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T390(E)(N22)T NOVEMBER EXAMINATION

NATIONAL CERTIFICATE

ELECTRICAL TRADE THEORY N2

(11041872)

22 November 2016 (X-Paper) 09:00–12:00

This question paper consists of 7 pages and 1 formula sheet.

(11041872) -2- T390(E)(N22)T


DEPARTMENT OF HIGHER EDUCATION AND TRAINING REPUBLIC OF SOUTH AFRICA

NATIONAL CERTIFICATE ELECTRICAL TRADE THEORY N2

TIME: 3 HOURS MARKS: 100

INSTRUCTIONS AND INFORMATION 1. 2. 3. 4. 5. 6. 7.

Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be given in accordance with the SABS (SANS) Code of Practice, SANS 10142-1:2003 for the wiring of premises. Sketches must be neat, labelled and large enough to show the required detail. Answers must be rounded to TWO decimal places. Write neatly and legibly.

(11041872) -3- T390(E)(N22)T


QUESTION 1: CONDUCTORS AND CABLES 1.1 Name THREE variables that must be considered when selecting a cable. (3) 1.2 The voltage between phases at the supply point of an installation is 380 V.

Calculate the following:

1.2.1 The voltage between supply and neutral at the supply point. (1) 1.2.2 The minimum allowable voltage between supply and neutral at any

point in the installation.

(2) 1.3 A 3 kW inductive load is connected to a 220 V AC supply.

Calculate the actual current drawn from the supply if the load has a power factor of 0,8.

(2)

1.4 Name the THREE methods of cable installation which are used in South

Africa.

(3) [11] QUESTION 2: SWITCHGEAR, CONTACTORS AND RELAYS 2.1 Indicate whether the following statements are true or false. Choose the

answer and write only 'true' or 'false' next to the question number (2.1.1–2.1.6) in the ANSWER BOOK.

2.1.1 In small applications it is allowable to join conductors inside

appliances and light fittings.

2.1.2 Both copper and aluminium crimping ferrules used for large

diameter conductors are crimped with dimple-indent crimping tools.

2.1.3 Never use soldered joints for flexible cables because they will not

flex anymore.

2.1.4 Once the shrinking sleeve of a cold shrink joint is in place, you

need to remove a spiral inside the sleeve.

2.1.5 The resin in a resin joint serves as a heat dissipater (prevents heat

build-up).

2.1.6 A conductor insulated with a silicon compound is good enough for

the internal wiring in stoves.

(6 × 1) (6)

(11041872) -4- T390(E)(N22)T


2.2 Explain the operation of a contactor. (2) 2.3 What are disconnectors used for? (2) 2.4 When the contacts of a circuit breaker open under load an electric arc can

form. Name TWO design features in the circuit breaker that minimises the destruction caused by the arc.

(2) [12] QUESTION 3: DC MOTORS AND STARTERS 3.1 Name ONE advantage of compound motors. (1) 3.2 Draw the load characteristics of a compound motor and label both axes. (4) 3.3 Draw a circuit diagram showing how the armature and field coils of a

compound motor are connected to the supply. Indicate the supply as well as the current flow direction in the circuit.

(3) 3.4 The circuit diagram you drew in QUESTION 3.3 represents ‘forward’.

Draw a circuit diagram that connects the motor for ‘reverse’. Indicate the supply polarity as well as the current flow direction in both circuits.

(4) [12] QUESTION 4: AC MOTORS AND STARTERS 4.1 Draw a circuit diagram of a capacitor start, capacitor run AC motor. Make use

of one capacitor only and label the components in the diagram.

(5) 4.2 A three-phase motor connected to a 3-phase 380 V, 50 Hz supply produces a

rotating magnetic field.

4.2.1 Explain how the rotating magnetic field is formed. (4) 4.2.2 Give the magnitude of the speed at which the resultant magnetic

field rotates around the stator.

(1)

(11041872) -5- T390(E)(N22)T


4.3 The universal series motor is widely used for small AC applications. 4.3.1 Name the DC motor that has the same load characteristic as the

universal series motor.

4.3.2 Name the type of rotor found in a universal motor. (2 × 1) (2) 4.4 Large induction motors need starters to limit the starting current.

Explain how an auto-transformer can be used to start a large squirrel cage induction motor.

(3) [15] QUESTION 5: EARTHING 5.1 Explain the main function of the following: 5.1.1 Cable gland 5.1.2 Earth tag washer (2 × 2) (4) 5.2 Give the stipulations that the wiring code (code of practice) has concerning

the following situations:

5.2.1 The earth continuity conductor does not form part of the cable. 5.2.2 The earth continuity conductor carries prospective fault currents. 5.2.3 The earth continuity conductor does not have the necessary

conductivity.

(3 × 2) (6) 5.3 Explain how overhead shielding against lightning strikes is provided for at

power stations.

(2) [12]

(11041872) -6- T390(E)(N22)T


QUESTION 6: PROTECTION 6.1 On a consumer's premises certain exposed metal parts need to be earthed

while others need not to be earthed. Name FOUR fixed installations or appliances in a household that have exposed metal parts, and state whether they need to be earthed or not.

(4) 6.2 Briefly explain what an earth leakage unit does, and what the rating or

sensitivity of the relay refers to.

(2) 6.3 Briefly explain what the rating or sensitivity of the earth leakage unit refers to. (2) 6.4 State which conductors in a single-phase supply should be disconnected by

the earth leakage unit when an earth fault occurs.

(2) [10] QUESTION 7: MEASURING INSTRUMENTS 7.1 State a disadvantage that a time-switch has when used to switch

non-essential loads off and on at certain times.

(1) 7.2 Apart from the method in QUESTION 7.1, give another method that

municipalities can use to reduce the maximum demand of consumers.

(1) 7.3 Explain how you can calculate your monthly consumption of electricity with

the aid of an energy meter.

(2) 7.4 An energy meter has two coils, similar to a wattmeter.

Illustrate by means of a circuit diagram how the instrument is connected in a single-phase circuit.

(2) [6]

(11041872) -7- T390(E)(N22)T

Copyright reserved

QUESTION 8: TRANSFORMERS A three-phase transformer has a delta-connected primary and a star-connected secondary. The transformer is connected to a 11 kV supply. The secondary line voltage is 132 kV. Calculate the following:

8.1 Primary phase voltage 8.2 Secondary phase voltage 8.3 Primary phase current when it draws full load current of 2 kA from the supply 8.4 Turns ratio 8.5 Secondary line current when it draws full load current from the supply. 8.6 Transformer rating in MVA (6 × 2) [12] QUESTION 9: ELECTRONICS 9.1 Answer the following questions about diodes: 9.1.1 Draw a circuit diagram of TWO diodes and a centre-tap

transformer used to give a DC output voltage from an AC source.

(2) 9.1.2 Draw a graph of the input and output waveforms of the diode circuit

drawn in QUESTION 9.1.1.

(2) 9.1.3 Explain the operation of the diode circuit drawn in

QUESTION 9.1.1.

(3) 9.2 A transistor can be used as an amplifier.

Name another application that transistors can be used for.

(1) 9.3 Use sine-wave waveforms to show how a thyristor can supply a controlled

amount of current to a circuit.

(2) [10]

TOTAL: 100

(11041872) T390(E)(N22)T

Copyright reserved

FORMULA SHEET

2

p p s

s s p

V N I

V N I

f

.60

r

fN

p

n ns

n

CABLES

Ifc =

I = V – E

Ra

Series motor

IL = Ise = Ia

Long shunt

Ise = Ia

IL = Ia + Ish/sj

Short shunt

IL = Ise

IL = Ia + Ish/sj

Series resistors

1 2 .....T nR R R R

Parallel resistors

1 2

1 1 1 1.....

T nR R R R

/

/

2 2

1

2 2

2

/

/

/

2

1

2

( )

( )

3

3

3

/

3

L

C

L C

T

ACTIVE AKTIEWE T

REACTIVE REAKTIEWE T

L

C

L C

R T

X T L

X T C

R X X

L L

L L

L PH F

L PH F

L PH

VI

Z

I I Cos

I I Sin

X fL

XfC

Z R X X

RCos

Z

V I R

V I X

V I X

V V V V

P I R

P V I Cos

S VI

S V I

DELTA

V V

I I

STAR STER

V V

/

F

L PH FI I



NOVEMBER EXAMINATION


22 NOVEMBER 2016

This marking guideline consists of 8 pages.

MARKING GUIDELINE

MARKING GUIDELINE -2- T390(E)(N22)T ELECTRICAL TRADE THEORY N2


QUESTION 1: CONDUCTORS AND CABLES 1.1 Permissable volt drop

Maximum current required

Permissable fault current and time of duration

(3) 1.2 1.2.1 Vphase = Vline/ 3

= 219,4 V

(1) 1.2.2 V allowed = 95 % of Vphase

= 0,95 × 219,4

= 208,4 V

(2)

1.3 P = V.I.cosΦ I = P/(V.pf)

= 3 000/(220 × 0,8)

= 17,05 A

(2)

1.4 In ducts, buried and overhead. (3) [11] QUESTION 2: SWITCHGEAR, CONTACTORS AND RELAYS 2.1 2.1.1 True 2.1.2 False 2.1.3 False 2.1.4 True 2.1.5 False 2.1.6 True (6 × 1) (6) 2.2 When the contactor coil is magnetised it attracts an iron armature.

This movement closes or opens contacts.

(2) 2.3 When maintenance needs to be carried out they isolate the circuit from the

supply.

(2) 2.4 Silver tungsten contacts and arc chutes (de-ionising arc extinguishers). (2) [12]



QUESTION 3: DC MOTORS AND STARTERS 3.1 An advantage of a compound motor is the altering of the characteristic curve

to suit the application.

(1) 3.2

(4) 3.3

+ Supply - Ish Ise (Any appropriate sketch) (3) 3.4

no change Ia reversed no change

- Supply + - Supply +

(4) [12]



QUESTION 4: AC MOTORS AND STARTERS 4.1

MARK ALLOCATION

Correctness 3

Labels 2

(5) 4.2 4.2.1 A three-phase motor produces a rotating magnetic field because

the stator coils are 120° apart and the current in them is also 120° out of phase. The resultant field is a combination of 3 fieldsand it rotates around the motor at a synchronous speed that is equal to the supply frequency.

(Any appropriate 4 × 1)

(4) 4.2.2 It rotates around the motor at a synchronous speed equal to the

supply frequency or 50 revolutions per second or 3 000 revolutions per minute.

(1) 4.3 4.3.1 The DC series motor has the same load characteristic as the

universal series motor.

4.3.2 A wound rotor is in a universal motor. (2 × 1) (2) 4.4 An auto-transformer can be used to start a large squirrel cage induction motor

by reducing the starting voltage that supplies the stator windings. At full speed the transformer is cut out of the circuit.

(3) [15]



QUESTION 5: EARTHING 5.1 5.1.1 To fix the cable to the equipmentand electrically connect the

armouring to the equipment.

5.1.2 To connect the earth continuity conductor to the cable armouring

or steel enclosure of the equipment.

(2 × 2) (4) 5.2 5.2.1 Must have at least the same cross-sectional areaas the largest

phase conductor.

5.2.2 Must not become too hotduring the disconnecting time. 5.2.3 A supplementary conductor must be installed or some of the steel

wires should be replaced with tinned copper wires.

(3 × 2) (6) 5.3 Tall earthed poles carry a spanned wire grid that covers the whole sub-

station.

This wire grid is earthed.

(2)

[12] QUESTION 6: PROTECTION 6.1 An electric stove needs to be earthed (except glass-top plates).

Metallic working surfaces in the kitchen need to be earthed.

Metal basins in bathrooms need not be earthed.

Hot and cold-water systems must be bonded together and earthed.

A fridge needs to be earthed. (Any 4 × 1)

(4) 6.2 When an earth fault occurs the relay detects a difference between the

current in the live and neutral conductorsand trips the supply.

(2) 6.3 The magnitude of the smallest leakage current that will activate the relay (2) 6.4 The liveand neutral conductors (2) [10]



QUESTION 7: MEASURING INSTRUMENTS 7.1 Time switches may be inaccurate in keeping time or the supply to the timer

may be intermittent, causing the switch-off time to be wrong.

The switch will always activate even though the demand for power is not high. (Any 1 × 1)

(1) 7.2 A relay that is activated by the electricity supplier using a high frequency

can be inserted in the supply lines to switch off (shed) non-essential loads such as geysers during peak periods (when demand for electricity is high).

Radio controlled relays may also be used.

Consumers can be requested to use energy saving appliances (e.g. lamps).

Certain areas of the network may be switched off by the supplier. (Any 1 × 1)

(1) 7.3 Subtract the reading at the beginning of the month from the reading at the

end of the month and multiply it with the price per kW.h.

(2) 7.4 energy meter

Load

(2) [6]



QUESTION 8: TRANSFORMERS 8.1 Primary phase voltage = Primary line voltage

Vp ph = 11 kV (must have units)

8.2 Secondary phase voltage = Secondary line voltage ÷

Vs L = 132 000 ÷ = 76,21 kV

8.3 Primary phase current = Ip L/

Ip ph = 2 000/

= 1 155 A (1,16 kA)

8.4 Turns ratio = Vp ph/Vs ph

= 11 000/76 210 = 1 : 7

8.5 Secondary phase current = Primary phase current × turns ratio

Is ph = 1 155 × 1/7 thus Is L = 165 A

8.6 Transformer rating in MVA = × Vp L × Ip L

= × 11 000 × 2 000 = 38,11 MVA

(6 × 2) [12] QUESTION 9: ELECTRONICS 9.1 9.1.1

Correctness

(2)


Copyright reserved

9.1.2

Input wave

Output wave

(2) 9.1.3 Use the middle-tap as neutral. The one side of the coil will then

give an output to the half-wave rectifier connected to it which will be 180° out of phase with the second side of the coil. Added together they become a full-wave rectification. NOTE: Also accept references to sketches. (3 × 1)

(3) 9.2 Also works as a switch (1) 9.3

(2) [10]

TOTAL: 100


T450(E)(A4)T

APRIL EXAMINATION



(11041872)

4 April 2016 (X-Paper) 09:00–12:00


(11041872) -2- T450(E)(A4)T






Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be in accordance with the SABS (SANS) Code of Practice SANS 10142-1 : 2003 for the Wiring of Premises. Sketches must be neat, labelled and large enough to show the required detail. Answers must be given to TWO decimal places. Write neatly and legibly.

(11041872) -3- T450(E)(A4)T


QUESTION 1: CONDUCTORS AND CABLES 1.1 Calculate the full load current which the cable must carry to deliver 380 V, 200

kW at a power factor of 0,9.

(3) 1.2 Calculate the maximum fault current that a 30 mm2 conductor insulated with

PVC can carry for 2 seconds. The conductor and insulation factor for this type of cable is 62.

(3) 1.3 Indicate whether the following statements are TRUE or FALSE. Choose the

answer and write only 'true' or 'false' next to the question number (1.3.1–1.3.5) in the ANSWER BOOK.

1.3.1

1.3.2 1.3.3 1.3.4 1.3.5

Cables in the open air are more dangerous to human beings and animals than cables installed underground. Lightning does not affect cables installed in the open. An EMF of self-inductance is when a coil induces voltage in another coil close by. In a purely inductive circuit, the phase angle between current and voltage is always above zero. As the power factor increases, the current needed to supply the same amount of true power will be less.

(5 x 1) (5) [11]

(11041872) -4- T450(E)(A4)T


QUESTION 2: SWITCHGEAR, CONTACTORS AND RELAYS 2.1 Indicate whether the following statements are TRUE or FALSE. Choose the

answer and write 'true' or 'false' next to the question number (2.1.1–2.1.6) in the ANSWER BOOK. Correct the statement if it is false.

2.1.1

2.1.2 2.1.3 2.1.4 2.1.5 2.1.6

A disconnector differs from a switch-disconnector because the disconnector cannot carry the on-load current. Switch-disconnectors are designed to open a highly inductive load at full voltage. A switch-disconnector must be connected to the neutral point of the supply. Overload current is defined as the current that flows when a short circuit occurs. An abnormally low ambient temperature will delay the tripping of thermal magnetic circuit breakers on overload. A circuit breaker may be used as a disconnector provided it complies with the standards of the relevant disconnectors.

(6 x 1)

(6) 2.2 2.3 2.4

Explain the operation of a relay. State the difference between relays and contactors. State what component of a relay is connected to the relay control circuitry

(3) (2) (1)

[12]

(11041872) -5- T450(E)(A4)T


QUESTION 3: DC MOTORS AND STARTERS 3.1 Briefly explain the purpose of the following parts of a DC motor: 3.1.1

3.1.2 3.1.3

The commutator The brushes The pole shoes

(3 x 1) (3) 3.2 3.3 3.4

State the advantage of a compound motor above other DC motors. State how the speed of a shunt motor is affected by a varying load. Name the application for which a series motor is most suitable.

(1) (1) (1)

3.5 Draw the load characteristics of the level-compounded motor. Label both

axes.

(3) 3.6 Draw a circuit diagram showing how the armature and field coils of a short

shunt compound motor is connected to the supply.

(3)

[12] QUESTION 4: AC MOTORS AND STARTERS 4.1 4.2

Draw a circuit diagram of a capacitor start, induction run AC motor. Explain why an induction motor will not run at synchronous speed.

(3) (2)

4.3 The wound rotor induction motor is widely used for large AC applications. 4.3.1

4.3.2

Explain how it is connected at start-up. Explain how it is connected when running at full speed.

(2 x 2) (4) 4.4 Large induction-motors need to be protected against electrical damage. 4.4.1

4.4.2

State what type of protection can be used to prevent the motor from being damaged. Name TWO criteria that the regulations stipulate about single and multi-phase motor protection devices. (2 x 1)

(1) (2)

4.5 Describe, with the aid of drawings, how to test the insulation resistance of the

windings to earth. Your explanation should include approximate values expected during the test.

(3) [15]

(11041872) -6- T450(E)(A4)T


QUESTION 5: EARTHING 5.1 Name TWO appliances that need to be earthed in households. (2 x 1) (2) 5.2 State why electrical appliances should be earthed (2) 5.3 State what must be done with conductive cable sheaths and cable armouring (1) 5.4 Define earth continuity conductor. (2) 5.5 Explain how the following equipment is earthed: 5.5.1

5.5.2 5.5.3

A double insulated hand-drill Metal roofs and gutters The metal face-plate of the light switch, knowing that the conduit going to the switch is made of plastic.

(3 x 1) (3) 5.6 5.7

Write why earth continuity conductors are erected above overhead lines. State where earth continuity conductors that are erected above overhead lines are earthed.

(1) (1)

[12] QUESTION 6: PROTECTION 6.1 6.2 6.3 6.4 6.5

Describe the purpose of surge arrestors Explain how over-current-protection protects equipment and property. Explain how a current transformer around each phase conductor can be used to protect against single-phasing. State in which conductor miniature circuit breakers should be installed in circuit wiring Except for protection, state one other purpose of an MCB.

(2) (2) (2) (1) (1)

6.6 Explain the following terms relating to circuit breakers: 6.6.1

6.6.2

Snap action Manual reset

(2 x 1) (2) [10]

(11041872) -7- T450(E)(A4)T


Load

Live

Neutral

Earth

QUESTION 7: MEASURING INSTRUMENTS Figure 7.1 shows a load connected to a supply. Draw a neatly labelled circuit diagram showing how the following instruments are connected in single phase circuits.

FIGURE 7.1 – LOAD CONNECTED TO A SUPPLY 7.1 7.2 7.3

A wattmeter An ammeter A frequency meter

(3 x 2) [6] QUESTION 8: TRANSFORMERS 8.1 A single-phase 220/24 V step-down transformer supplies a secondary full-

load current of 70 A at a power factor of 0,8. Calculate the following:

8.1.1

8.1.2

The transformer rating The transformer ratio

(3) (3)

8.2 Show how three coils can be connected together to form a delta connection.

Also show how a 3-phase supply is connected to these coils.

(6) [12]

(11041872) -8- T450(E)(A4)T

Copyright reserved

QUESTION 9: ELECTRONICS 9.1 Draw a circuit showing a step down transformer 240/12 Volts AC that is

connected to a full wave rectifier.

(6) 9.2 Name TWO applications of transistors.

(2 x 1)

(2) 9.3 Draw and label the symbol of a thyristor. (2) [10]

TOTAL: 100

(11041872) T450(E)(A4)T

Copyright reserved

ELECTRICAL TRADE THEORY FORMULA SHEET

2

p p s

s s p

V N I

V N I

f

.60

r

fN

p

n ns

n

CABLES/KABELS

Ifc =

I = V – E

Ra

Series motor/ Seriemotor

IL = Ise = Ia

Long shunt/Langsjunt

Ise = Ia

IL = Ia + Ish/sj

Short shunt / Kortsjunt

IL = Ise

IL = Ia + Ish/sj

Series Resistors/Serieweerstande

1 2 .....T nR R R R

Parallel Resistors/

Paralelleweerstande

1 2

1 1 1 1.....

T nR R R R

/

/

2 2

1

2 2

2

/

/

/

2

1

2

( )

( )

3

3

3

/

3

L

C

L C

T

ACTIVE AKTIEWE T


L

C

L C

R T

X T L

X T C

R X X

L L

L L

L PH F

L PH F

L PH

VI

Z

I I Cos

I I Sin

X fL

XfC

Z R X X

RCos

Z

V I R

V I X

V I X

V V V V

P I R

P V I Cos

S VI

S V I

DELTA

V V

I I

STAR STER

V V

/

F

L PH FI I



APRIL EXAMINATION


4 APRIL 2016


MARKING GUIDELINE

MARKING GUIDELINE -2- T450(E)(A4)T ELECTRICAL TRADE THEORY N2


QUESTION 1: CONDUCTORS AND CABLES 1.1 P = x V x I x cosΦ

I = 200 000 ÷ (380 x 0, 9)

= 584,8 a

(3)

1.2 Ifc =

= 62 x 30 ÷ = 1,32 kA

(3) 1.3 1.3.1

1.3.2 1.3.3 1.3.4 1.3.5

True False False False True

(5 x 1) (5) [11] QUESTION 2: SWITCHGEAR, CONTACTORS AND RELAYS 2.1 2.1.1

2.1.2 2.1.3 2.1.4 2.1.5 2.1.6

False, a disconnector is known as an off-load isolator because it cannot open a highly inductive load at full voltage. True, a switch-disconnector is known as an on-load isolator because it can open a highly inductive load at full voltage. False, a switch-disconnector must be connected in the supply’s live conductor. False, overload current is defined as the current that flows when a fault other than a short circuit occurs. True, an abnormally low ambient temperature will delay the tripping of thermal magnetic circuit breakers on overload. True, a circuit breaker may be used as a disconnector provided it complies with the standards of the relevant disconnectors.

(6 x 1)

(6)



Sp

ee

d

Ia – Armature Current

Differentially Compounded

Level Compounded

Award mark for shape of curve

}Cumulatively Compounded

Series

2.2 2.3 2.4

When the relay coil is magnetised, it attracts an iron armature. This movement closes or opens contacts. Relays need a small current to be activated. Contactors need a slighty larger current to be activated, but their contacts can carry a much larger current. The coil.

(3) (2) (1)

[12] QUESTION 3: DC MOTORS AND STARTERS 3.1 3.1.1

3.1.2 3.1.3

The commutator is the termination of each field winding and it keeps the current of each field winding flowing in the same direction OR it connects the moving coil to the stationary brushes. The brushes transfer currents to the rotating commutator. The pole shoes increase the magnetic efficiency.

(3 x 1) (3) 3.2 3.3 3.4

An advantage of a compound motor is its high starting torque and safe no-load speed. The speed of a shunt motor is relatively constant with a varying load. Cranes and lifts, or where a high starting torque is required.

(1) (1) (1)

3.5

(3)


Copyright reserved

3.6

Sketch must show:

Series Field Correctly drawn

Shunt field correctly connected across armature

Armature correctly connected to Short Shunt and Series Field

(3)

[12] QUESTION 4: AC MOTORS AND STARTERS 4.1

Sketch must show:

Starter Winding, Capacitor and Centrifugal Switch in Series with each other.

Run Winding mus t be parallel with the supply

Centrifugal switch shown as connected to the Squirrel cage rotor / Rotor drawn in relation to the centrifugal switch.

(3) 4.2 When the rotor speed and the synchrone speed is the same, there is no

induction from the rotating magnetic field to the rotor.

(2)


Copyright reserved

4.3 4.3.1

4.3.2

At start-up, 3 resistors are connected to the star-connected rotor windings. When running at full speed, the resistors now measure 0 ohms or the rotor windings are short circuited.

(2 x 2) (4) 4.4 4.4.1 Phase imbalance, thermal, overload and surge protection. (1) 4.4.2 Multi-phase motors should be protected against single phasing,

people should be protected against automatic start-up once supply has tripped. (2 x 1)

(2)

4.5

An insulation resistance tester (megger) is connected between terminals a1 and earth, then between b1 and earth, then between c1 and earth.

Readings are taken and in each case they must be at least as high as recommended by the manufacturer (normally higher than 500 000 ohms).

(3) [15] QUESTION 5: EARTHING 5.1 Stove

Washing machine

Fridge

Freezer

Metal exterior appliances (Any 2 x 1)

(2)

5.2 To minimise the danger of being electrocuted

reduce touch voltage

Route fault currents down to earth.

(2) 5.3 It must be bonded to earth. (1) 5.4 A conductor, including any clamp or terminal, that connects the consumers'

earth terminal electrically to the exposed conductive parts of an installation.

(2)


Copyright reserved

5.5 5.5.1

5.5.2 5.5.3

Has a floating earth. Braided conductor bolted onto the roof and gutters and clamped onto an earth rod in the ground. A bare earth conductor must be drawn from the earth bar in the DB through the conduit and bonded to the face-plate

(3 x 1) (3) 5.6 5.7

Earth continuity conductors protect the phase lines against lightning and falling trees/objects. Earth continuity conductors that are erected above overhead lines are connected to conducting structures (pylons) and earthed via an earth continuity conductor or earth lead connected to an earth electrode in the ground.

(1) (1)

[12] QUESTION 6: PROTECTION 6.1 To discharge excessive / surge voltages down to earth and to short circuit

so that the surge does not damage any equipment

(2) 6.2 In preventing excessive currents from flowing the possibility of equipment

melting and property catching fire / being damaged becomes less.

(2) 6.3 When current in one phase is significantly less than the other two phase

currents, the secondary winding's EMF will be less and this can be used to trigger a tripping mechanism.

(2) 6.4 In the live supply wire that feeds the circuit (1) 6.5 As a switch. (1) 6.6 6.6.1

6.6.2 The rapid opening of current carrying contacts Reset that is done by hand.

(2 x 1) (2) [10]


Copyright reserved

Load

Live

Neutral

Earth

7.1Wattmeter

A

7.2 Ammeter

F

7.3 Frequency

Meter / Counter

L1

L2

L3

U1

U2

V1

V2

W1

W2

QUESTION 7: MEASURING INSTRUMENTS

NOTE: Sketches of candidates may look different (3 x 2) [6] QUESTION 8: TRANSFORMERS 8.1 8.1.1

(3) 8.1.2

(3) 8.2

(6)

[12]


Copyright reserved

Live

240 / 12 V AC

Neutral

+

-

Laminated

Core

Coils

QUESTION 9: ELECTRONICS 9.1

(6)

9.2 A switch

An amplifier (2 x 1)

(2)

9.3 Correct Symbol = Correct Labelling =

(2) [10]

TOTAL: 100


T440(E)(N12)T




(11041872)

12 November 2014 (Y-Paper) 13:00–16:00


(11041872) -2- T440(E)(N12)T






Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be in accordance with the SABS (SANS) Code of Practice SANS 10142-1:2003 for the Wiring of Premises. Sketches must be neat, labelled and large enough to show the required detail. Formulae used in Electrical Trade Theory N2 can be found at the end of the question paper. Answers must be given to TWO decimal places. Write neatly and legibly.

(11041872) -3- T440(E)(N12)T


QUESTION 1: CONDUCTORS AND CABLES 1.1 Name THREE variables to consider when selecting the cable for a specific

application.

(3) 1.2 Name THREE advantages of installing high-voltage cables by suspending

them in the open air.

(3) 1.3 A 220 V single-phase inductive load uses 20 kW at a lagging power factor of

0,8. Calculate the full load current that flows through the supply cable.

(3)

1.4 Explain the term armoured cable. (2)

[11] QUESTION 2: SWITCHGEAR, CONTACTORS AND RELAYS 2.1 Compare disconnectors and circuit breakers under the headings below.

Present your answer in table format.

2.1.1 Function 2.1.2 Operation

(2 x 2)

(4) 2.2 Explain how single-phasing is prevented when a 3-phase circuit breaker trips. (2) 2.3 A circuit breaker has an inverse time delay. Explain this terminology. (2) 2.4 When a breaker begins to open, the current could still flow as a result of

arcing. Name THREE measures manufacturers use to minimize the destructive power of the arc.

(3)

2.5 State how the geyser sub-circuit is identified the distribution board. (1)

[12]

(11041872) -4- T440(E)(N12)T


QUESTION 3: DC MOTORS AND STARTERS 3.1 Explain the function of the following protective devices used in DC motor

starters:

3.1.1 Overload relay 3.1.2 No-volt release coil 3.1.3 Starting resistors

(3 x 2)

(6) 3.2 Draw a neat, fully labelled circuit diagram of a short shunt compound DC

motor.

(4) 3.3 Name TWO methods of reversing the direction of rotation of a series motor. (2)

[12] QUESTION 4: AC MOTORS AND STARTERS 4.1 The universal series motor is widely used for small AC applications. 4.1.1 Explain how it operates with an AC supply. (2) 4.1.2 Name TWO appliances using universal motors. (2) 4.2 Name TWO types of induction motors. (2) 4.3 Explain how current flow is established in the rotor bars of an induction type

squirrel cage motor.

(3) 4.4 Large induction motors need starters to limit the starting current. State the

type of starter that should be used with a large squirrel-cage induction motor.

(2) 4.5 Figure 4.1 shows an electric motor and insulation tester. Describe, with the

aid of drawings, how to test the insulation resistance between the windings. The explanation should include approximate values expected during the test.

FIGURE 4.1: INSULATION RESISTANCE TESTING OF A MOTOR

(4) [15]

(11041872) -5- T440(E)(N12)T


QUESTION 5: EARTHING 5.1 Between supplier and consumer, the ground is being used as the earth-

continuity conductor.

5.1.1 State which conductors on the supplier’s side are connected to

earth.

(1)

5.1.2 State which conductors on the consumer’s side are connected to earth.

(1)

5.2 Explain the purpose of earthing of an electrical installation. (2) 5.3 Give a definition of the 'consumer's earth terminal'. (1) 5.4 Name THREE protective measures that will protect people, animals and

property from harmful earth fault currents.

(3) 5.5 An outdoor substation contains overhead conductors, switch disconnectors,

surge arrestors, overload protection, star-delta transformers, etcetera. Explain how the following are earthed:

5.5.1 All equipment earthing points, metal tanks, metal support

structures

(2)

5.5.2 The overhead earth wires stretching above the substation (2) [12]

QUESTION 6: PROTECTION 6.1 State the purpose of an earth-leakage protection device (2)

6.2 Explain where the earth leakage relay must be installed in a domestic electrical installation.

(2)

6.3 Refer to FIGURE 6.1. Identify the conductors that should pass through the core of the toroid transformer inside a three-phase core balance earth leakage relay.

FIGURE 6.1: THREE PHASE BALANCED CORE EARTH LEAKAGE

(1) 6.4 Explain the term 'phase imbalance' with reference to three phase mains

supply.

(2)

6.5 Make a sketch to show a single-phase installation can be protected against voltage surges using low-voltage surge protectors.

(3) [10]

L1 L2 L3 N E

(11041872) -6- T440(E)(N12)T


QUESTION 7: MEASURING INSTRUMENTS Measuring instruments are important for circuit diagnostics and recording. Give the name of the instrument you would use to determine:

7.1 The amount of electric power consumed (1) 7.2 Reasons why the alarm clock (operating on AC) is losing time and the electric

grinder seems to be turning slower

(1) 7.3 House earthing continuity within acceptable limits (stipulated in the Code of

Practice)

(1) 7.4 If the supply at a point falls within the permissible 5% drop (1) 7.5 Whether the month's billing from the supplier would be high because of

excessive power usage over long periods

(1) 7.6 If the current rating of the supply cable is being exceeded

(1)

[6] QUESTION 8: TRANSFORMERS A three-phase transformer has a star-connected primary and a delta-connected secondary. The transformer is connected to a 2,2 kV supply. The secondary-phase voltage is found to be 220 V. Calculate the following:

8.1 Primary-phase voltage (3) 8.2 Secondary-line voltage (1) 8.3 Secondary-line current when at full-load (4) 8.4 Transformer rating in kVA (4)

[12] QUESTION 9: ELECTRONICS 9.1 Explain how a diode is tested when it is suspected to be faulty. (2) 9.2 Explain how a Zener diode can be used as a voltage regulator. (2) 9.3 Draw the circuit symbol for a PNP transistor. Correctly label all the terminals. (4) 9.4 State TWO uses of a transistor. (2) [10]

TOTAL: 100

(11041872) -7- T440(E)(N12)T


FORMULA SHEET

Z

VIT

CosII TACTIVE

SinII TREACTIVE

Lf2πXL

fC2π

1CX

2CL

2 )X(XRZ

Z

RCos 1

RIV TR

LTXL XIV

CTXC XIV

2XCXL

2R )V(VVV

R IP 2

osIV3P LL C

VIS

LL I V3S

DELTA

V FPH /L V

FPHI /L 3I

STAR

V FPH /L V3

FPHI /LI

CABLES

t

ACIFI

t

ACIFI fs

rfc

/

p

s

s

p

s

p

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N

N

V

V

f2ω

p

fN

60.

n

nns r

I =aR

EV

Serie motor

aI

seI

LI

Long shunt

ase II

sjsh

Ia

IL

I/

Short shunt

seL II

sjshI

aI

LI

/

Series Resistors n21T R...RRR

Parallel Resistors

n21T R

1...

R

1

R

1

R

1





12 NOVEMBER 2014


MARKING GUIDELINE



INSTRUCTIONS AND INFORMATION NOTE: There is NOT only one answer or one method (approach) of answering the questions. This marking guideline gives only one answer or one possible method (approach). Examiners must analyse the student's solution to determine if the question has been answered and must not adhere strictly to solutions in this marking guideline.



QUESTION 1 1.1 The load to be supplied The type of conditions where installed

The permissible voltage drop The protective circuitry available The magnitude of possible fault currents (Any 3 x 1)

(3)

1.2 Easier to install Easier to repair a fault

Easier to locate a fault Easier to make joints Easier to replace Easier to identify and trace a cable Easier to add a cable to the same route Cheaper to install (less labour costs) (cable is cheaper) The air acts as a good dissipater of heat (Any 3 x 1)

(3)

1.3 P = VIcosΦ OR S=P/cosΦ = 25 000 VA

I = P/(V(cosΦ) I = S/V Formula manipulated = 20 000/(220x0,8) = 25 000/220 Substitution = 113,64 A = 113,64 A Answer(Must include units)

(Any 3 x 1)

(3)

1.4 By armoured cable we mean that the conductors and their insulation are

protected by metal wires and a waterproof covering.

(2) [11]

QUESTION 2 2.1

Comparison Disconnector Circuit Breaker

2.1.1 Function Used to isolate a circuit for maintenance or in the case of an emergency

Used to isolate a circuit during abnormal circuit conditions such as overload or short circuit

2.1.2 Operation Operated manually Operated automatically

(Any 2) (4) 2.2 The three contacts are interlocked, so all three phases are opened and

closed simultaneously.

(2) 2.3 The time it takes to open is inversely proportional to the amount the current

exceeds the rated value. The larger the current, the quicker it trips.

(2) 2.4 (a)

(b) (c)

The manufacturer inserts de-ion arc extinguishers. The manufacturer inserts interpole barriers. The contacts are made of silver tungsten.

(3)

2.5 Each sub-circuit must be labelled at its respective circuit breaker. (1)

[12]



QUESTION 3 3.1 3.1.1 Overload relay cuts the supply to the motorwhen the current is

too high.

(2) 3.1.2 No-volt release coil cuts the supply to the motor when the supply

voltage is too low.

(2) 3.1.3 Starting resistors limit the starting current through the motor.

(2)

3.2

Correct circuit coils armature (4) 3.3 Reversing the direction of current through the field winding or reversing the

direction of current through the armature winding.

(2) [12]

QUESTION 4 4.1 4.1.1 Since the current changes direction every half cycle, the force on

the conductors will remain in the same direction if both armature and field coil are connected in series.

(2)

4.1.2 Hand drill, angle grinder, hair dryer, shaver, et cetera.

(Any 2)

(2) 4.2 Wound rotor- and squirrel-cage induction motor. (2) 4.3 The stator coils generate a rotating magnetic field through the motor.

When a changing magnetic field cuts the rotor bars, an EMF is induced inside the rotor bars. Current is generated inside the rotor bars because the bars are shorted at the rotor ends. (Any 3 feasible facts)

(3)



4.4 Auto-transformer, Star-delta starting, Solid state (thyristor) voltage

controllers (Any 2 x 1)

(2) 4.5

An insulation resistance tester (megger) is connected between terminals a1 and b1, then between a1 and c1, then between b1 and c1. Readings are taken and in each case they must be at least as high as recommended by the manufacturer (normally higher than 500 000 ohms).

(4) [15]

QUESTION 5 5.1 5.1.1 The neutral conductor 5.1.2 Only earthing conductors (2) 5.2 To guard electrical installations, machines, appliances or apparatus against

the effects of leakage currents, static charges and lightning. To be used with protective equipment to protect humans and animals.

(2)

5.3 A terminal that is effectively and permanently earthed and to which the

earth-continuity conductor of the consumer's installation is permanently connected.

(1)

5.4 Earthing and bonding, electrical separation of circuits, double insulation,

earth-leakage protection. (Any 3 x 1)

(3) 5.5 5.5.1 With an earthing conductor connected to an array of earth rods

and earth mats and a bare wire grid laid in the ground

(2) 5.5.2 With an earthing conductor or conductive metal structures from

the overhead earth wires to the earth rods mentioned above

(2) [12]



QUESTION 6 6.1 To isolate the circuit from the supplyas soon as a dangerous earth-fault

current is detected

(2) 6.2 The earth-leakage relay must be as close as possible to the supply point of

the circuit, normally directly after the consumer's disconnector or main circuit breaker

(2)

6.3 The three live conductors (1) 6.4 When the currents in each phase are very different (2) 6.5

one mark per sketch

(3) [10]

QUESTION 7 7.1 Watt-hour or energy meter (1) 7.2 Frequency meter (1) 7.3 Megger or multimeter set on resistance scale or ohmmeter (1) 7.4 Multimeter set on voltage scale or voltmeter (1) 7.5 Maximum demand meter (1) 7.6 Ammeter (possible to measure high currents) (1)

[6]



QUESTION 8 8.1

(3)

8.2 For Delta The secondary-line voltage = Secondary Phase Voltage

Phase Voltage is given as 220V thus:

VLS = 220 V

(1)

8.3

(4)

8.4

(4)

[12] QUESTION 9 9.1 Use a multimeter on the ohms scale. If the diode indicates a low resistance

OR open circuit in both directions, then it is faulty.

(2) 9.2 A Zener diode will have a constant voltage across it when reverse biased,

even when the supply varies.

(2)



9.4

Symbol Emitter Base Collector

(4)

9.5 A transistor can be used as a switch or as an amplifier. (2)

[10] TOTAL: 100


T440(E)(J22)T

AUGUST EXAMINATION



(11041872)

22 July 2014 (Y-Paper) 13:00–15:00


(11041872) -2- T440(E)(J22)T





INSTRUCTIONS AND INFORMATION 1. 2. 3. 4. 5. 6. 7. 8.

Answer ALL the questions. Read ALL the questions carefully. Number the answers according to the numbering system used in this question paper. Where applicable, answers must be in accordance with the SABS (SANS) Code of Practice SANS 10142-1:2003 for the Wiring of Premises. Sketches must be neat, labelled and large enough to show the required detail. Formulae used in Electrical Trade Theory N2 can be found at the end of the question paper. Answers must be given to two decimal places. Write neatly and legibly.

(11041872) -3- T440(E)(J22)T


QUESTION 1: CONDUCTORS AND CABLES

1.1 Cables can carry fault currents that are much higher than their rated current. Name TWO factors that will determine the permissible fault current.

(2)

1.2 Give TWO disadvantages of installing high-voltage cables, by suspending them

in open air.

(2)

1.3 State a variable that must be considered when selecting a cable for the following applications;

1.3.1 Supplying an inductive load (1)

1.3.2 Connecting an outlet which is very far from the point of supply (1)

1.3.3 Handle an expected fault current (1)

1.4 Answer the following questions on inductance: 1.4.1

1.4.2 1.4.3 1.4.4

State how magnetic flux brings about inductance in a conductor. State the difference between self-induced EMF in a coil and back EMF. Name a load which has inductance when coupled to an AC supply. Name a load which has no inductance when coupled to an AC supply.

(4 x 1)

(4)

[11]

QUESTION 2: SWITCHGEAR, CONTACTORS AND RELAYS

2.1 Explain the purpose of a relay. (2)

2.2 For the efficient distribution of power from power stations to consumers, electrical energy from a 3-phase 11 kV alternator, is transformed to 132 kV for long-distance transmission. At the substation it is transformed back to 11 kV, and then to a 3-phase 380 V 4-wire and to single-phase 220 V 2-wire systems. Show by means of a sketch how this is done in practice. Assume that there are consumers that need a 3-phase supply while others require single-phase supply.

(6)

2.3 Overhead conductors are not covered with insulating material. State the two main

purposes of the insulators used in this type of electrical distribution systems.

(2) 2.4 Name TWO methods to minimize damage caused by arcing in circuit breakers. (2) [12]

(11041872) -4- T440(E)(J22)T


QUESTION 3: DC MOTORS AND STARTERS 3.1 Name TWO components that need regular replacement in DC motors. (2) 3.2 Write 3.2.1 to 3.2.4 down in your ANSWER BOOK and correctly name all the

components identified by the arrows, selecting only FOUR of the following names:

stator windings

armature windings,

slip rings,

commutator

shaft

spacers

end plates

3.2.1 3.2.2 3.2.3 3.2.4

(4)

3.3 Explain the need for a motor starter. (2) 3.4 Explain what happens to the speed of a series motor as the load decreases. (2) 3.5 Name TWO methods of reversing the direction of rotation of DC motors. (2)

[12] QUESTION 4: AC MOTORS AND STARTERS 4.1 For a single-phase capacitor-start, capacitor-run motor in which the capacitor

values are not the same:

4.1.1

4.1.2 4.1.3

Draw a neat, fully labelled circuit diagram of the above. State one application of this type of motor. State why single phase motors are not all similar to this type of motor.

(4) (1) (1)

(11041872) -5- T440(E)(J22)T


4.2 The overcurrent protection devices used for motors must meet certain

requirements. Discuss these requirements under the following headings:

4.2.1

4.2.2 4.2.3 4.2.4

The tripping value The time delay Multiphase motors Automatically controlled motors

(2) (2) (1) (1)

4.3 Give a short description of how a squirrel-cage rotor is constructed. (3) [15] QUESTION 5: EARTHING 5.1 State which TWO parts of the earthing system should be connected to the

consumer's earth terminal.

(2) 5.2 5.3

The cable sheath is being used as the earth continuity conductor. The cable is to be joined. Explain how earth continuity is maintained if the joint is made inside a metal joint box. State what must be done with the earth conductor of a three-core cable that is connected to electrical appliances that have floating earths.

(2) (1)

5.4 Explain how the following items in an outdoor substation is earthed. 5.4.1

5.4.2 5.4.3

The transformer windings The metal enclosure of the transformer The lightning rods that are mounted on the highest points above the substation

(3 x 2)

(6)

[11]

QUESTION 6: PROTECTION 6.1 Answer the following questions on phase-imbalance protection:

6.1.1 6.1.2 6.1.3

State the purpose of phase-imbalance protection State what equipment requires phase-imbalance protection Explain how three-phase overload relays protect circuits against single phasing.

(1) (1) (2)

(11041872) -6- T440(E)(J22)T


6.2 6.3 6.4

State on which conductor in a circuit a fuse should be installed. Name the application and purpose of a valve arrestor. List the FOUR main components of the bimetal type overload relay.

(1) (2) (4)

[11] QUESTION 7: MEASURING INSTRUMENTS 7.1 Explain how the watt-hour meter is able to give a reading which is proportional to

the energy consumed.

(3) 7.2

Draw a neat, fully labelled circuit diagram to show how a wattmeter is connected to a single-phase system. Assume that the wattmeter is rated for the high voltage and current present in the system. Label the coils of the meter.

(3)

[6] QUESTION 8: TRANSFORMERS A three-phase transformer has a delta-connected primary and a star-connected secondary. The transformer is connected to a 132 kV supply. The secondary-phase voltage is found to be 11 kV. Calculate the following:

8.1 8.2 8.3 8.4

The primary-phase voltage The secondary-line voltage The secondary-phase current when it draws its full load current of 120 A from the supply The transformer rating in MVA

(1) (2) (6) (3)

[12] QUESTION 9: ELECTRONICS 9.1 Explain the operation of a diode. (2) 9.2 9.3 9.4

Explain the construction of a transistor. Explain how a SCR is triggered. Draw a neat, fully labelled circuit diagram to show how full-wave rectification can be obtained using two diodes and a centre-tap transformer.

(2) (2) (4)

[10]

TOTAL:

100

(11041872) -7- T440(E)(J22)T


FORMULA SHEET

2

p p s

s s p

V N I

V N I

f

.60

r

fN

p

n ns

n

CABLES/KABELS

Ifc =

Series Resistors/Serieresistors

1 2 .....T nR R R R

Parallel Resistors/ Parallelresistors

1 2

1 1 1 1.....

T nR R R R

I = V – E

Ra

Series motor/Seriemotor

IL = Ise = Ia

Long shunt/Langsjunt

Ise = Ia

IL = Ia + Ish/sj

Short shunt / Kortsjunt

IL = Ise

IL = Ia + Ish/sj

/

/

2 2

1

2 2

2

/

/

/

2

1

2

( )

( )

3

3

3

/

3

L

C

L C

T

ACTIVE AKTIEWE T


L

C

L C

R T

X T L

X T C

R X X

L L

L L

L PH F

L PH F

L PH

VI

Z

I I Cos

I I Sin

X fL

XfC

Z R X X

RCos

Z

V I R

V I X

V I X

V V V V

P I R

P V I Cos

S VI

S V I

DELTA

V V

I I

STAR STER

V V

/

F

L PH FI I



AUGUST EXAMINATION


22 JULY 2014


MARKING GUIDELINE

MARKING GUIDELINE -2- T480(E)(J22)T ELECTRICAL TRADE THEORY N2


QUESTION 1 1.1 The conductor and insulation factor

The conductor cross sectional area

The time duration of the fault current (Any 2 x 1)

(2)

1.2 Exposed to mechanical damage

Exposed to the elements

Exposed to sabotage & vandalism

Exposed to theft

Unsightly (Any 2 x 1)

(2)

1.3 1.3.1 The total current the cable has to carry (not the true power, but the kVA).

(1)

1.3.2

Make sure the voltage drop across the cable does not exceed 5% of the supply voltage.

(1)

1.3.3 The timethat the cable has to carry the expected fault current without any damage.

(1)

1.4 1.4.1

1.4.2 1.4.3 1.4.4

A change in flux around the coil / conductor induces an emf It is the same phenomenon Any coil of conductive material OR any conductor under the influence of a changing magnetic field A pure resistor or capacitor

(4 x 1)

(4) [11]



QUESTION 2 2.1 A relay is able to switch circuits on or off with the aid of a secondary circuit

that uses less voltage and current (control circuit).

(2) 2.2

(6)



2.3 To tie the conductors to a support structure. To separate conductors from

each other so that they do not touch.

(2) 2.4 Arc extinguishers and silver-tungsten contact points. (2)

[12] QUESTION 3 3.1 The brushes and the bushes (bearings). (2) 3.2 3.2.1

3.2.2 3.2.3 3.2.4

Shaft. Spacers. Commutator. Armature windings.

(4 x 1)

(4)

3.3 Motor starters are used to connect the motor to the supply and to protect the

motor against overloading. The motor starter also protects the operator against the motor restarting after power outages.

(2)

3.4 3.5

The speed increases exponentially to dangerous levels . Reverse either the field or the armature connections .

(2) (2) [12]

QUESTION 4 4.1 4.1.1

Sketch correct (must have 2 caps) Any 2 labels

(4)

4.1.2

The motor supplies a more constant torque during starting and running.

(1) (1)



4.1.3

The motor is too expensive and not necessary for certain applications. OR Higher maintenance is required because of the centrifugal switch.

4.2 4.2.1

4.2.2 4.2.3 4.2.4

The tripping value must be about 20% above the motor full load current rating. The time delay must be shortened according to the size of the overload, no time delay for short circuit, very little time delay if rotor locked. Protection prevents multiphase motors from operating if a phase has no voltage. Automatically controlled motors must be manually reset before allowing restart.

(2) (2) (1) (1)

4.3 The rotor has a shaft and the metal core is made up of steel laminations. There

are conductive bars on the surface of the core. These bars are shorted together by conductive plates at both ends.

(3)

[15] QUESTION 5 5.1 5.2 5.3

The consumer earth continuity conductors and the earth lead (protective conductor) providing continuity to the suppliers earth/earth electrode. The cable sheath is tightened to the metal joint box by means of a metal gland. The metal gland puts enough pressure on the wire armouring to ensure continuity. The joint box’s paint must be removed under the gland to ensure continuity. (The answer must show how earth continuity is achieved) Electrical appliances that have floating earths need not be earthed. Rather use a two core cable or tie off earth conductor to prevent it from touching live conducting parts.

(2) (2) (1)

5.4 5.4.1

5.4.2 5.4.3

The transformer windings can only be earthed at the star point of the star winding. The metal enclosure of the transformer must be connected with an earth continuity conductor to the earth terminal or directly to the earth electrode/ mat. The lightning rods that are mounted on the highest points above the substation must be connected with an earth continuity conductor to the earth terminal or directly to the earth electrode/ mat. (answers must ensure earth continuity to earth) (3 x 2)

(6) [11]



QUESTION 6 6.1 6.1.1

6.1.2 6.1.3

The purpose of phase imbalance protection is to disconnect all phases when the currents differ by a specified amount. (2) Motors and supply lines require phase imbalance protection. Phase overload relays protect circuits against single phasing because they trip all phases when current in one phase is excessive.

(1) (1) (2)

6.2 6.3 6.4

Fuses should be installed in the live supply wire that feeds the circuit. Valve arrestors are found in high voltage distribution networks as protection against voltage-surges caused by lightning strikes. The four main components that make up the bi-metal type relay are a heating element, a bi-metal strip, snap action contacts and a reset device. (4)

(1) (2) (4) [11]

QUESTION 7 7.1 The product of power and time (energy) determined when a disc connected

to a metering unit is rotated through a magnetic field The speed of rotation is proportional to the power consumed. The number of rotations in a certain time represents the energy consumed.

(3)

7.2 current coil

voltage coil

Award ONE mark for either supply OR load

(3) [6]



QUESTION 8 8.1 The primary phase voltage = The primary line voltage

Vp ph = 132 kV (must have units)

(1) 8.2 The secondary line voltage = x The secondary phase voltage

Vs L = x 11 = 19,05 kV

(2)

8.3 The secondary phase current = The primary phase current x turns ratio

turns ratio = Vp ph/Vs ph = 132/11 = 12:1

The primary phase current = Ip L/

Ip ph = 120/ = 69,28 A Thus Is ph = 69,28 x 12

= 831,38 A

(6)

8.4 The transformer rating in MVA = x Vp L x Ip L

= . 132 000. 120 . 10-6 = 27,44 MVA

(3) [12]

QUESTION 9 9.1 A diode conducts when the anode is more positive than the cathode (The diode

will have a low resistance when forward biased) . High resistance when reverse biased.

(2)

9.2 9.3

9.4

A trsnistor is made up of three layers of P- and N-type semiconductor material. Connecting leads are connected to the 3 layers to connect to external circuitry. The leads are identified as a Collector, Base and Emittor. A pulse on the gate will switch it on, provided the anode is more positive than the cathode. Transformer

correct

Direction correct

Sketch correct

Wiring correct

(2) (2) (4)

[10]

TOTAL: 100

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