🧵 Selection and Erection of Wiring Systems (Part 5, Chapter 52)

1. A cable is concealed in a plastered wall less than 50 mm from the finished surface and runs in a permitted safe zone. According to BS 7671, what additional protection must be provided?

1. Additional protection by a 30 mA RCD
2. A separate earthed bonding conductor
3. A 100 mA time-delayed RCD
4. Double insulation of the circuit conductors

Regulation 522.6.202 requires a cable concealed at less than 50 mm depth and not otherwise protected to be in a safe zone and given additional protection by a 30 mA RCD. (BS 7671:2018+A2:2022, Regulation 522.6.202)

2. At what depth from any surface does Regulation 522.6.202 stop applying its safe-zone and RCD requirements, so that a cable buried deeper needs no additional protection on this basis?

1. 25 mm
2. 50 mm
3. 75 mm
4. 100 mm

Regulation 522.6.202 applies where a cable is concealed at less than 50 mm; cables buried deeper than 50 mm need no additional protection on this basis. (BS 7671:2018+A2:2022, Regulation 522.6.202)

3. Which of the following describes a permitted safe zone for a cable concealed at less than 50 mm depth in a wall?

1. Within 150 mm of the top of the wall or within 150 mm of an angle formed by two walls
2. Within 300 mm of any door opening
3. Within 150 mm of the floor only
4. Anywhere provided the cable is run diagonally

Permitted zones include horizontally within 150 mm of the top of the wall and within 150 mm of an angle formed by two adjoining walls, in addition to runs to a point or accessory. (BS 7671:2018+A2:2022, Regulation 522.6.202)

4. What is the maximum rated residual operating current (I delta n) of an RCD used to provide additional protection for a concealed cable?

1. 10 mA
2. 30 mA
3. 100 mA
4. 300 mA

Additional protection by an RCD requires a rated residual operating current not exceeding 30 mA, per Regulation 415.1.1 as referenced by 522.6.202. (BS 7671:2018+A2:2022, Regulation 415.1.1)

5. A concealed cable is run in earthed metal conduit that satisfies the protective-conductor requirements. Which statement is correct regarding RCD additional protection?

1. A 30 mA RCD is still mandatory in all cases
2. RCD additional protection is not required on this basis
3. A 100 mA RCD must be fitted instead
4. The conduit must additionally be sleeved in PVC

Regulation 522.6.204 does not require RCD additional protection where the cable is enclosed in earthed metal conduit meeting the protective-conductor requirements. (BS 7671:2018+A2:2022, Regulation 522.6.204)

6. Under Regulation 522.6.204, which cable feature removes the need for RCD additional protection of a concealed cable?

1. A cable having mechanical protection sufficient to prevent penetration by nails and screws, such as cable to BS 8436
2. Any cable coloured grey rather than white
3. A cable with a cross-sectional area of at least 4 mm squared
4. A cable installed using plastic clips at 250 mm centres

Regulation 522.6.204 permits omission of RCD protection where the cable has an earthed metallic covering, is in earthed metal enclosure, or has mechanical protection against nail/screw penetration such as BS 8436 cable. (BS 7671:2018+A2:2022, Regulation 522.6.204)

7. A cable is concealed in a metal-stud (internally metallic) partition. What does BS 7671 require regardless of the cable's depth in the partition?

1. No additional measures, provided the cable is more than 50 mm deep
2. Protection by a 30 mA RCD or by the means of Regulation 522.6.204
3. Use of single-core steel-wire-armoured cable
4. Installation outside any recognised safe zone

Regulation 522.6.203 requires a concealed cable in an internally metallic wall/partition to be protected by a 30 mA RCD or by the means of 522.6.204, irrespective of depth. (BS 7671:2018+A2:2022, Regulation 522.6.203)

8. Which method of supporting a wiring system would FAIL to meet the requirement that it not be liable to premature collapse in the event of fire?

1. Metallic cable clips at the manufacturer's recommended spacing
2. Steel saddles fixed to the building structure
3. Plastic cable ties used as the sole means of support
4. Non-combustible metallic support brackets

Regulation 521.10.202 precludes plastic clips, ties or trunking as the sole means of support; non-combustible (metallic) fixings are required throughout the installation. (BS 7671:2018+A2:2022, Regulation 521.10.202)

9. The requirement to support wiring so it is not liable to premature collapse in a fire applies to which parts of an installation?

1. Only designated escape routes
2. Only commercial premises
3. Throughout the installation
4. Only circuits above 1000 V

Regulation 521.10.202 applies the fire-resistant support requirement throughout the installation, not only along escape routes. (BS 7671:2018+A2:2022, Regulation 521.10.202)

10. A cable is to be buried directly in the ground. Which requirement applies under BS 7671?

1. It must incorporate an earthed armour or metallic sheath, or be in conduit/duct giving equivalent protection, and be marked by covers or marker tape
2. It may be ordinary flat twin-and-earth PVC cable laid directly without covering
3. It must be a single-core steel-wire-armoured cable on an a.c. circuit
4. It needs no identification provided it is at least 300 mm deep

Regulation 522.8.10 requires a directly buried cable to have suitable earthed armour/sheath or equivalent conduit/duct protection, and to be marked by cable covers or marker tape. (BS 7671:2018+A2:2022, Regulation 522.8.10)

11. How should the route of a cable buried directly in the ground be identified?

1. By cable covers or suitable marker tape, with buried conduits/ducts suitably identified
2. By a painted line on the ground surface only
3. No marking is required at any depth
4. By a single warning sign at the property boundary

Regulation 522.8.10 requires the buried run to be marked by cable covers or suitable marker tape, with any buried conduits or ducts suitably identified. (BS 7671:2018+A2:2022, Regulation 522.8.10)

12. Why does BS 7671 prohibit the use of single-core cables armoured with steel wire or steel tape on an a.c. circuit?

1. The steel is too heavy to support reliably
2. Eddy-current and hysteresis losses arise in the magnetic armour
3. Steel armour cannot be earthed
4. The cable colour cannot be identified

Regulation 521.5.2 prohibits single-core steel-wire/tape-armoured cables on a.c. circuits because the magnetic armour causes eddy-current and hysteresis losses. (BS 7671:2018+A2:2022, Regulation 521.5.2)

13. A wiring system must be selected and erected to avoid harmful external influences. Which Appendix 5 classification code corresponds to the presence of water?

1. AD
2. AE
3. AG
4. BA

External influences are classified by code in Appendix 5; AD denotes presence of water, AE solid foreign bodies and AG impact. (BS 7671:2018+A2:2022, Appendix 5)

14. In the BS 7671 external-influence classification, which code relates to the risk of mechanical impact?

1. AD
2. AE
3. AG
4. AF

In Appendix 5, AG denotes impact, AD water and AE the presence of solid foreign bodies. (BS 7671:2018+A2:2022, Appendix 5)

15. What reference ambient air temperature is used as the basis for the current-carrying capacities tabulated for cables in air in Appendix 4?

1. 20 degrees C
2. 25 degrees C
3. 30 degrees C
4. 40 degrees C

Appendix 4 bases the tabulated current-carrying capacities for cables in air on a reference ambient temperature of 30 degrees C. (BS 7671:2018+A2:2022, Appendix 4, Section 2.1)

16. What reference temperature is used in Appendix 4 for the current-carrying capacities of cables buried in the ground?

1. 10 degrees C
2. 20 degrees C
3. 30 degrees C
4. 45 degrees C

Appendix 4 uses a reference temperature of 20 degrees C for cables buried in the ground, as opposed to 30 degrees C for cables in air. (BS 7671:2018+A2:2022, Appendix 4, Section 2.1)

17. The ambient temperature surrounding a cable installed in air is higher than the reference value. Which rating factor must be applied to the tabulated current-carrying capacity?

1. Ca, the ambient temperature factor
2. Cg, the grouping factor
3. Ci, the thermal insulation factor
4. Cc, the BS 3036 fuse factor

Where ambient temperature differs from the reference value, the rating factor Ca (Table 4B1 for cables in air) is applied; Cg corrects grouping and Ci thermal insulation. (BS 7671:2018+A2:2022, Appendix 4 and Regulation 523.5)

18. Which rating factor accounts for the effect of grouping several circuits together?

1. Ca
2. Cg
3. Ci
4. Ct

Grouping is corrected by the factor Cg (e.g. from Table 4C1); Ca corrects ambient temperature and Ci thermal insulation. (BS 7671:2018+A2:2022, Appendix 4 and Regulation 523.5)

19. What is the maximum permitted normal operating conductor temperature for general-purpose thermoplastic (PVC) insulated cables?

1. 60 degrees C
2. 70 degrees C
3. 85 degrees C
4. 90 degrees C

Table 52.1 gives a maximum normal operating conductor temperature of 70 degrees C for general-purpose thermoplastic (PVC) insulated cables. (BS 7671:2018+A2:2022, Table 52.1)

20. What is the maximum permitted normal operating conductor temperature for thermosetting (XLPE) insulated cables?

1. 70 degrees C
2. 80 degrees C
3. 90 degrees C
4. 105 degrees C

Table 52.1 gives a maximum normal operating conductor temperature of 90 degrees C for thermosetting (XLPE) insulated cables. (BS 7671:2018+A2:2022, Table 52.1)

21. A 90 degrees C thermosetting cable is connected to an accessory not confirmed as suitable for temperatures above 70 degrees C. What does BS 7671 require?

1. The conductor must not operate above 70 degrees C at that equipment unless the manufacturer confirms suitability
2. The accessory may be used at 90 degrees C without restriction
3. The cable must be downsized to one rated at 60 degrees C
4. The accessory must be earthed via a separate conductor

Per Regulation 523.1 (Note b to Table 52.1), equipment must not be connected to conductors operating above 70 degrees C unless the manufacturer confirms suitability for the resulting temperature. (BS 7671:2018+A2:2022, Regulation 523.1 (Note b to Table 52.1))

22. Under typical public-supply conditions, what is the recommended maximum voltage drop for a lighting circuit between the origin of the installation and the point of utilisation?

1. 1%
2. 3%
3. 5%
4. 8%

Appendix 4 Table 4Ab recommends a maximum voltage drop of 3% for lighting circuits and 5% for other circuits under typical public-supply conditions. (BS 7671:2018+A2:2022, Appendix 4 Table 4Ab)

23. Under typical public-supply conditions, what is the recommended maximum voltage drop for a power (non-lighting) circuit?

1. 3%
2. 4%
3. 5%
4. 6%

Appendix 4 Table 4Ab recommends a maximum voltage drop of 5% for circuits other than lighting under typical public-supply conditions. (BS 7671:2018+A2:2022, Appendix 4 Table 4Ab)

24. When coordinating a conductor with its protective device, which relationship must be satisfied?

1. Ib must be greater than In
2. Ib <= In <= Iz
3. Iz must be less than In
4. In must equal Ib exactly

Regulation 433.1.1 requires Ib <= In <= Iz, so the device rating sits between the design current and the corrected current-carrying capacity of the conductor. (BS 7671:2018+A2:2022, Regulation 433.1.1)

25. What reference ambient air temperature are the current-carrying capacities tabulated in Appendix 4 of BS 7671 based on, for cables installed in air?

1. 20 degrees C
2. 25 degrees C
3. 30 degrees C
4. 70 degrees C

Appendix 4 tabulated values for cables in air assume a reference ambient temperature of 30 degrees C, irrespective of the installation method. (BS 7671:2018+A2:2022, Appendix 4, Section 2.1)

26. Which rating factor is applied to the tabulated current-carrying capacity to correct for an ambient temperature that differs from the reference value?

1. Cg
2. Ca
3. Ci
4. Cc

Ca is the ambient temperature correction factor, taken from Table 4B1 (cables in air) or Table 4B2 (buried cables). (BS 7671:2018+A2:2022, Appendix 4 and Regulation 523.5)

27. Which rating factor accounts for the reduced heat dissipation when several circuits are bunched or grouped together?

1. Cg (grouping)
2. Ca (ambient temperature)
3. Ci (thermal insulation)
4. Cc (BS 3036 fuse)

Cg, the grouping factor (e.g. from Table 4C1), corrects for the mutual heating effect of cables grouped together. (BS 7671:2018+A2:2022, Appendix 4 Table 4C1 and Regulation 523.5)

28. What is the maximum permitted normal operating conductor temperature for general-purpose thermoplastic (PVC) insulated cables according to Table 52.1?

1. 60 degrees C
2. 70 degrees C
3. 90 degrees C
4. 105 degrees C

Table 52.1 gives 70 degrees C as the maximum normal operating conductor temperature for thermoplastic (PVC) insulation. (BS 7671:2018+A2:2022, Table 52.1 (and Appendix 4 Table 4D))

29. What is the maximum permitted normal operating conductor temperature for thermosetting (XLPE) insulated cables according to Table 52.1?

1. 70 degrees C
2. 80 degrees C
3. 90 degrees C
4. 120 degrees C

Table 52.1 gives 90 degrees C as the maximum normal operating conductor temperature for thermosetting (XLPE) insulation. (BS 7671:2018+A2:2022, Table 52.1 (and Appendix 4 Table 4E))

30. What is the reference temperature on which Appendix 4 bases the current-carrying capacities of cables buried directly in the ground?

1. 10 degrees C
2. 15 degrees C
3. 20 degrees C
4. 30 degrees C

For cables buried in the ground, Appendix 4 uses a reference ground temperature of 20 degrees C, whereas cables in air use 30 degrees C. (BS 7671:2018+A2:2022, Appendix 4, Section 2.1)

31. A protective device must coordinate with the cable's current-carrying capacity. Which inequality correctly expresses the required coordination between design current Ib, device rating In and corrected current-carrying capacity Iz?

1. In <= Ib <= Iz
2. Ib <= In <= Iz
3. Iz <= In <= Ib
4. Ib <= Iz <= In

The design current must not exceed the device rating, which in turn must not exceed the corrected current-carrying capacity: Ib <= In <= Iz. (BS 7671:2018+A2:2022, Regulation 523.1 and Regulation 433.1.1)

32. When sizing a cable protected by a fuse to BS 3036, which additional rating factor must be applied because of the device's fusing characteristics?

1. Ca
2. Cg
3. Ci
4. Cc

Cc, the correction factor for semi-enclosed (BS 3036) fuses, accounts for their relatively high fusing factor and is included only when such a fuse is used. (BS 7671:2018+A2:2022, Regulation 433.1 and Appendix 4)

33. A cable is grouped with others (Cg = 0.8) in an ambient of 40 degrees C (Ca = 0.87) and is protected by a 20 A type B circuit-breaker. Using It >= In / (Ca x Cg x Ci x Cc) with Ci = Cc = 1, what minimum tabulated current-carrying capacity It is required?

1. About 16 A
2. About 23 A
3. About 29 A
4. About 35 A

It >= 20 / (0.87 x 0.8) = 20 / 0.696 = approximately 28.7 A, so a tabulated rating of about 29 A is required. (BS 7671:2018+A2:2022, Regulation 433.1.1 and Appendix 4)

34. A thermoplastic cable rated It = 27 A is installed where the only applicable factors are an ambient of 35 degrees C (Ca = 0.94) and grouping (Cg = 0.7). What is the cable's effective current-carrying capacity Iz?

1. About 14.4 A
2. About 17.8 A
3. About 21.1 A
4. About 25.4 A

Iz = It x Ca x Cg = 27 x 0.94 x 0.7 = approximately 17.8 A. (BS 7671:2018+A2:2022, Appendix 4 and Regulation 523.1)

35. When the ambient temperature surrounding a cable in air is below the reference 30 degrees C, what is the effect on the ambient rating factor Ca and the cable's capacity?

1. Ca falls below 1, reducing the capacity
2. Ca rises above 1, increasing the capacity
3. Ca stays at 1 with no change
4. Ca must be ignored for low temperatures

Cooler-than-reference conditions allow more heat dissipation, so Ca exceeds 1 and the effective current-carrying capacity increases. (BS 7671:2018+A2:2022, Appendix 4 Table 4B1 and Regulation 523.5)