Re: 2330 level 3 smartscreen questions/answers

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#SmartScreen

Level 3 Certificate in Electrotechnical Technology Unit 4 Sample questions answers

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#Unit 4: Electrical maintenance: Inspection, testing and commissioning

Sample questions

Answer guide

1. List six methods of making an area safe before commencing work

Solution

use barriers and tapes

place warning signs in appropriate locations

Inform other workers in the area

isolate the supply

obtain a permit to work

shut down machinery in the area

(6 marks)

2. Describe the following types of maintenance:

a. preventative

b. corrective

Solution

a. Maintenance carried out to a schedule to try to prevent a breakdown of machinery during normal operation. (3 marks)

b. Corrective maintenance is work carried out when machines or equipment fails during operation and is not planned. (3 marks)

3. List six elements of a maintenance work plan

Solution

Definition of the task

Estimation of costs

Estimation of times taken

Safety procedures

Shut down arrangements

Permit to work

Access

Need for standby power supplies

(any 6) (6 marks)

4. Describe briefly:

a. a three-phase four-wire distribution system

b. the method of obtaining a single-phase supply from (a)

c. the importance of balancing loads on a three-phase system.

Solution

a. The three-phase four-wire system is taken from the secondary side of a

Delta-Star connected distribution transformer. The star point is connected to earth and neutral conductor. A four-wire supply provides choice of three-phase and Neutral, three-phase three-wire and single-phase supplies. Very rarely a

two-phase and neutral supply is used. (2 marks)

b. Single-phase loads are taken from the supply by connecting between any phase and neutral. (2 marks)

c. A balanced load will i) limit the neutral current, keep the cable size to a minimum as well as reducing to a minimum the switchgear sizes. (2 marks)

5. For a three-phase cage rotor induction motor describe the STAR-DELTA method of starting the motor.

Solution

The two ends of each phase are brought out to the starter. On starting, the windings are connected in star and when the motor reaches a set speed (often determined by a timer for automatic starting), the winding connections are re-made in delta for normal running. The main features of Star/Delta starting are

1. the start voltage is reduced to approximately 58% of its full load (run) value

2. the starting current is limited to between two and four times the full load current, rather than five to ten times for direct on-line starting

3. the motor torque is reduced during the starting

 

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4. Identify, for each of the following a typical use of the following drawings etc.

a. Assembly drawings

b. Block diagrams

c. Circuit diagrams

Solution

a. Shows the component parts (perhaps in an exploded view) of the equipment which is to be worked on. Could be used for dismantling and reassembling equipment/plant.

(2 marks)

b. A block diagram shows the sequence of operation/control set out as blocks on a plan etc. It does not show the actual position of the items. (2 marks)

c. Shows the wiring system of the plant including controls, isolation etc. May not necessarily show the connections of components. (2 marks)

5. Identify six items which would form part of a maintenance procedure work plan

Solution

Any from: definition of the task, shut-down arrangements, permits to work, coordination with stores/suppliers, maintenance schedules/documentation, communications, safety procedures, cost estimates, time estimates. (6 marks)

6.

Figure 1

Distribution RCD 3 pole switch Starter Motor

Board

Using the drawing, Figure 1 above, identify which of the items provides or contains the following

a. short circuit protection

b. overload protection

c. mechanical maintenance switching

d. functional control

e. isolation

f. earth fault protection.

Solution

a. Fuses in distribution board

b. Overload device in starter

c. Three-pole switch

d. Starter

e. Three-pole switch

f. RCD (6 marks)

7. For each of the following situations, identify a suitable wiring system, giving one reason for your choice.

a. Overhead supply to machines in a factory. Machines may need to be relocated.

b. Connection between fixed controls and a motor mounted on slide rails.

c. Connections to terminals of a furnace.

Solution

a. Overhead busbar system, allows supplies to be instantly disconnected and reconnected.

b. Flexible conduit or armoured cable. Allows for vibration and movement of motor.

c. High temperature connections such as porcelain sleeving, high temperature would burn out high temperature rubber/pvc.

(3 x 2 marks, one mark only for naming cable or reason, two for both)

8. a. For each of the following earthing arrangements, state the item/position that the earthing conductor is connected to at the opposite end to the main earthing terminal.

i) TN-C-S

ii) TT

iii) TN-S

b. State the purpose of bonding.

Solution

a. i) Neutral at service head. (1 mark)

ii) Earth electrode (1 mark)

iii) Cable sheath (1 mark)

b. To create a zone of equal potential where all exposed metalwork is connected together. This prevents the risk of shock between exposed and extraneous parts.

(3 marks)

9. A three-phase 400v motor is supplied via an automatic star