Download 6035 Introduction to electrical principles GD

Unit 202: Electrical principles and processes for
building services engineering
Introduction to Electrical
Principles
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.
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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.
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Electricity
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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.
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Principles of electricity
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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.
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Principles of electricity
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Principles of electricity
• There are three important variables relating to electrical circuits:
– Current
– Voltage
– Resistance
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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)
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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.
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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.
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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)
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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.
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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.
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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)
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Ohms law
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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.
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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.
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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.
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Traditional fuse type and mcb type
consumer unit
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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.
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Fuse rating
• Ensuring the appropriate size
• 100÷240(volts) =0.426amps
fuse is used or fuse rating
• Therefore a 1 amp fuse would
can be worked out using this
formulae:
• watts÷volts=amps
• Often fuses in houses are
overrated i.e. a lamp with a
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.
100 watt bulb should be rated
thus.
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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.
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Circuits
• There are two very basic types of electrical circuits:
• Series circuits
• Parallel circuits
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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.
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Series circuit
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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.
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Parallel circuit
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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.
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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.
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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.
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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)
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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?
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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.
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Generation of electricity
• The principles of electricity generation
were discovered by Michael Faraday in
1831. 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.
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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.
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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.
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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.
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Generating electricity
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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!
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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.
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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.
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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.
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Distribution
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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.
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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.
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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.
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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.
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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.
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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.
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Basic Lighting circuit
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Basic Ring main circuit
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Wiring to a ring circuit and spur
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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?
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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.
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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.
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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.
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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.
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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.
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Metal pipework bonding
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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.
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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.
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Temporary continuity bonding
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Temporary continuity bonding
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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.
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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.
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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?
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