4 Electricity

Question 1

Formula for Ohms Law ( Resistance )

Answer 1

V = A X R

Question 2

How to calculate resistance in Parallel

Answer 2

Calculate resistance for each branch Add both currents apply Ohms law formula again using the total

Question 3

How to calculate resistance in Series

Answer 3

Add all resistance and use formula

Question 4

Transformer formula

Answer 4

Vp / Vs = np /ns

Question 5

Resistivity accounting for variables

Answer 5

R = pL / A where p is resitivity, L is length and A is cross sectional area

Question 6

Formula for Power

Answer 6

P - V X A ( Joules )

Question 7

Describe electric current as a flow of electrons

Answer 7

When electric current flows in a conductor its actually the electrons moving. Electrons have a negative charge they move from a crowded area to an area without many electrons. IE from negative areas to positive areas Amperes is the measurement of the number of electrons passing a point in a conductor

Question 8

Describe the characteristics of DC current Give 2 examples

Answer 8

Flows in 1 direction only Don’t travel far before current is depleted Not as easy to change voltage Frequency is 0 Batteries DC Generators in wind turbines

Question 9

Describe the characteristics of AC current

Answer 9

Changes direction. The electrons keep switching directions Easy to change the voltage with a transformer Easier to carry over long distances Frequency is 50htz

Question 10

Define a transformer

Answer 10

Transforms the alternating current or Voltage from 1 value to another using the principle of electromagnetic induction

Question 11

Describe a transformer

Answer 11

2 soft iron cores with coils wrapped around. Not touching Primary winding has the AC supplied to it When electricity is supplied the PC creates a changing magnetic field around it the changing field induces an AC in secondary coil

Question 12

In a transformer what dictates the size of the induced current

Answer 12

The number of coils in secondary coil

Question 13

Describe step up transormer

Answer 13

Increases Voltage More turns on secondary coil than Primary coil

Question 14

Describe step down transormer

Answer 14

Decreases Voltage Less turns on secondary coil than Primary coil

Question 15

Draw a diagram of a transformer

Answer 15

.

Question 16

Can transformers work on AC and DC

Answer 16

No only AC

Question 17

Give 2 examples of step up transformers

Answer 17

Starter coils in fluorescent lights Electron gun in TV’s

Question 18

Give 3 examples of step down transformers

Answer 18

Mains clock Stereo Sub station

Question 19

What is considered high voltage

Answer 19

1000v AC and 1500V DC

Question 20

Define Ohms law

Answer 20

The value of a current passing thru a conductor at a constant temperature, is directly proportional to the potential difference between the ends of the conductor and indirectly proportional to the resistance of the conductor

Question 21

Explain Ohms law

Answer 21

Electricity flowing thru a wire is called a current, measurement of the electrons passing at any point is measured in Amps. A pressure is needed which is Volts Resistance is the opposition to this flow creating a change in the current Measured in Ohms

Question 22

drescribe electricity

Answer 22

when flowing along a wire (known as a conductor) is called a current, and this is a measure of the number of electrons passing a particular point in a conductor. This rate of flow is measured in units called amperes (symbol A). Pressure must be provided to cause the electrons to flow and this pressure, which may be derived from a number of sources, is termed the applied voltage or electromotive force (EMF). This is measured in volts (symbol V); the greater the applied voltage, the greater the current flowing.

Question 23

describe short circuit

Answer 23

If a breakdown occurs in the insulation separating adjacent conductors or a conductor from the earth, what is known as a short circuit takes place. That is, the current, instead of following its normal path, finds a quicker return path. The electrical resistance in such a case is generally negligible, whereupon a heavy current will flow and cause intense local heating combined with overloading of the cables. They may then become dangerously overheated unless the circuit is broken.

Question 24

a breakdown in the insulation may take place in many ways. Name and describe some

Answer 24

Such a breakdown in the insulation may take place in many ways. Insulating material will deteriorate with age or from other causes, and a condition may be reached where their insulating properties are insufficient to prevent a short circuit. The perishing of rubber, is a good example of this, and is one of the reasons PVC has superseded rubber as an insulating medium. Cables or wiring may be subjected to mechanical stress through vibration caused by external influences, whilst dampness is a frequent cause of the breakdown in insulating properties. Alternatively, excessive heat through external causes e.g., steam pipes, industrial processes for which the system has not been designed, will also lead to rapid deterioration. Furthermore, insulation is often destroyed by nails driven into walls and penetrating the wiring; workers picks, pneumatic drills etc., striking cable runs; abrasion and (although rarely) rodents.

Question 25

If a breakdown of insulation occurs describe what will happen asnd how this may start a fire

Answer 25

If a breakdown of insulation occurs, excessive current will probably flow through the fault and, if the fuse or circuit breaker fails to operate, overheating will result. For a fire to occur in such circumstances, it is only necessary that there should be combustible material in close proximity to an over-heated wire or a hot spark. Fire can readily be started through a short circuit whether or not a cable is insulated

Question 26

Describe how electrical energy is generated and distributed

Answer 26

A generator is a machine which produces electricity. Electricity is generated, transmitted and distributed as alternating current and can be converted to direct current for specific purposes. Direct current flows from the positive to the negative terminal of the conductor. But, with alternating current there is a rapid change (or alternation) in the direction of flow which occurs many times a second. The number of changes per second is called the frequency and is expressed in so many Hertz (Hz) (cycles per second); this is standardised at 50 Hz in the U.K. Alternating current is generally used for transmission as the voltage can be increased or decreased according to the requirements by means of apparatus called a transformer. Alternating current is particularly suitable for transmission over very long distances for which very high voltages are required. This is because if the voltage is increased, the current is reduced and so the equivalent resistance for any given length of conductor passing the same amount of power is less, so enabling smaller conductors to be used. Electricity is also distributed almost entirely in the form of alternating current which can, if necessary, be converted (or rectified) into direct current for any specialised use.

Question 27

Voltages are classified as either low voltage or high voltage. The classifications are:

Answer 27

Low voltage - greater than 50 volts but not exceeding 1000 volts ac or 1500 volts dc. High voltage - anything greater than low voltage i.e., greater than 1000 volts ac or 1500 volts dc.

Question 28

list the types of substations and describe

Answer 28

1..A pole-mounted rural distribution substation (Figure 2.10) does not usually exceed 11 kV, has a transformer with exposed high voltage terminals and open or enclosed low voltage terminals. The high voltage supply may be fed to the transformer from overhead or underground cables, and the low voltage local distribution may also be either overhead or underground 2.Other Outdoor substations (i) Primary substations (132 kV/33 kV or 11 kV) which may have exposed live conductors (Figure 2.11) or, (ii) Secondary substations (11 kV/400v) which generally consist of a transformer and switchgear with no accessible live parts, and a low voltage (LV) distribution unit (Figure 2.12). Secondary substations, which are the most common type of substation, are used to distribute electricity at 230/400 volts to domestic, commercial and light industrial premises. 3.Indoor Distribution Substations These usually take the form of separate buildings or rooms within or on the roof of larger buildings. They will contain one or more transformers, switch gear and LV distribution units. Voltages do not normally exceed 33 kV.

Question 29

The majority of fires of electrical origin occur due to poor installation, poor or lack of maintenance or the mis-use of electrical systems and apparatus. Electricity is capable of igniting insulation or other combustible material if the power is misused, equipment or cables are overloaded or are not properly insulated and maintained. The most common electrical causes of fires are:

Answer 29

(i) short circuits caused by insulation failure or during work on an installation; (ii) overheating of cables and equipment due to overloading, lack of adequate ventilation or high resistance joints; (iii) the ignition of flammable gases, vapours or dusts by sparks or heat generated by electrical equipment; (iv) the ignition of combustible substances by electro-static discharges.

Question 30

an electric current is

Answer 30

the movement of electrons by continuous displacementthru a substance . Measured in amperes.

Question 31

describe and state the type of cables that may be used in fire alarm or hot areas of cabling

Answer 31

A mineral powder (generally magnesium oxide) within a copper sheathing is also used as an insulating medium for cables which are laid in hot places, such as near furnaces or boilers or in situations where circuit integrity is vital e.g., alarms. These cables are known as MICS (mineral insulated copper-sheathed) or alternatively MICC (mineral insulated copper-clad) cables and are also used in general situations where extra physical protection is required.

Question 32

What is SF6

Answer 32

Sulphur Hexafluoride (SF6) This gas is used as an insulating and interrupting medium in many types of electrical apparatus throughout the voltage range. The types of equipment within which SF6 may be found varies from large gas-filled enclosures where entry by personnel is possible, to small items where access to the gas-filled enclosure is not possible, even during maintenance. Because it is heavier than air and will not disperse easily it -can be a hazard in confined or low lying areas as it is an asphyxiant. Under fault conditions toxic and corrosive byproducts can be produced, both as a gas and as powdery products. Under catastrophic failure, this powder can be blown over and contaminate the immediate area. BA and full protective clothing if a need to enter

Question 33

volts for primary and secondary substations

Answer 33

Primary substations (132 kV/33 kV or 11 kV) which may have exposed live conductors (Figure 2.11) or, (ii) Secondary substations (11 kV/400v) which generally consist of a transformer and switch gear with no accessible live parts, and a low voltage (LV) distribution unit (

Question 34

Indoor substations voltage and describe

Answer 34

Indoor Distribution Substations These usually take the form of separate buildings or rooms within or on the roof of larger buildings. They will contain one or more transformers, switch gear and LV distribution units. Voltages do not normally exceed 33 kV.

Question 35

where is and descibe a 3 phase system

Answer 35

(a) Commercial/Industrial The electrical demand of commercial or small industrial premises may be such that the electricity company has to provide a three-phase supply. The connection from the distribution system will be similar to a single phase supply, except that the service cable has either three cores and a concentric neutral, or, on older installations, four separate cores encased in a lead sheath. A cut-out with fuses in all three line conductors and a solid neutral is installed. The cut-out is connected to a three-phase meter by four single core insulated and sheathed conductors. After the meter the supply is fed through a three-phase main switch to a distribution fuse board. In larger installations the main switch may be connected to a bus-bar chamber from which separate single-phase or three-phase circuits controlled by their own switches may be taken to distribution fuse boards at remote parts of the premises. (b) Residential A block of flats is often supplied by a single threephase service cable which terminates in a 'multiway' cut-out or fuse board (Figures 3.2 and 3.3). A number of separate fuses (usually not more than eight) are connected to each line conductor and single-phase 'rising mains' taken to each flat. If meters are installed in the individual flats the rising main may terminate in another cut-out or a main switch. Alternatively, the meters may all be installed at a single meter position adjacent to the service terminal equipment and the rising main terminates in the consumer unit for each flat. 30 Fire Service Manual In some larger blocks there may be more than one service cable. The first three-phase cable may terminate on the ground floor and single-phase rising mains will service the flats on that floor and the next few floors above. Another three-phase cable will be terminated at a higher level to service the flats on that level and the next few floors above, and so on.

Question 36

Three-phase High Voltage Systems

Answer 36

Where the demand for power is high the electricity company may supply power at high voltage (typically 11 kV) to a substation on the consumers' premises. For a medium sized customer a transformer may be installed at this point to supply a normal three-phase low voltage installation, that is a three-phase four-wire bus-bar with separate subcircuits connected to it.

Question 37

briefly describe what is meant by Earth Leakage

Answer 37

Electricity can only be safely used if the conductors, or windings of apparatus which carry it, are insulated, not only against contact with other conductors, but also against contact with any metal in which they are encased. Insulation may fail as a result of ageing, moisture, mechanical damage, heat or corrosion, and precautions have to be taken to protect personnel and installations against consequent damage. Insulation failures could result in severe or fatal shock, or equipment overheating to a sufficient degree to constitute a fire risk. Electrical systems are designed to provide two lines of defence against electrical shock. The first step is to give protection to the user during the normal working of the installation (e.g., by insulation) and then, in the event of a failure of the first stage, to give protection by automatic disconnection of the supply and earthing.

Question 38

What is earthing

Answer 38

Earthing is the usual safeguard provided to give protection against electric shock. An 'earth' is an electrical connection between a piece of equipment and the general mass of earth such that a fault on that equipment will cause sufficient current to flow to operate the circuit protective devices and, if the equipment has a metal enclosure, to prevent a dangerously high voltage appearing on the casing.

Question 39

Earthing Arrangements List and describe four examples of providing an earth connection for electrical circuits and an example of providing shock protection

Answer 39

(1) Water pipe earthing: Because of the extensive use of plastic piping, this method is now forbidden as the sole means of earthing in the Institution of Electrical Engineers' (IEE) Wiring Regulations. (2) Local earthing (other than water mains pipe): In this system plates, rods or steel frames of buildings are used. (3) Cable sheathing and/or armouring: This is the most reliable system available and is used wherever possible, unless the system of supply is as provided in (4) below. (4) Protective multiple earthing: In this system the earth-continuity conductor connecting all exposed metal work of the electrical installation is itself connected to the local supply neutral (5) Residual Current Devices (RCD): These are fittings designed to help prevent electricshock due to faulty electrical appliances or wiring. An RCD can detect changes in the proper flow of electric current and when it does so it disconnects the power supply in milliseconds. This protects against electrocution and other possible damage

Question 40

what is protective mulpiple earthing

Answer 40

Protective multiple earthing: In this system the earth-continuity conductor connecting all exposed metal work of the electrical installation is itself connected to the local supply neutral

Question 41

Fires caused by earth faults Due to some high impedance in the circuit, the current may take an alternative route which can set up arcing which in turn may lead to ignition of adjacent flammable material. The following are some examples of circumstances where there is a high impedance which prevents the protective gear from operating:

Answer 41

Failure of insulation allowing a leakage to occur between a phase conductor and an earthing conductor or earthed metal-work, e.g., persistent arcing between a conductor and a conduit; • Local overheating at a point of high resistance in the earth fault path itself, e.g., at loose or corroded joints in the earth-continuity conductor; • An earth fault current, unable to dissipate due to high impedance in the earth fault loop, which finds an alternative path to earth by tracking or arcing to adjacent metal-work, e.g., arcing between an earthed conductor (conduit) and a composite gas pipe. This could puncture the gas pipe and cause the escape and ignition of gas.