1. Introduction to Electrical Principles
Unit 202: Electrical principles and processes for building services engineering
Introduction to Electrical Principles

2. Principles of electricity
At its simplest, an electric current is a flow of electrons.
In order for an electric current to flow in a simple circuit two requirements are necessary.
A source of chemical energy
A continuous loop of a conducting metal which will allow transfer of that energy.

3. Principles of electricity
The source of chemical energy most commonly used is the electric cell (a group of cells forms a battery).
Batteries are relatively safe because they produce small amounts of electricity and causes a one way electron flow.
They are said to produce a direct current as opposed to the alternating current.

4. Electricity

5. Principles of electricity
The chemical reaction which takes place in the battery produces an excess of electrons at one pole of the battery, and since electrons have a negative electrical charge, this is the negative terminal or cathode.
Similarly, there’s a net positive charge at the other pole of the battery, and this is the positive terminal or anode.

6. Principles of electricity

7. Principles of electricity
If the two terminals of the battery are connected to a continuous loop (or circuit) of materials that will allow the transfer of those electrons (electrical conductors), the difference in electrical potential between the two battery terminals will cause the electrons to “flow” through the material, as a current of electricity.
This will flow from the negative (cathode) to the positive (anode) poles of the battery.

8. Principles of electricity

9. Principles of electricity
There are three important variables relating to electrical circuits:
Current
Voltage
Resistance

10. Current
The electric current in a circuit “the flow of electrons” can be measured as the quantity of charge passing through any particular circuit in a given time.
The unit of electrical charge is known as the coulomb, and when one coulomb of charge flows in one second, the current is said to be one ampere (or 1 amp or 1 A)

11. Current This can be shown by the formulae:
Charge in coulombs (Q)= current in amperes (l) x time (t): Q=lt
And current in amps (l) = charge flowing in coulombs (Q) per second (t): l=Q/t
What effects the flow of current in a simple electric circuit?
For the current of electrons to flow at all there must be a difference in electrical potential between different parts of the circuit.

12. Current
The potential is measured in Volts, and a potential difference of one Volt (V) between two points will allow one joule of work to be done per coulomb of electric charge passing between the points.
The voltage in a battery is therefore a measure of how much energy it can provide.
A 12 volt battery will therefore transfer twice as much energy (12 joules per coulomb) as a 6 volt battery.

13. Current
If an electric current can be represented by the rate of flow of water in a pipe, the voltage would correspond to the water pressure.
This, in a closed circuit such as a central heating system, would be governed by the size and power of the water pump ( or in an electrical circuit by the battery)

14. Current
The second factor affecting the rate of flow of electrons in a circuit is the resistance of materials in the circuit.
Some materials, even though they allow the passage of electrons (i.e. they are conductors of electricity) nevertheless they can slow down electron transfer.
Such materials are known as resistors, and their resistant properties is measured in ohms.
A resistance of one ohm will need a voltage of one volt to drive a current of one amp through it.

15. Current
The resistance of different materials can be compared to pipes of a different diameter through which water must pass in a closed system, the smaller the diameter of the pipe the greater the resistance to the flow.
These relationships between current, volts, resistance is summarised in the principle known as ohms law.
This states that the current flowing in a circuit is proportional to the potential difference (the voltage), providing the temperature of the conductor remains constant.

16. Ohms law
The formulae in ohms law can be written three different ways in order to isolate each of the variables in turn.
Current (l) = voltage (V)
resistance (R)
Voltage (V) = current (l) x resistance (R)
= V=lR
Resistance (R) = voltage (V)
Current (l)

17. Ohms law

18. Fuses
Fusing is a safety measure which aims to prevent high electrical current passing through wires that are not designed to carry such high charges.
This is important because if a current is too high for the wire it passes through, overheating of the wire presents a serious risk of fire.

19. Fuses
The various types of fuse that exist all contain fuse wire, the fuse wire will melt or blow if electric current above the specified amount is passed through the wiring.
Fuses come in various sizes to protect against different levels of current.

20. Miniature circuit breaker
Another form of circuit protection is the miniature circuit breaker or (mcb).
This device will trip a switch to break the electrical current if an excessively high current is detected.
These are far more accurate and more expensive than fuses but they can be reset and are found in all new domestic properties.

21. Traditional fuse type and mcb type consumer unit

22. Residual current device
The Residual current device (rcd) this is a highly sensitive device providing a high degree of protection to high risk parts of electrical systems such as plug socket outlets and electric showers.
The rcd measures the difference between the different electrical conductors in the system e.g. live and neutral and measure changes in the electrical current, if a small change occurs then the system is isolated.

23. Fuse rating
Ensuring the appropriate size fuse is used or fuse rating can be worked out using this formulae:
watts÷volts=amps
Often fuses in houses are overrated i.e. a lamp with a 100 watt bulb should be rated thus.
100÷240(volts) =0.426amps
Therefore a 1 amp fuse would be sufficient for use, though usually a 3 amp fuse would be fitted.
If a 13 amp fuse is used it does not give sufficient protection and is unsafe.

24. Fuse rating
There's a potential for confusion here because 1amp of current is more than sufficient to kill at 240 volts.
Remember the purpose of the fuse is to protect the wiring.
The earth is to protect you the user.

25. Circuits There are two very basic types of electrical circuits:
Series circuits
Parallel circuits

26. Series circuits
This describes a system where the current flow is made to pass through each component (e.g. a bulb) in a circuit.
The current should be the same in any part of the circuit, but the voltage will vary depending upon the resistance of each component.
The total voltage of all components must not exceed the total available voltage; otherwise the bulbs will not glow sufficiently.

27. Series circuit

28. Parallel circuit
In a simple parallel circuit however there are alternative routes open to the flow of electrons, and the current will flow along both.
This results in a very different effect to a series circuit when two electric lamps are connected in parallel.
With this system, when a bulb blows the other stays alight, where as a series system, when one bulb blows, it breaks the circuit all the bulbs go out. E.g. lighting in a house is wired in parallel and a Christmas tree lights are wired in series.

29. Parallel circuit

30. Direct and alternating current
Direct current (d.c.), in a d.c. electrical circuit, the electron flow is in the same direction all the time.
One example would be from cathode to anode of a battery around a simple circuit.

31. Alternating current (a.c.)
Alternating current is found in the majority of domestic properties, the usual rate at socket level usually being 240V a.c.
Within the alternating current, electrons travel continually back and forth. The reason for this is a result of the way the electricity is produced.
Alternating current is produced as a result of electromagnetism.

32. Alternating current (a.c.)
All electrical current produces magnetic force: this is a basic fact that underpins the creation of almost all the electricity used in today's world.
The application of this fact was first demonstrated by Michael Faraday in the 1830’s who discovered that electricity could be generated by moving a magnet in and out or around a coil of wire, which is wound around a soft iron core.

33. Alternating current (a.c.)
Electric generators at power stations still produce a.c. electricity on this principle today.
When a.c. electricity is used it’s essential that appliance be “earthed” as this completes the formation of a circuit necessary for current flow.
The way this works is that the current flows to an appliance from the phase (live) wire and then from the neutral wire (which is in effect connected to earth) the current flows continuously back and forth in the UK at a rate of 50 times a second (50 Hertz)

34. Recap Q. what is the unit for the measurement for electrical current?
Q. the positive terminal on a battery is called what?
Q. what three issues is ohms law concerned with?
Q. What type of circuit protection is used on a new build circuit?
Q. a.c. current is produced as a result of what?

35. Electricity supply and control
At the end of this section you should be able to:
State the main principles behind the generation and supply of electricity
Explain the main features of domestic circuits.

36. Generation of electricity
The principles of electricity generation were discovered by Michael Faraday in He found that moving a bar magnet through a wire coil generated electricity. Modern generators are more complex, but the difference is mainly one of scale.

37. Generating electricity
Power stations range in size from single wind driven devices to major industrial sites, employing many hundreds of staff, but what they are all doing is converting one kind of energy into another. Different stations use a variety of energy sources but they all generate electricity in the same way.

38. Generating electricity
Simplified to its essentials, a power station consists of just two major items. First, there is a machine that generates electricity when its shaft is turned - the generator. Secondly, there is some kind of engine to turn the shaft. The generated voltage can be up to 25,000 volts, which is transformed to a higher voltage for transmission on the grid.
Generators need to turn fast and continuously, and the most efficient type of engine for this is the turbine. In the United Kingdom, most power stations use steam-driven turbines.

39. Generating electricity
In a power station generator, the equivalent of Faraday’s bar magnet is a powerful electromagnet - a coil energised by direct current to produce a magnetic field. This is mounted on the central rotating shaft, and is called the rotor. Around the rotor is a series of coils called the stator, in which the electrical voltage is generated by the rotating magnetic field. Both rotor and stator may weigh several hundred tonnes.

40. Generating electricity

41. Generating electricity
The rotor turns at 3000 revolutions per minute - 50 per second - to produce alternating current with a frequency of 50 hertz (cycles per second). Modern generators typically produce 500 megawatts of power, the largest generating up to 700 megawatts - enough to light seven million 100 watt bulbs!

42. Distribution
Electricity arrives in your area from the national supply network (the National grid) at 275,000 or 400,000 volts. It is reduced to 132,000 volts at a substation for distribution within each area of the country, travelling to further substations known as grid supply points. From these it is distributed on overhead lines or underground cables at 33,000 volts - the primary distribution networks - to the intermediate substations.

43. Distribution
At the intermediate substations, electricity at 33,000 volts is reduced to 11,000 volts for secondary distribution. The secondary distribution networks then carry it at 11,000 volts to individual towns, industrial areas and groups of villages.
Particularly heavy users such as manufacturing industries are supplied at 33,000 volts. Electrified railways have their own substations which draw electricity direct from the grid supply point - the latest overhead-line systems run at 25,000 volts.

44. Distribution
At the final substations, transformers reduce the 11,000 volt supply to 230 volts for small scale customers such as homes and shops. A typical substation serves 200 to 300 houses. Larger users such as farms take electricity at 415 volts.

45. Distribution

46. Distribution
Upon entering the customers home you will find the following.
A sealed over current device that protects the supply companies cable.
An energy metering system to determine the customers usage.
This is then fed into the consumer unit.

47. Basic domestic circuits
The next section will show the final stage from production to consumer unit to domestic output devices such as sockets and electric lighting.
Lighting circuit: this is a radial circuit which feeds each overhead light or wall in turn.
To stop the light being on continuously the live or phase wire is passed through a wall mounted switch, used by the property owner to turn lights on and off at will.
Two way switches are used usually on stairways and these require special switch controls.

48. Basic domestic circuits
The lighting circuit is usually fed by a 1.5mm2 twin and earth pvc insulated cable and is protected by a 6amp fuse or mcb at the consumer unit.
Commonly lighting is split into an upstairs and downstairs circuit.

49. Ring main circuit – 13amp socket outlets
The sockets you will see in domestic properties feeding televisions and stereos will normally be 13 amp socket outlets fed from a continuous ring circuit.
As with the lighting circuit, cables circulate from the consumer unit round each socket and then return to the consumer unit, hence the term ring main.
The ring main permits the cables to be kept to an optimum size as electricity is permitted to flow in two directions to reach the socket.

50. Basic domestic circuits
The ring main circuit is fed using a 2.5mm2 twin and earthed pvc cable and is protected by a 32 amp mcb or fuse.

51. Spur outlets
Spur outlets are usually connected into a ring circuit on an existing system (you would not usually encounter spurs on new installations) where its inconvenient to place a socket from the ring main using two conventional cables.
The spur is connected to the ring main through a joint box, or is wired directly from the back of an existing socket. Spurs can be either fused or non fused.

52. Basic Lighting circuit

53. Basic Ring main circuit

54. Wiring to a ring circuit and spur

55. Recap
Q. Who discovered the principles with regard to generating electricity?
Q. Name three fuel sources of capable of generating electricity?
Q. What is the frequency electricity is brought into the home at?
Q. What are the two types of circuit found in a domestic dwelling?
Q. What size cable is used to feed a socket ring main?

56. What is earthing? 
The earth, or ground in America, in electrical terms, carries no current, and it is this that electricity will make a dash for when it is allowed to escape from its secure home in an electric cable or flex.
This is because one side of the electrical supply, the neutral, is intentionally connected to earth.
If someone touches a live conductor then a current will flow through the person, their shoes, the floor, the wall, via earth and back to the supply transformer via one or more earth connections of the transformer neutral.

57. What is earthing? 
The person has completed the electrical circuit. Should any fault develop in an electrical system the electricity will always head for earth, taking the easiest route there.
The electrical appliances and supplies in the home are of a much higher potential and if any of these become available to touch and are electrically charged at a different voltage to earth the possibility of an electric shock exists, with the current passing through the connection between the charged parts and earth.

58. What is earthing? 
If, for instance, a person comes into contact with a conductive part that is at a potential difference to earthed metalwork and that metalwork; then a very serious shock can result
In order to eliminate this possibility, all electrical earths of circuits supplying equipment in the bathroom and all extraneous conductive parts are bonded together.

59. What is earthing? 
In this way, even if a potential does develop, such as during an earth fault on one of the electrical circuits, all the conductive parts that someone could touch will be at substantially the same voltage.
No dangerous shock current can then flow.

60. What is earthing? 
On its way to earth, leaking current may pass through walls, floors or anything capable of carrying it.
This is made much easier when the connecting substance is wet. Water is an excellent conductor of electricity which is why special care must be taken in the bathroom. 

61. Metal pipework bonding

62. Earthing
The bonding of all exposed metal components in a dwelling that are not part of the electrical are known as equipotential bonding.
The equipotential bonding conductor should be found close to the consumer unit.
In certain areas of domestic property supplementary bonding may be required.
Supplementary bonding is required to link sections of central heating or cold water together as well as metallic surfaces such sink unit tops, steel baths etc.

63. Earthing
When maintenance processes are being undertaken and it is necessary to remove a length a of metal pipework, its essential that the earth continuity be maintained.
This achieved by “bridging” the gap exposed by the removed section of pipe with a temporary bonding wire.
Its vital that the temporary bonding wire is securely in place before the length of pipe is removed.

64. Temporary continuity bonding

65. Temporary continuity bonding

66. Earthing
Earth clips should be used when connecting bonding wire to pipework.
These are designed to clearly inform the importance of the connection and to show that it ensures safe electrical connection.

67. Plastic pipework bonding
Plastic is not a good conductor of electricity and where the use of plastic pipework systems occur the earth continuity to components is broken therefore provision for this must be made.

68. Recap Q. Why is the earth cable so important?
Q. What is the correct size cable required for equipotential bonding?
Q. What is the correct size cable required for supplementary bonding?
Q. Why is it important to use temporary continuity bonding?