Electricity

Fundamentals:

Electricity is form of energy, which is capable of doing works, e.g. lighting lamps, producing heat, driving machines, operating television-radio-communication systems etc.

Electrical conductors: materials, which allow flow of electricity, are conductors of electric. E.g. silver, copper, aluminium, iron, earth, human body etc.

Insulators: materials that put resistance to flow of electricity are called insulator. E.g. porcelain, ebonite, glass, rubber, wood, mica etc.

Current: rate of flow of electricity or in simple electricity in motion. Unit used ampere and is measured by instrument ammeter.

Potential difference: the terminal from which electric currents tends to flow is positive (+) one, while the terminal to which current tend to flow is negative (-) one. So positive terminal is at higher potential, potential indicates “electric pressure”. The difference between higher and lower potential is called potential difference which is expressed by term “Volts”, measured by “voltmeter”.

Resistance: is the opposition offered by conductors to the flow of electricity. Units in “ohms” and measured by “ohmmeter”.

Work: when an object is moved against force, work is said to be done, measured by ‘Force x Distance’.

Power: is rate of doing work. So power = work/time. Electrical unit of power is watt. Power = EI watts where E = Volts, I = Amperes (Amps). 1 Horse Power (H.P.) = 746 watts=3/4 Kw. 1 K.W. = 1.33 H.P.

Energy: means capacity to do work, during certain time. Electrical energy unit- K.W.H (Kilowatt hour) is energy supplied in one hour by an electrical circuit, which is measured by Energy meter.

A.C. and D.C. Current:

A.C. means alternating current and D.C. means Direct current.

A.C.

When conductors is moved in a magnetic field so that the conductor cuts the magnetic lines of force then E.M.F. (electro magnetic force) or say electrical pressure is generated in the conductor, which can cause flow of electrical current in it. Actually the E.M.F. produced is alternating type direction wise. So the A.C. current direction wise (in alternating manner) is produced in the external electrical circuit.

The alternating current can be made to flow in same direction by connecting a device called “commutator” in the circuit. Then the current produced is known as D.C. (Direct Current).

Phase: it has reference to the manner the winding is made in A.C. machines. We say A.C. machines to be single phase, two phases, three phases etc. if they are wound with single, two or three phase winding respectively.

Alternators (generators producing alternating current) are usually of single or three phase. Three phases gives greater power at less cost than single phase for the same size.

Advantages of A.C. over D.C.:

Simpler generating machine.
Economy in transmission of electrical energy over long distances.
Voltage can be raised or lowered by using transformers.
By raising the voltage, the current is reduced for transmission of a given supply of energy and so the size of conductor is reduced and thus economy is affected.

Load on single and three phase supply:

Most of the domestic loads like light, fan, fridge, iron, heater, oven, washing machine etc. run on single phase supply.

In case of houses/buildings when load demand is high three phase connection made to distribute loads equally amongst the supply phase and this will effect less voltage drop in the load circuits. When voltage around 440v required for loads, this type supply is necessary.

Specification:

D.C. generator specification is given in term of volts, current, kilowatt, speed etc. provided in name plate.

In case of A.C. generator these are volts, phase, frequency, K.V.A., current, speed etc.

Electric circuits:

The electric current path is called electric circuit. When current flow is interrupted the circuit is open circuit. Other wise it is closed circuit and hence electric current flow exists.

When there occurs any connection in between a closed circuit there is short circuits in the system.

Circuit element symbols:

voltage – V or E,
Resistance – R,
Current – I,
Ohms law – I=V/R or E/R
Resistance﹏
Fuse
Earth connection
Switch
Two way switch
Battery

Reason of placing switches on the Live wire.

If the switch is on ‘N’ wire (Neutral wire ) the use of lamp is not safe as even if the switch is off then also the current is there to the lamp unit, hence repairs or replacement works are unsafe. But if the switch is on ‘L’ wire (Live wire) the use of lamp is safe, as if the lamp is not in use, or needs repairs and the switch is off, then there is no current in the lamp fitting.

Electric wires and types of wiring:

Electric conductors are termed as wires and cables. Mainly two types of wires – (i) Bare wire, & (ii) Insulated wire. Bare wires are used for overhead transmission and distribution of electric energy. Insulated wires and cables are used for indoors and underground connection mainly.

The types of insulated wires are (a) lead covered, (b) vulcanized India Rubber (VIR), (c) Tough Rubber Sheathed (TRS) or Covered Tough Sheathed (CTS), (d) Polyvinyl chloride compound wire (PVC), (e)Weather proof wires (f)flexible wire, (g)Insulated cable wire etc.

Wiring types:

1. Cleat wiring. It is VIR wire in porcelain cleat,

2. Casing capping. VIR wire in wooden casing,

3. TRS or CTS wiring run over wooden battens,

4. Metal sheathed wiring,

5. Conduit wiring. VIR wires run in metallic or plastic conduits.

Conduit type is considered to be the best wiring system. Metal conduit provides better protection and safety. However, whether metallic or non-metallic, conduit wiring gaining preference as wiring system.

Calculation of electric energy consumption:

In a house (i) 20 lamps of 100 watts each working 6 hrs./day, (ii) a 1500 w heater working 8 hrs./day and (iii) a 2 HP motor of 85% efficiency works 4 hrs./day. Calculate total no. of units consumed during a month of 30 days. And sum up the total cost for the month (Rs. 5/unit).

Energy taken by lamp/day= 20 x100 x6 watt/hr.=12000w.hr.

Energy taken by heater/day=1500 x8 = 12000w.hr.

Energy taken by motor/day= (2x746x4)/0.85=7021w.hr.

So, total energy /day=12000+12000+7021=31021w.hr.

Hence the units consumed during the month= (31021×30)/1000=930.63 k.w.h.

Total cost for the month=930.63×5=Rs. 4653.15

Safety precautions while using electrical appliances:

There is no gain saying that electrical appliance and switches all need careful handling. Slightest carelessness can result in nasty shock and burn injuries which may even lead to fatal accident or instant death.

Electrical accidents normally may be of three types-

fire,
burns,
Shocks.

Fire Danger:

Electrical short circuits can cause fire. The wiring need to be oveserved and tested from time to time for leakage and short circuits. Electrical sparks may also be another cause of occurring fire. So, faulty wiring must be replaced as and when necessary.

Fuse of correct rating to be used. Amperage of electric appliances to be noted and proper fuses fixed in circuit.

Electric heaters are always to be used with guard over heater bars.

Inflammable materials to be kept at safe place to avoid probable electric spark nearby.

Burns:

When burns caused due to electrical fire, first aid is of immediate necessity.

Shocks:

Short circuit or leakage of electric in appliances or naked wires may be responsible for it. Apart from being fatal, painful shocks may be felt at lower voltage, even down to 25 volts.

For rescue from shocks expert treatment is very necessary.

Some safety hints:

· Switch off before touching appliances for any purpose.

· Replace frayed flex and not to put these under carpet of floors.

· Avoid sharp bends or kink, in the flexible wires.

· Avoid joining extra flex with insulating tape.

· Never to touch switch with wet hands.

· Beware of bare or un-insulated electric conductors.

· Before switching on current to nay appliance better make sure that it is properly earthed and wire insulation in good condition.

· Always switch off the circuits before any repair works undertaken.

· Connect live wire through switch only.

· Earthening system to be in good condition.

· If there is fire, immediately to disconnect supply system.

· At the time of doing any minor repair work even, switching off the circuit to be done and the jobs done with proper insulated tools and instruments.

Lamp Types:

1. Carbon filament: consists of carbon filament in evacuated glass bulb. Life about 800hrs.

2. Metal filaments: here tungsten is used as filament. Life about 850 hrs.

3. Gas filled coil filament: inert gas such as nitrogen, argon etc. put inside the bulb and coil filament incorporated. Light much whiter than vacuum bulbs.

4. Mercury or sodium vapour: these are mainly used for street lighting. The colour of light depends on gas used.

5. Neon-tube lights: used for sign boards and advertisement purpose.

6. Fluorescent tubes: this is similar to mercury vapour type with a coating of fluorescent material on the inside of the tube. 40 watt power, in effect like 100 watt of metal filament lamp. Gives glare less shadows, cool light. Useful for all general lighting. Life about 2000 hrs.

7. Incandescent lamps: here light produced by passing electric current through tungsten filament inside the bulb and light comes from the glowing white hot filament.

Device to be properly selected for efficient and purposeful indoor and outdoor lighting maintaining lower light level for energy conservation.

Electric lighting:

In lighting system the heights are to be located to get uniform illumination. Both direct and indirect lighting can be used, but then the indirect type is preferable. In this the light rays are reflected from ceilings and walls, thus does not strike the eyes of the people around directly.

Illumination: one foot candle unit is the illumination produced by one candle at a distance of one foot.

Lumen: one lumen means one foot candle on one square foot area.

Consumption of lamps given in watts and the efficiency by the amount of lumens produced per watts consumed.

External Lighting:

Good internal light create good atmosphere and mood, thus making hotel inside quite comfortable and congenital for use and even food also more appetizing rather.

External lighting should be such that the hotel stands out significantly from the surrounding lighted areas. It should make the hotel sign clearly visible from a distance. The hotel entrance to be illuminated at a level higher than street lights as far as practicable, having proper brightness. For external lighting mercury vapour lamps on aluminium poles could be gainfully arranged. To use outdoor (external) lights only when needed. For this they may be put on a photocell unit that will turn them off automatically during daylight hours.

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