C&G 2391 MOCK Exam One With Answers

Answers:

1. Any THREE from the following:
- Safety of persons and livestock against electric shock and burns
- Protection from damage to property due to fire or heat from a defective installation
- Ensuring the installation is not damaged or deteriorated with age which may impair safety
- To identify any defects or non-compliance of the installation which may cause danger?

2.
i) 500 V and  1 M ohm
ii) 1000 V and 1.0 M ohm
iii) 250 V and 0.50 M ohm

3.
i) Between the circuit phase connection and the earth bar in the consumer unit
ii) Between the phase and earth connection at the socket outlet
iii) The R1 + R2 value for the circuit to be entered on the schedule of test results

4.
i) Resistance increases
ii) Resistance decreases
iii) Resistance decreases

5. Any three from the following:
- Bathrooms
- Swimming pools
- Hot air saunas
- Construction sites
- Agricultural and horticultural premises
- Restrictive conductive locations
- Installations having high protective conductor currents
- Caravan, motor caravans, caravan sites
- Installation of highway power supplies, street furniture and street located equipment

6. Any three from the following:
- Check all switches are closed
- Check all lamps and fluorescent tubes removed
- Check for any electronic equipment which may be vulnerable to
- Damage from the test voltage i.e. PIR's Electronic door openers,
- Automatic light switches.
- Check there are no neon's, capacitors, or current using equipment in the circuit
- Check that no danger to persons or livestock will occur during the test
- Check meter probes are in a sound condition

7.>
- IP 2X (412-03-01)
- IP XXII (412-03-01)
- IP 4X (412-03-02)

8. From Part 3 BS 7671 Assessment of general characteristics (300-01-01). BS:7671 sections(311) (312) (313)
- Maximum demand in amperes (311)
- Arrangement of live conductors and type of earthing (312) 
- Nature of supply (313)
- Design, Construction, Inspection and Testing

10.
i) Insulation Resistance Ohm-meter
ii) Prospective Fault Current Tester
iii) RCD Tester

11.
a) Statutory: The Electricity at Work regulations
b) Non-statutory: Any two from the following:
- BS 767l 17th Edition Wiring Regulations
- The IEE On-Site Guide
- IEE Guidance Note 3 booklet

12. Any three of the following: sight, touch, smell, hearing.

13.
a) Finger contact protection only
b) Small foreign body i.e. 1 mm csa wire
c) Total submersion

14. Operation of main isolating switch, operation of light switch, operation of dimmer switch.

15.
a) External Phase-Earth Fault Loop Impedance
b) Resistance of the CPC
c) Multiplier for conductor operating temperature

16.
I) Continuity of protective conductors
2) Continuity of ring final circuit conductors
3) Insulation resistance

17. Any three of the following:
- Earthing conductor
- Main equipotential bonding conductor
- Circuit protective conductors CPC

18.
a) TN-C-S
b) TN-S
c) TT

19.
1) All single pole switching devices are connected to the phase conductor only
2) All accessories are correctly connected
3) Centre contact of Edison screw lamp-holders connected to the phase conductor

20.
a)
i) 0.15 ohm
ii) 0.15 ohm

b) (R1+R2) for the circuit

21.
a) The results of the assessment of General Characteristics, sections 311, 312, 313 and diagrams, charts, and similar information regarding the installation
b) Maximum demand / Rating of the consumer unit / Earthing and bonding arrangements / Capacity of main protective device, etc.
c) In or adjacent to the distribution board
d) Reference to the electronic door sensor and the PIR controlled external luminaries as these could be vulnerable to the test voltage

22.
Socket 1: Satisfactory as reading is approx 1/2 the 0.8 ohm loop reading (3 marks)
Socket 2: Cross polarity between P and N, or twisted CPC not in terminal (3 marks)
Socket 3: Loose neutral connection (3 marks)
Socket 4: Cross polarity between phase and CPC, or twisted N not in terminal (3 marks)
Socket 5: Satisfactory as reading is approx 1/2 the 0.8 ohm loop reading (3 marks)

23.
a) Steps:
1. Check condition of test lead probes prior to test (ensure they are sound)
2. Consumer unit main switch off i.e. safely isolate supply. (Prove dead)
3. All fuses can be left in but breakers (MCB`s) must be off or bypassed.
4. All functional switches ON (where practical).
5. Two-way switches will need to be tested in both positions.
6. Some electrical equipment may suffer from damage when the test voltage is applied, such equipment should be disconnected before the test i.e. PIR electronic door sensors / security lamps sensors etc, electronic starters for fluorescent lamps, dimmer switches, touch switches, power controllers, delay timers, flood-lamp controllers.(All of the above are not in this installation, but if they were they would need disconnecting)
7. All lamps removed
8. Where the removal of lamps and equipment is impractical the local switch controlling the equipment should be left open i.e. switched off. (But this should be recorded on the schedule of test results)
9. Current using equipment will affect the insulation resistance value, and should be disconnected i.e. pilot or indicator lamps, neon indicators in socket or fused spur outlets, capacitors etc.
10. The incoming phase and neutral should be disconnected to avoid connections with earth, turning off the distribution main switch to isolate the installation will accomplish this.
11. Select correct tester i.e. Insulation Resistance Ohm-meter
12. Set test voltage to 500 V
13. For a single-phase installation measure between Phase and Neutral, Phase and Earth, and Neutral and Earth.
14. Circuit must measure at least 1M ohm to be in compliance with BS 7671.
15. lf any voltage sensitive equipment could not be disconnected it is acceptable instead of measuring between Phase and Earth to connect the Phase and Neutral conductors together and measure between the connected conductors and earth.

b)
Since the test results indicate an overall value of 1.60 MQ, this is below the acceptable 2 M ohm value and therefore requires further investigation.
Each circuit in the detached workshop should be checked individually and if the individual readings are 1 M ohm or greater than the installation is acceptable.
Any individual circuit showing a test reading below 1 M ohm would not be in compliance with regulations, and would require further investigation.

24.
a)
1.) The point of the fault
2.) The circuit CPC
3) The steel wire armour of the workshop supply cable
4) The main earthing terminal
5) The earthing conductor
6) The metallic earth return path of the supply cable
7) The earthed neutral of the supply transformer (substation)
8) The transformer phase winding (substation)
9) The supply phase conductor

b)
 = 1.15 ohm
The impedance Zs is 1.159 which is below the maximum value specified for the circuit (i.e. 3 ohm) so the circuit protection will operate fast enough and disconnect the fault within the prescribed time. i.e. 5 seconds for fixed equipment.

25. Any five from the following:
1) Connection of conductors
2) Identification of conductors
3) Routing of cables in sate zones to provide protection against mechanical damage
4) Selection of conductors for current carrying capacity and voltage drop
5) Connection of single pole devices for protection or switching in phase conductor only
6) Correct connection of accessories and equipment
7) Presence of tire barriers, suitable seals and protection against thermal effects
8) Methods of protection against electric shock
9) Prevention of mutual detrimental influences
l0) Presence of appropriate devices for isolation, and switching correctly located
11) Presence of under-voltage protective devices
l2) Choice and setting of protective and monitoring devices for protection against indirect contact and/or protection against over-current
I3) Labelling of protective devices, switches and terminals
14) Selection of equipment and protective measures appropriate to external influences
I5) Adequacy of access to switchgear and equipment
16) Presence of danger notices and other warning signs
I7) Presence of diagrams, charts, instructions, and similar information
I8) Erection methods

26.
a) An earth fault loop impedance tester delivers a high current for a short period, so it is not unusual for sensitive MCB’s (i.e. Type B’s) with low ratings to trip out on overload.
b) The earth fault loop impedance in such cases will have to be determined by a combination of calculation and measurement as follows:
Zs = Ze + (R1+R2)
Or use an instrument with a “no trip test” function.