ET 45 - Electrician Theory Examination Marking Schedule

ET 45 - Electrician Theory Examination Marking Schedule Notes:1. (1 mark) means that the preceding statement/answer earns 1 mark. 2. This schedule se...
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ET 45 - Electrician Theory Examination Marking Schedule Notes:1. (1 mark) means that the preceding statement/answer earns 1 mark. 2.

This schedule sets out the accepted answers to the examination questions. A marker can exercise their discretion and decide on the overall accuracy of any answer that is presented in the candidate’s own words.

3.

Symbols and terms - alternatives Power

W or P

Voltage

V or E or U

Phase

Active

Question 1

Reference

Marking notes

Marks (a)

Any ● ● ● ●

(b)

● ●

(c)

N = =

(d)



(f)

(2 marks)

To limit the current flowing through the electrode and tube. To produce a high voltage when the starter opens to start the tube glowing.

(2 marks)

=

(½ mark)

60f P 60 x 50 24 125 rpm

(½ mark) (1 mark)

Any ONE of: * There is an imbalance between the neutral current and phase current * There is earth leakage current in the circuit Greater than the residual tripping current of the RCCB

(1 mark)

(i)

A bi-polar junction transistor

(1 mark)

(ii)

A device that changes energy from one form to another

(1 mark) (e.g., changing a mechanical action into an electric signal).



(e)

TWO of: Earth rod Buried electrode Metallic reinforcing Earth mat

To ensure that the protective device can safely clear the prospective short-circuit ET45 ANSWERS – 22 JUNE 2013

(1 mark)

(2 marks) 1

Question 1

Reference

Marking notes

Marks current of the installation. (g)

(i)

(ii)



The earth bar

(½ mark)



The earth electrode

(½ mark)

Any ONE of: ● (A) A notice warning disconnection (B) At the earth electrode ●

(h)

(i)

(½ mark) (½ mark)

(A) A notice at the switchboard stating (B) The position of the earth electrode

(i)

This is the time it takes the fuse to interrupt the current flow

(1 mark) Any answer relating to the fuse operating is acceptable

(ii)

This is the time it takes to interrupt the flow of current and extinguish the arc.

(1 mark)



The thermistor detects a rise in temperature in the windings It opens the control circuit when a preset temperature has been exceeded.

(1 mark) How the motor stopped must (1 mark) described

P

(1 mark) Alternative:



(j)

against

(i)

=

√3 x VL x IL x pf

is be

√3 x VL x IL x cosΦ (ii)

P

=

VxI

(1 mark)

ET45 ANSWERS – 22 JUNE 2013

2

Question 2 (a)

Ifault = = = Itotal = = =

(b)

● ●

(c)

(d)

Reference Marks V R 230 (0.37 + 14.7) 15.26A

(½ mark)

Ifault + Iload 26.08 + 15.26 41.34A

(1 mark)

(½ mark) (1 mark)

(1 mark)

The 40A fuses have a fusing factor (gG Utilisation Category) of 1.5 Fusing current = 1.5 x 40 = 60A. Because the fault current is 41.34 the fuses will not operate.

(1 mark) (1 mark)

Vd across the frame equals touch voltage VdE = IxR = 15.26 x 0.1 = 1.53V ● ●

Marking notes

No The voltage across the frame of the appliance is well below the touch voltage of 50V.

(½ mark) (½ mark) (1 mark) (½ mark) (1½ marks)

ET45 ANSWERS – 22 JUNE 2013

3

Question 3

Reference

Marking notes

Marks (a)

0.82

(b)

kW

(c)

(½ mark) =

HP x 0.746

(½ mark)

=

5.36 x 0.746

(½ mark)

=

4kW

(1 mark)

Any ONE of: ● Input power

=

= ●

(d)

(i)

(ii)

P

Output kW Efficiency

(½ mark)

4000 0.85 4705.88W

(½ mark) (1 mark)

=

√3 x VL x IL x cosΦ

=

√3 x 415 x 8.52 x 0.82

=

5021.65W (1 mark)

Any ONE of: ●

International protection



Ingress protection

Protection against the ingress of solid objects

(iii) Protection liquids.

against

the

ingress

of

(1 mark)

(1 mark)

(e)

Delta connection

(½ mark)

(f)

Star connection

(½ mark)

(g)



Short circuit

HRC fuse or MCB

(½ mark)



Over-temperature

Thermistor

(½ mark)



Phase polarity

Phase relay

reversal

ET45 ANSWERS – 22 JUNE 2013

(½ mark)

4

Question 4

(a)

● ● ●

(b)

Reference Marks

Use an ohmmeter. Test between the cable ends. A low resistance indicates a winding. Each winding is of a similar resistance.

(i)

An earth continuity (protective earthing conductor) test.

(ii)





The earth continuity (protective earthing conductor) test * Use an ohmmeter * Test between the protective earthing conductor at beginning of the cable and the frame of the motor. * A test result of 1Ω or less The insulation resistance test * Use an insulation resistance tester * 500V d.c. test voltage * Test between each of the three windings. * Test between each of the three windings and the motor framework * A test result for each test of 1MΩ or more.

ET45 ANSWERS – 22 JUNE 2013

Marking notes

(1 mark) (1 mark) (1 mark) (½ mark)

(½ mark) (1 mark)

(½ mark) (½ mark) (½ mark) (½ mark) (1 mark) (1 mark) (1 mark)

5

Question 5

Reference

Marking notes

Marks (a)

● ●

(b)

(i)

(ii)

Conduct a voltage test using the provetest-prove method. To confirm that the final subcircuit is isolated. Testing the final subcircuit ● Turn off both fixed-wired appliance ● Select the insulation resistance function ● Select 500V range ● Testing between load side of the MCB and the neutral bar. ● Near 0Ω for the fault

(1 mark) (1 mark)

● (1 mark) (½ mark) (½ mark) (1 mark)



An alternative approach is to disconnect the phase an neutral The removal of the MEN link is hazardous – no marks for part (b)

(1 mark)

Testing the fixed-wired-appliances: ●

Turn on the fixed-wired appliance

(1 mark)



Selecting the resistance function

(½ mark)



Selecting the lowest range or the auto range Testing between load side of the MCB and the neutral bar. Much less than 8.54Ω.

(½ mark)

● ●

ET45 ANSWERS – 22 JUNE 2013

(1 mark) (1 mark)

6

Question 6

Reference

Marking notes

Marks (a)

(i)

Multiple Earthed Neutral or MEN system

(2 marks)

(ii)

Any TWO of: ● Limits the voltage to 230V to earth under fault conditions. ● Limits the voltage between neutral and earth to about 0V. ● Ensures that under fault conditions, the voltage between any earthed metal and the mass of earth never rises above 0V

(2 marks)

(b)

Any ● ● ●

TWO of: The MCB is unlikely to clear the fault. The MCB may be destroyed. A current path may be created through the MCB

(c)

The MCB will interrupt the fault as intended

(d)

ISC

= =

(2 marks)

(2 marks) (1 mark)

5000 kVA √3 x 400v 7.22 kA

(1 mark)

ET45 ANSWERS – 22 JUNE 2013

7

Question 7

Reference

Marking notes

Marks (a)

(i)

Sec VPH

(½ mark) For (i) and (ii), the use of 230V and 400V (½ mark) values is acceptable

=

Pri VL NP 66000 287 229.97V

=

VPh x √3

(½ mark)

=

229.97 x √3

(½ mark)

=

398.31 V

=

kVA √3 x Pri. VL 150000 √3 x 66000 1.31 A

(½ mark)

kVA √3 x Sec. VL 150000 √3 x 398.36 217.4A

(½ mark)

= =

(ii)

Sec. VL

(iii) Pri. IL

= = (iv) Sec. IL

= = =

(b)

(1 mark)

(1 mark)

(½ mark) (1 mark)

Any TWO of: ● A high voltage is induced in secondary circuit ● The flux density in the CT rises. ● The CT could be damaged

(½ mark) (1 mark) (2 marks) the

ET45 ANSWERS – 22 JUNE 2013

8

Question 8

Reference

Marking notes

Marks (a)

(b)

(i)

The fusing current

= =

1.5 x 20 30A

(1 mark) Accept the use of a fusing factor between 1.25 and 1.5

(ii)

The fusing current

= =

2 x 20 40A

(1 mark) Accept the use of a fusing factor between 1.6 and 2.5



The protective device nearest the fault operates

(1 mark)



Before any other protective device.

(1 mark)

(c)

This is the maximum current that the fuse is designed to carry continuously

(1 mark)

(d)



(1 mark)

(e)

160A 2



The 160A fuse has a pre-arcing time of 62A (I 10 3 ).

x

(1 mark)



which is greater than the total clearing time rating of the 100A fuse.

(1 mark)

(i)

Any ONE of: ● Rewireable fuses are not rated for more than 1 kA ● Rewireable fuses will not safely rupture under the stated fault conditions

(1 mark)

(ii)

Any ONE of: ● MCBs are rated up to 10kA ● MCBs will safely open under the stated fault conditions

(1 mark) Any answer that shows why an MCB is more advantageous than a rewireable fuse is acceptable

ET45 ANSWERS – 22 JUNE 2013

9

Question 9 (a)

(i)

An insulation resistance tester with 100V d.c. and 250V d.c ranges. * Appliance has to be disconnected * Select the 250V d.c. range

(½ mark)

*

(½ mark)

* (ii)

Marks

Any ONE of: Test between the phase pin and the frame of the appliance and between the neutral pin and the frame of the appliance Bridge the phase and neutral pins and test between the bridge and the frame of the appliance A test result of 1MΩ or more

An insulation resistance tester with 500V d.c. and 1000V d.c ranges. * Appliance has to be disconnected * Select the 500V d.c. range *

*

Any ONE of: Test between the phase pin and the frame of the appliance and between the neutral pin and the frame of the appliance Bridge the phase and neutral pins and test between the bridge and the frame of the appliance A test result of 1MΩ or more

(iii) A clip-on (clamp) ammeter with mA ranges. * Appliance has to be live * * (b)

Clamp the meter around the protective earthing conductor A test result of 5 mA or less

(i)

An ohmmeter

(ii)

Test between the earth pin of the plug and the frame of the dishwasher

Reference

Marking notes

(½ mark) No marks for this part of the answer if testing between phase and neutral is stated

(1 mark)

(½ mark) (½ mark) (1 mark)

No marks for this part of the answer if testing between phase and neutral is stated

(1 mark)

(½ mark) (½ mark) (1 mark) (½ mark) (1 mark)

ET45 ANSWERS – 22 JUNE 2013

10

(ii)

1Ω or less

(1 mark)

ET45 ANSWERS – 22 JUNE 2013

11

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