From Intermediate Substation :: 11kV current supplied to light industry and city substation

City Substation :: Step down from 11kV to 220V, supply to domestic consumers.

Power distribution to a house :: Pole/ source - high rated fuse (company fuse) connected in the live wire - kWh meter - main switch connected in both live and neutral - main fuse (MCB) connected in live wire - different parts of the house.

Ring system :: System where a separate ring is used for each portion of the house. Wires starting from the distribution box run around all the rooms of that portion before returning back to the distribution box forming a ring. A separate connection is taken from the ring for each appliance.

Advantages of ring system :: 1) Current from mains can take two paths to reach each appliance, therefore lower current carrying capacity wire can be used, reducing the cost. 2) Each appliance has a separate fuse and the breaking of that fuse doesn’t affect the other appliances. 3) All plugs and sockets can be of the same size. 4) While installing a new appliance, a new line up the the distribution box is not necessary, it can be connected to the ring circuit of that room.

Advantages of connecting appliances in parallel :: 1) Each appliance gets connected to 220V supply. 2) Each appliance operates independently without being influenced by the presence of other appliances.

Disadvantages of connecting appliances in series :: 1) The voltage of the source gets divided in all the appliances in the ratio of their resistances. 2) On connecting one more appliance in the same circuit, the resistance will increase and each appliance will get less power. 3) All appliances operate simultaneously and none can operate independently.

Electric fuse :: 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. Generally, al alloy of lead and tin is used as it has a low melting point. Always connected in live wire before the appliance.

Rise in temperature of a fuse depends on :: current rating I and radius r. Does not depend on length of wire. change in temperature = I^2/r^2

Why is fuse connected in live wire? :: The current stops flowing when the fuse breaks, but the appliance remains connected to the high potential point of supply through live wire. If they touch the faulty appliance, the person comes in direct contact of the mains and gets an electric shock.

Current rating of fuse :: Total power of appliances in circuit / voltage of supply

MCB :: modern alternative to the traditional fuse, it trips to switch off the circuit in a very short time (25ms) It is more convenient than fuse wire because it does not need to be replaced, and it is safer due to the quick response.

Switches :: on-off device for current in a circuit, connected in live wire. Single pole switch disconnects only live wire. Double pole switch like main switch disconnects both neutral and live wires.

Reasons for switch to be connected in live wire :: If the switch was connected in the neutral wire, the appliance would remain connected to the high potential point of the live wire even when the switch was off, and if a person touched the appliance they could get an electric shock.

Local earthing :: done near the kWh meter. A hole nearly 2-3m deep is dug and a copper rod covered by a hollow insulating pipe is inserted. A thick copper plate is welded to the end of the rod and buried surrounded by a mixture of charcoal and salt and water is poured to keep the ground damp.

Earthing of an appliance :: Earth wire of the cable is connected to the metallic case of the appliance. In case the live wire comes in contact with the case, the current flows through the earth wire, the fuse wire melts and the person doesn’t get an electric shock when touching the appliance.

Three pin plug :: plug where the top pin is for earthing, left is for live and right is for neutral. Earth pin is made longer so that the earth connection is made before the live connection, protecting the user in case of a faulty appliance.

High tension wires :: for high voltage and heavy current, high tension wires are used which have low resistance and large surface area to reduce heating.