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MAINS ELECTRICITY

Mains electricity comes from a power station and its current is the alternating current which can either be stepped up or down by a transformer.

A.c is produced when a conductor is rotated in a magnetic field or when a magnetic field is rotated near a conductor.

This method is known as electromagnetic induction. The source of energy for rotating the turbine is the actual source of electrical energy.

Most of the electricity in East Africa is generated from water.

 Power transmission

This is the bulk transfer of electric power from one place to another. A power transmission system in a country is referred to as the national grid.

This transmission grid is a network of power generating stations, transmission circuits and sub-stations. It is usually transmitted in three phase alternating current.

 Grid input

At the generating plant the power is produced at a relatively low voltage of up to 25 kV then stepped up by the power station transformer up to 400 kV for transmission. It is transmitted by overhead cables at high voltage to minimize energy losses.

The cables are made of aluminium because it is less dense than copper.

Metallic poles (pylons) carry four cables, one for each phase and the fourth is the neutral cable which is thinner and completes the circuit to the generator.

 Grid exit

At sub-stations transformers are used to step down voltage to a lower voltage for distribution to industrial and domestic users.

The combination of sub-transmission (33 kV to 132 kV) and distribution (11 kV to 33 kV) which is then finally transformed to a voltage of 240 V for domestic use.

 Electricity distribution

This is the penultimate process of delivery of electric power. It is considered to include medium voltage (less than 50 kV) power lines, low voltage (less than 1,000 V) distribution, wiring and sometimes electricity meters.

 Dangers of high voltage transmission

1. They can lead to death through electrocution
2. They can cause fires during upsurge
3. Electromagnetic radiations from power lines elevate the risk of certain types of cancer

 Electrical power and energy

Work done = volts × coulombs = VQ,

but Q = current × time = I t.

 So work done = V I t

 Other expressions for work may be obtained by substituting V and I from Ohms law as below

V = I R and I = V / R, 

work done = I R × I t = I2 R t,

 or work done = V × V t / R = V² t / R.

 The three expressions can be used to calculate work done. Electrical power may be computed from the definition of power.

 Power = work / time = I2 R t /t = I2 R

or V2 t / R t = V2 / R

Using work done = V I t,

then Power = V I.

 These expressions are useful in solving problems in electricity. Work done or electrical energy is measured in joules (J) and power is measured in watts (W). 1 W = 1 J/s.

Example

An electric heater running on 240 V mains has a current of 2.5 A.

1. a) What is its power rating?
2. b) What is the resistance of its element?

Solution

1. Power = V I

= 240 × 2.5

= 600 W.

 Rating is 600 W, 240 V.

1. Power = V / R = 600 W.

R = V / I. R

= 240 / 2.5

 = 96 Ω.

 Costing electricity

 The power company uses a unit called kilowatt hour (kWh) which is the energy transformed by a kW appliance in one hour.

 1 kW = 1,000 W × 60 × 60 seconds = 3,600,000 J. The meter used for measuring electrical energy uses the kWh as the unit and is known as joule meter.

 Examples

1. An electric kettle is rated at 2,500 W and uses a voltage of 240 V.
2. a) If electricity costs Ksh 1.10 per kWh, what is the cost of running it for 6 hrs?
3. b) What would be its rate of dissipating energy if the mains voltage was dropped to 120 V?

 Solution

2. Energy transformed in 6 hrs = 2.5 × 6 = 15 kWh.

 Cost = 15 × 1.10 × 6 = Ksh 99.00

2500. Power = V2 / R = 2500.

R = (240 × 240) /2500 = 23.04 Ω.

  Current = V / R = (240 × 2500) / (240 × 240) = 10.42 A

 Power = V I = (2500 × 120) / 240 = 1,250 W.

2. An electric heater is made of a wire of resistance 100 Ω connected to a 240 V mains supply.

 Determine the;

1. a) Power rating of the heater
2. b) Current flowing in the circuit
3. c) Time taken for the heater to raise the temperature of 200 g of water from 230C to 950C. (specific heat capacity of water = 4,200 J Kg-1 K-1)
4. d) Cost of using the heater for two hours a day for 30 days if the power company charges Ksh 5.00 per kWh.

Solution

1. Power = V2 / R

= (240 × 240) / 100

= 576 W

1. P = V I =>> I = P / V

= 576 / 240 = 2.4 A

1. P × t = heat supplied

= (m c θ) = 576 × t = 0.2 × 4200 × 72.

 Hence t = (0.2 × 4200 × 72) / 576 = 105 seconds.

1. d) Cost = kWh × cost per unit = (0.576 × 2 × 30) × 5.0 = Ksh 172.80
2. A house has five rooms each with a 60 W, 240 V bulb. If the bulbs are switched on fro 7.00 pm to 10.30 pm, calculate the;
3. a) Power consumed per day in kWh
4. b) Cost per week for lighting those rooms if it costs 90 cents per unit.

 Solution

1. a) Power consumed by 5 bulbs = 60 × 5 = 300 W = 0.3 kWh. Time = 10.30 – 7.00 = 3 ½ hrs.Therefore for the time duration = 0.3 × 3 ½ = 1.05 kWh.
2. b) Power consumed in 7 days = 1.05 × 7 = 7.35 kWh. Cost = 7.35 × 0.9 = Ksh 6.62

  Domestic Wiring System

Electrical power is transferred from generating stations to consumers at different voltage levels. Electrical power can be considered just like gas or water and same rule applies to its distribution i.e. proper pipeline and associated valves/switches.

Electrical wiring is a proper calculation oriented process and wiring of every installation/facility varies according to its requirements and expectations. 

A typical domestic wiring should be capable of fulfilling all performance and safety regulations and therefore must be planned and approved before its implementation.

                                  Wiring scheme of a household

Following components are necessary for an efficient electrical wiring system:

1. Live and neutral connections from energy meter to the main consumer unit. A split consumer unit is also installed for more redundancy and user friendly operations.
2. Proper circuit breakers are part and parcel of a successful electrical supply system. Ring type circuits are installed for feeding the main sockets of consumer units.
3. Lighting circuits are considered an important part of wiring system and they are radial in nature (most of the time). Miniature circuit breakers of proper rating (usually 6 amperes) are also installed for the protection purpose.
4. Incomer to consumer unit grounding connections.
5. Heavy duty electrical wiring backed by dedicated breakers (radial) for high power equipment like HVAC, cookers, heavy duty geysers etc.
6. Two way switches as desired.
7. Outdoor connections (outside the premises) are also used for lighting purposes. Rating of the breaker governing these circuits should be tightly close to the maximum load to avoid power theft (in case).
8. Very high power equipment like storage heating equipment should be powered directly by the consumer unit.

Consumer unit / fuse box

A consumer unit comprises of an isolator, dedicated miniature circuit breakers (separate for every major circuit), grounding terminals and well defined circuit paths.

Grounding connections

Proper grounding connection is a vital part of every electrical installation. Basic purpose of earthing is to ensure that fault current flows through the system immediately after the fault has occurred resultantly tripping the main circuit. This prevents the floating voltage condition and thus fatal/non-fatal accidents.

A functional earthing system is a must have for every domestic/industrial wiring system. In case the power company does not provide it, it should be arranged locally for the greater good.

Electric shock prevention/minimization

Recent wiring regulations emphasize on RCBOs or RCDs (residual current devices) in an electrical wiring circuit.

Residual Current Device

These devices are crucial for electric shock prevention but relying on them for personnel safety is highly not recommended. Especially sockets and vulnerable switches must be protected by residual current devices. Separate RCBOs or RCDs are recommended for every ring or radial circuit.

Sockets

Recommended number of sockets are crucial for residents’ satisfaction and safety. Number and location of sockets in a wiring system should be precisely taken care of. Less and widely spread sockets encourage the use of circuit extensions which pose a safety risk.

Fuses/MCBs

Fuses and MCBs serve the same purpose i.e. disconnection of fault from the circuit. Fuses are recommended for lighting circuits as they are less sensitive to false alarms and bulb failure or other extremely transient faults do not cause trouble or unnecessary nuisance. Some MCBs are lesser sensitive and they can be used for lighting purpose.

Fuses and miniature circuit breakers are backbone of domestic electrical protection. Failure to design and implement an effective fuse/breaker sequence always results in catastrophic scenarios.

Voltage fluctuation protection

Voltage provided by a power company is prone to fluctuate no matter how well designed a system is. There are documented incidents where domestic electric wiring was exposed to high voltages. This is a very undesirable consequence and should be countered with the installation of voltage sensors.

Voltage fluctuation control system identifies voltage abnormalities and trips the circuit. Over voltage exposure can cause the weakening or failure of cables’ insulation. Voltages below the rated values can also damage the equipment and can render the protection devices useless. A well designed voltage controller is always recommended for domestic electric wiring.

Color coding

Color coding is an agreed upon standard and should be followed in every electrical wiring. A separate and distinguishable color is assigned to every conductors’ insulation and it aids in identification and fault clearance of a circuit.

IEC standards (for three phase system) use brown, black and grey as live wire identifiers.

 Photoelectric Effect

 Photoelectric effect was discovered by Heinrich Hertz in 1887. Photoelectric effect is a phenomenon in which electrons are emitted from the surface of a substance when certain electromagnetic radiation falls on it.

 Metal surfaces require ultra-violet radiation while caesium oxide needs a visible light i.e. optical spectrum (sunlight).Work function

 A minimum amount of work is needed to remove an electron from its energy level so as to overcome the forces binding it to the surface.

 This work is known as the work function with units of electron volts (eV). One electron volt is the work done when one electron is transferred between points with a potential difference of one volt; that is,

 1 eV = 1 electron × 1 volt

 1 eV = 1.6 × 10^-19 × 1 volt

 1 eV = 1.6× 10^-19 Joules (J)

 Threshold frequency

 This is the minimum frequency of the radiation that will cause a photoelectric effect on a certain surface. The higher the work function, the higher the threshold frequency.

  Factors affecting the photoelectric effect

1. Intensity of the incident radiation – the rate of emission of photoelectrons is directly proportional to the intensity of incident radiation.
2. Work function of the surface – photoelectrons are emitted at different velocities with the maximum being processed by the ones at the surface.
3. Frequency of the incident radiation – the cut-off potential for each surface is directly proportional to the frequency of the incident radiation.

 Planck’s constant

 When a bunch of oscillating atoms and the energy of each oscillating atom is quantified i.e. it could only take discrete values.

 Max Planck’s predicted the energy of an oscillating atom to be E = n h f, where n – integer, f – frequency of the source, h – Planck’s constant which has a value of 6.63 × 10^-34 Js.

 Applications of photoelectric effect

1. Photo-emissive cells – they are made up of two electrodes enclosed in a glass bulb (evacuated or containing inert gas at low temperature).

  The cathode is a curved metal plate while the anode is normally a single metal rod)

  They are used mostly in controlling lifts (doors) and reproducing the sound track in a film. Photoconductive cells – some semi-conductors such as cadmium sulphide (cds) reduces their resistance when light is shone at them (photo resistors).

Other devices such as photo-diodes and photo-transistors block current when the intensity of light increases.

 Photo-conductive cells are also known as light dependent resistors (LDR) and are used in alarm circuits i.e. fire alarms, and also in cameras as exposure metres.

2. Photo-voltaic cell– this cell generates an e.m.f using light and consists of a copper disc oxidized on one surface and a very thin film of gold is deposited over the exposed surfaces (this thin film allows light). The current increases with light intensity.

They are used in electronic calculators, solar panels etc.