electrical installer theory examination - 20 june 2015 ... · electrical installer theory...

EIN 10

Candidate Code No.

For Board Use Only

Result Date Int

Result Date Int

ELECTRICAL INSTALLER THEORY

EXAMINATION

20 June 2015

QUESTION AND ANSWER BOOKLET

Time Allowed: Three hours INSTRUCTIONS – READ CAREFULLY You have 10 minutes to read this paper but do not start writing until you are told to do so by the supervisor. Write your Candidate Code Number in the box provided above. Your name must NOT appear anywhere on this paper. Answer all questions. The pass mark for this examination is 60 marks. Use a pen for written answers. Do not use a pencil or a red pen. Drawing instruments and pencils may be used when diagrams are required. Marks are allocated on the basis of correctness. Do not use correcting fluid or correcting tape. For calculation questions all workings, including formulae, must be shown to gain full marks. Non-programmable calculators may be used. Warning – You could get 0 marks for any question, or part of a question, if you show anything hazardous or dangerous in your answer. Candidates are not permitted to use any Act, Regulation, Standard, Code of Practice, Handbook or other reference text in this examination. PLEASE HAND THIS PAPER TO THE SUPERVISOR BEFORE LEAVING THE ROOM.

(turn over)

Question 1 (a) Explain why mechanical interlocks are incorporated in a star/delta

starter. (2 marks)

____________________________________________________________

____________________________________________________________

____________________________________________________________

(b) (i) State the reason why a capacitor would be connected in series with

the start winding in a single-phase a.c. split-phase induction motor. (1 mark)

________________________________________________________

________________________________________________________ (ii) For what reason would you connect a resistor across a circuit after

the electricity supply to that circuit has been isolated. (1 mark)

________________________________________________________

________________________________________________________ (c) (i) State the main consideration when selecting a protective device for a

switchboard. (1 mark)

________________________________________________________

________________________________________________________ (ii) A residual current device (RCD) that protects a final subcircuit and a

fixed-wired Class I electrical appliance has operated. What has the RCD detected that would cause it to operate?

(1 mark)

________________________________________________________

________________________________________________________

(turn over)

EIN10-QUESTIONS – 20 JUNE 2015 2

Question 1 continued (d) In relation to HRC fuses: (i) Define the term cut off characteristic as it applies to HRC fuses.

(1 mark) ________________________________________________________ ________________________________________________________ (ii) Define the term current rating as it applies to HRC fuses.

(1 mark) ________________________________________________________ ________________________________________________________ (e) State ONE reason why the neutral in the MEN system is multiple-earthed.

(2 marks) ____________________________________________________________

____________________________________________________________ ____________________________________________________________ (f) An electrical installer used a voltmeter to test for isolation on the supply

side of a three-phase isolator using the following procedure: A test on an independent live source confirmed that the meter worked

correctly. A test between each phase showed no reading on the meter. Another test on an independent live source confirmed that the meter

still worked correctly. The installer proceeded to dismantle the isolator and received an electric

shock. State ONE reason why the person received the shock. (2 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________

____________________________________________________________

(turn over)


Question 1 continued (g) (i) A three-phase motor has: Six terminals on the terminal block A six-core and earth flexible cable is connected to the terminal

block. To what type of motor starter would the three-phase motor be

connected? (1 mark)

________________________________________________________

(ii) A three-phase motor has: Six terminals on the terminal block A three-core and earth flexible cable is connected to the

terminal block. To what type of motor starter would the three-phase motor be

connected? (1 mark)

________________________________________________________ (h) (i) What device is used to provide starting torque in a shaded pole motor?

(1 mark) ________________________________________________________ (ii) State TWO applications for a shaded-pole motor.

(1 mark) (1) ____________________________________________________ (2) ____________________________________________________

(turn over)


Question 1 continued (i) A star-connected squirrel cage motor that has been operating normally

develops a fault. The tests on the motor show the following readings: L1 – L2 20Ω L1 – L3 open circuit L2 – L3 open circuit State the most likely fault indicated by these readings.

(2 marks) ____________________________________________________________

____________________________________________________________

(j) One reason the metal frame of a Class I electrical appliance is earthed is to ensure the operation of a protective device under fault conditions.

State ONE other reason why the metal frame of a Class I electrical

appliance is earthed. (2 marks)

____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

(turn over)


Question 2 (a) State the TWO reasons why the prove-test-prove method is used for taking

voltage tests for the purpose of isolation. (2 marks)

(1) ________________________________________________________

________________________________________________________ ________________________________________________________ (2) ________________________________________________________

________________________________________________________ ________________________________________________________ (b) (i) Define the term switching off as it relates to three-phase equipment.

(1 mark) ________________________________________________________ ________________________________________________________ (ii) Define the term isolation as it relates to three-phase equipment.

(2 marks)

________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________

(turn over)


Question 2 continued (c) A three-phase, 10 kW pump motor has been isolated and the motor cables

have been disconnected from the motor. The motor cables are still connected to the DOL starter.

State TWO precautions which must be taken to ensure the safety of

persons and property before you leave the work site. (2 marks)

(1) ________________________________________________________

________________________________________________________ ________________________________________________________ (2) ________________________________________________________

________________________________________________________ ________________________________________________________ (d) A three-phase fixed-wired machine is protected by MCBs. The machine has

been isolated from the supply and is being repaired. The repair will take some time.

Describe THREE different methods of ensuring the machine remains

isolated. (3 marks)

(1) ________________________________________________________

________________________________________________________ ________________________________________________________ (2) ________________________________________________________

________________________________________________________ ________________________________________________________ (3) ________________________________________________________

________________________________________________________ ________________________________________________________

(turn over) EIN10-QUESTIONS – 20 JUNE 2015 7

Question 3 (a) (i) State the permitted maximum residual current rating of an RCD used

for personal protection in a domestic electrical installation. (1 mark)

________________________________________________________ (ii) State the permitted maximum residual current rating of an RCD used

for personal protection in a child-care centre. (1 mark)

________________________________________________________ (iii) State the permitted maximum residual current rating of an RCD used

for the protection of property. (1 mark)

________________________________________________________ (b) An RCD rated for personal protection is a safeguard designed to operate if

earth leakage current flows through a person’s body to earth. State why that person does not receive a severe shock from the earth

leakage current? (2 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________

____________________________________________________________ ____________________________________________________________

____________________________________________________________

(turn over)


Question 3 continued (c) A Residual Current Circuit Breaker (RCCB) is installed in a single phase

circuit. Explain why the RCCB does not operate when there is a short circuit

between active (phase) and neutral in the single phase circuit? (2 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________

____________________________________________________________ (d) Explain why a PRCD is required to be voltage dependent.

(2 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________

____________________________________________________________ ____________________________________________________________

____________________________________________________________ (e) The phase and neutral on the supply side of a single phase RCD have been

transposed. Would the RCD operate if it detected an earth leakage fault on the circuit it

protects? State a reason to support your answer. (1 mark)

____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

(turn over)


Question 4 Introduction Another electrical installer has isolated, disconnected and repaired a 400V three-phase circular saw in a joinery factory. You are required to: Reconnect the machine by connecting the flexible cord to the DOL (direct-

on-line) starter mounted on an adjacent pillar, and Recommission the circular saw There are no danger tags or lock attached to the isolator controlling the DOL starter. The isolator has a locking facility. The only test instrument available is a multi-function test instrument. Use the information in the introduction to this question to answer parts 4(a), 4(b), 4(c) and 4(d). (a) State the TWO instrument tests that need to be carried out on the circular

saw to ensure it is safe to connect to the supply. (1 mark)

Test No. 1 ________________________________________________

Test No. 2 ________________________________________________ (b) For Test No. 1 stated in (a) State the range selected on the test instrument State the function selected on the test instrument State the test voltage (if applicable) Describe how the test is carried out with the function and range

selected. State the permitted test result that allows the saw to be returned to

service. (3 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

____________________________________________________________

(turn over)


Question 4 continued (c) For Test No. 2 stated in (a) State the range selected on the test instrument State the function selected on the test instrument State the test voltage (if applicable) Describe how the test is carried out with the function and range

selected. State the permitted test result that allows the saw to be returned to

service. (3 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

____________________________________________________________ (d) The saw has been connected to the DOL starter but not livened. A test needs to be carried out to ensure that the saw’s protective devices

will operate under fault conditions. For this test: State the function selected on the test instrument State the range selected on the test instrument Describe how the test is carried out with the function and range

selected. The permitted test result that allows the saw to be returned to service.

(3 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________ ____________________________________________________________

(turn over)


Question 5 (a) A factory is supplied by a 150 kVA three phase, delta-star connected

transformer with a turns ratio of 47.8 to 1. The primary is connected to an 11kV three phase supply and the transformer is fully loaded.

(i) Calculate the secondary phase voltage.

(2 marks) (ii) Calculate the secondary line voltage.

(2 marks) (iii) Calculate the primary line current.

(2 marks)

(turn over)


Question 5 continued (iv) Calculate the secondary line current.

(2 marks) (b) How is the output of a transformer rated?

(1 mark)

____________________________________________________________ (c) State TWO typical winding configurations found in transformers.

(1 mark)

(1) ________________________________________________________

(2) ________________________________________________________

(turn over)


Question 6 Introduction The figure below represents a 400V, three-phase commercial oven comprising three 6 kW single-phase elements. Each phase is protected by a 32A HRC fuse with a fusing factor (gG Utilisation Category) of 1.5.

An earth fault of 6Ω has developed between L1 and the oven frame while the oven is operating. The fault occurred at point X. The earth fault loop impedance is 0.38Ω. Use the information in the introduction to this question to answer parts 6(a), 6(b) and 6(c)

(turn over)

X N

L1

L2

L3

Element 1

Element 2

Element 3

E


Question 6 continued (a) Calculate the total current that will flow in L1 under the fault conditions.

(6½ marks)

(turn over)


Question 6 continued (b) Refer to the following graph and determine how long it with take the 32A

HRC fuse to operate with the fault current flowing. (1 mark)

____________________________________________________________

(turn over)


Question 6 continued (c) The protective earthing conductor resistance is 0.28Ω. Use calculations to determine whether any touch voltage hazard exists to

the operator of the oven. (2½ marks)

(turn over)


Question 7 (a) Explain why rewireable fuses are classed as coarse excess current

protection. (2 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________

____________________________________________________________ ____________________________________________________________ (b) Explain why Gg type HRC fuses and MCBs are classed as close current

protection. (2 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________

____________________________________________________________ ____________________________________________________________ (c) HRC fuses and thermal overloads provide protection for a final subcircuit

and motor. State the main reason why the HRC fuses are not expected to provide

overload protection for the motor. (1 mark)

____________________________________________________________

____________________________________________________________ ____________________________________________________________

(turn over)


Question 7 continued (d) Apart from cost, state TWO technical advantages that HRC fuses have

when compared to MCBs. (1 mark)

(1) ________________________________________________________ (2) ________________________________________________________ (e) An MCB provides protection for a final subcircuit. State the main reason

why HRC fuses are installed as back-up protection on the same final subcircuit.

(2 marks) ____________________________________________________________

____________________________________________________________ ____________________________________________________________ ____________________________________________________________ (f) Explain how correct discrimination between protective devices occurs in

an electrical installation. (2 marks)

____________________________________________________________

____________________________________________________________ ____________________________________________________________ ____________________________________________________________

(turn over)


Question 8 (a) The figure below represents the components of the power circuit of a DOL

forward and reverse starter. The components are: The three-phase supply The forward contactor contacts The reverse contactor contacts The thermal overloads The motor On the figure draw the conductors so the motor can be operated in the

forward and reverse directions. Overload protection for the motor must operate in either direction.

(5 marks)

(turn over)

L1 L2 L3

M

W

V

U

Forward contactor

Reverse contactor


Question 8 continued (b) A 400V, three-phase motor is started with a DOL (direct-on-line) starter.

When started, the motor draws 90A and produces 200% full-load torque. (i) Calculate the starting current if the same motor is started with a

star/delta starter. (2 marks)

(ii) Calculate the percentage (%) of starting torque if the same motor is

started with an auto-transformer starter with an 80% tap. (2 marks)

(c) Explain the term ramp time as it relates to an a.c. variable speed

controller. (1 mark)

____________________________________________________________

____________________________________________________________

____________________________________________________________

____________________________________________________________

(turn over)


Question 9 (a) When carrying out testing on a low voltage electrical installation, which

document details the tests and checks required to be carried out? (1 mark)

____________________________________________________________ (b) The two figures below represent the front view of two single-phase socket

outlets. Label the polarity of the socket outlet connections on the figures.

(3 marks)

IEC 60309AS/NZS 3112

Note: You must show the polarity completely correct to gain

marks.

(turn over)


Question 9 continued (c) You are using a voltage tester on the front of a 230V switched socket

outlet to ensure power is present. Note: The voltage tester is operating correctly.

(3 marks)

(i) What voltage you would expect to find with the switch OFF: Between Active – Neutral ________________________________

Between Active – Earth ________________________________

Between Neutral – Earth ________________________________ (ii) What voltage you would expect to find with the switch ON: Between Active – Neutral ________________________________

Between Active – Earth ________________________________

Between Neutral – Earth ________________________________

(d) You are using a voltage tester on the front of a 230V switched socket

outlet to ensure power is present and get the following results.

Test between Socket Terminals

Voltages Switch OFF

Voltages Switch ON

Active - Neutral 0V 230V Active - Earth 0V 0V Neutral- - Earth 0V 0V

State TWO reasons why you would get these readings? Note: The voltage tester is operating correctly.

(2 mark)

(1) ________________________________________________________ ________________________________________________________ (2) ________________________________________________________ ________________________________________________________

(turn over)


Question 9 continued (e) You are using a voltage tester on the front of a 230V switched socket

outlet to ensure power is present and get the following results:

Test between Socket Terminals

Voltages Switch OFF

Voltages Switch ON

Active - Neutral 0V 230V Active - Earth 0V 0V Neutral- - Earth 230V 230V

State the reason why you would get these readings. Note: The voltage tester is operating correctly.

(1 mark)

____________________________________________________________ ____________________________________________________________


For Candidate’s Use For Examiner’s Use Only

Questions Answered

Marks

In the box, write the number of EXTRA sheets you have used. Write NIL if you have not used any

1

2

3

4

5

6

7

8

9

TOTAL


electrical installer theory examination - 20 june 2015 ... · electrical installer theory...

Documents