📖 Definitions and Key Terminology (Part 2)

1. According to Part 2 of BS 7671, extra-low voltage (ELV) is defined as a voltage normally not exceeding which value for AC?

1. 50 V AC (rms)
2. 120 V AC (rms)
3. 230 V AC (rms)
4. 1000 V AC (rms)

Extra-low voltage is defined as a voltage normally not exceeding 50 V AC (rms) or 120 V ripple-free DC, whether between conductors or to Earth. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Extra-low voltage')

2. What is the maximum ripple-free DC value for a voltage to be classed as extra-low voltage (ELV) under Part 2 of BS 7671?

1. 120 V ripple-free DC
2. 50 V ripple-free DC
3. 150 V ripple-free DC
4. 60 V ripple-free DC

ELV is defined as not exceeding 50 V AC (rms) or 120 V ripple-free DC, whether between conductors or to Earth. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Extra-low voltage')

3. For low voltage (LV) under Part 2 of BS 7671, what is the upper limit for AC between conductors?

1. Not exceeding 1000 V AC
2. Not exceeding 600 V AC
3. Not exceeding 230 V AC
4. Not exceeding 400 V AC

Low voltage is defined as exceeding ELV but not exceeding 1000 V AC (or 1500 V DC) between conductors. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Low voltage')

4. Under the Part 2 definition of low voltage, what is the maximum AC value permitted between conductors and Earth?

1. 600 V AC
2. 1000 V AC
3. 900 V AC
4. 230 V AC

Low voltage does not exceed 600 V AC (or 900 V DC) between conductors and Earth, with the higher 1000 V AC limit applying between conductors. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Low voltage')

5. What is the maximum DC value between conductors for a system to remain classed as low voltage under Part 2 of BS 7671?

1. 1500 V DC
2. 1000 V DC
3. 900 V DC
4. 120 V DC

Low voltage does not exceed 1000 V AC or 1500 V DC between conductors; the 900 V DC figure applies between conductors and Earth. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Low voltage')

6. In which voltage band does a standard 230/400 V supply to a household or commercial installation fall?

1. Band II
2. Band I
3. Band III
4. Band 0

Voltage Band II contains the low-voltage supplies to household, commercial and most industrial installations, such as 230/400 V. (BS 7671:2018+A2:2022, Part 2 — Voltage Bands (Band II))

7. A telecommunications and signalling circuit operates at 24 V AC where the value of voltage itself provides protection against electric shock. Which voltage band does this fall within?

1. Band I
2. Band II
3. Band III
4. Outside any band

Voltage Band I covers extra-low voltage circuits, including telecommunications, signalling and control, where protection is afforded by the value of voltage. (BS 7671:2018+A2:2022, Part 2 — Voltage Bands (Band I))

8. A system supplies 230 V single-phase to a domestic property. How should this voltage be correctly described under Part 2 of BS 7671?

1. Low voltage, Band II
2. Extra-low voltage, Band I
3. High voltage, Band III
4. Reduced low voltage, Band I

230 V exceeds the ELV limits and is supplied to a household, so it is low voltage in Voltage Band II. (BS 7671:2018+A2:2022, Part 2 — 'Low voltage' and Voltage Bands (Band II))

9. How does Part 2 of BS 7671 define 'earthing'?

1. Connection of the exposed-conductive-parts of an installation to the main earthing terminal of that installation
2. Connection of all live conductors to the supply neutral
3. The provision of a residual current device on every final circuit
4. Connection of extraneous-conductive-parts to the consumer's main switch

Earthing is defined as the connection of the exposed-conductive-parts of an installation to the main earthing terminal of that installation. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Earthing')

10. What is a PEN conductor as defined in Part 2 of BS 7671?

1. A conductor combining the functions of both a protective conductor and a neutral conductor
2. A conductor used solely for protective bonding of gas and water pipes
3. A conductor that connects the main earthing terminal to an earth electrode only
4. A line conductor in a reduced low voltage system

A PEN (protective earthed neutral) conductor combines the functions of both a protective conductor and a neutral conductor, as used in TN-C and TN-C-S (PME) arrangements. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'PEN conductor')

11. In which type of supply arrangement would you expect to find a PEN conductor?

1. TN-C-S (PME)
2. TT
3. IT with the neutral not distributed
4. SELV

A PEN conductor, combining protective and neutral functions, is used in TN-C and TN-C-S (PME) supply arrangements. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'PEN conductor')

12. How is an 'earthing conductor' defined in Part 2 of BS 7671?

1. A protective conductor connecting the main earthing terminal to an earth electrode or other means of earthing
2. A conductor bonding two extraneous-conductive-parts together
3. Any conductor that carries the neutral current back to the source
4. The conductor connecting an exposed-conductive-part to a socket-outlet

An earthing conductor is a protective conductor connecting the main earthing terminal of an installation to an earth electrode or other means of earthing. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Earthing conductor')

13. What does the main earthing terminal (MET) provide for in an installation?

1. Connection of protective conductors, including protective bonding and functional earthing conductors, to the means of earthing
2. Connection of all line conductors to the busbar
3. Isolation of the supply under fault conditions
4. Connection of the neutral to the line conductor

The MET is the terminal or bar for connecting protective conductors, including protective bonding conductors and functional earthing conductors, to the means of earthing. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Main earthing terminal')

14. A metallic gas pipe entering a property is liable to introduce Earth potential into the installation but is not part of the electrical installation. What term best describes it?

1. Extraneous-conductive-part
2. Exposed-conductive-part
3. Protective conductor
4. PEN conductor

An extraneous-conductive-part is a conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation, such as metallic water and gas pipes. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Extraneous-conductive-part')

15. The metal casing of a Class I appliance can be touched, is not normally live, but could become live under fault conditions. Which term applies?

1. Exposed-conductive-part
2. Extraneous-conductive-part
3. Earthing conductor
4. Functional earth

An exposed-conductive-part is a conductive part of equipment which can be touched and which is not normally live but can become live under fault conditions. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Exposed-conductive-part')

16. A SELV system is best described as which of the following?

1. An extra-low voltage system electrically separated from Earth and from other systems
2. An extra-low voltage system connected directly to the main earthing terminal
3. A low voltage system with reduced line-to-Earth voltage of 55 V
4. A functional extra-low voltage system not relied upon for shock protection

A SELV (separated extra-low voltage) system is an extra-low voltage system that is electrically separated from Earth and from other systems. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'SELV system'; Section 414)

17. Which statement best distinguishes a PELV system from a SELV system?

1. A PELV system is not electrically separated from Earth, but otherwise satisfies the requirements for SELV
2. A PELV system uses voltages above the ELV limits
3. A PELV system does not provide any protection against electric shock
4. A PELV system must always operate above 120 V DC

A PELV (protective extra-low voltage) system is an ELV system that is not electrically separated from Earth but otherwise satisfies the requirements for SELV. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'PELV system'; Section 414)

18. An extra-low voltage is used for functional reasons only and is not relied upon for protection against electric shock. Which system does this describe?

1. FELV
2. SELV
3. PELV
4. Reduced low voltage

In a FELV (functional extra-low voltage) system the ELV is used for functional reasons and is not relied upon for shock protection, and it does not meet all the SELV or PELV requirements. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'FELV'; Regulation 411.7)

19. Under the current terminology in Part 2 of BS 7671, what is 'basic protection'?

1. Protection against electric shock under fault-free (normal) conditions
2. Protection against electric shock under single-fault conditions
3. Protection provided only by a residual current device
4. Protection of cables against thermal overload

Basic protection is protection against electric shock under fault-free conditions, i.e. against contact with parts intended to be live (formerly 'direct contact'). (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Basic protection')

20. What term now replaces the former expression 'protection against indirect contact'?

1. Fault protection
2. Basic protection
3. Additional protection
4. Functional earthing

Fault protection is protection against electric shock under single-fault conditions, i.e. contact with exposed-conductive-parts made live by a fault (formerly 'indirect contact'). (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Fault protection')

21. In BS 7671, what does the symbol U0 represent?

1. The nominal voltage to Earth (line to Earth)
2. The nominal voltage between line conductors
3. The prospective fault current at the origin
4. The design current of a circuit

U0 is the nominal voltage line to Earth, whereas U denotes the nominal voltage between line conductors. (BS 7671:2018+A2:2022, Part 2 (Definitions) — nominal voltage symbols U and U0)

22. On a standard 230/400 V three-phase supply, which values correspond to U0 and U respectively?

1. U0 = 230 V, U = 400 V
2. U0 = 400 V, U = 230 V
3. U0 = 400 V, U = 400 V
4. U0 = 230 V, U = 230 V

U0 is the line-to-Earth (line-to-neutral) nominal voltage of 230 V, while U is the line-to-line nominal voltage of 400 V. (BS 7671:2018+A2:2022, Part 2 (Definitions) — nominal voltage symbols U and U0)

23. In BS 7671, what does the abbreviation SELV stand for?

1. Separated extra-low voltage
2. Safe extra-low voltage
3. Secondary earthed low voltage
4. Single-ended low voltage

SELV is defined in Part 2 as a separated extra-low voltage system, electrically separated from Earth and from other systems. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'SELV system')

24. Which statement correctly describes a SELV system according to Part 2 of BS 7671?

1. An extra-low voltage system electrically separated from Earth and from other systems
2. An extra-low voltage system that is connected to Earth at one point
3. A low voltage system supplied at 230 V and earthed at the transformer
4. A system where the extra-low voltage is provided purely for functional reasons

A SELV system is an extra-low voltage system that is electrically separated from Earth and from other systems. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'SELV system'; Section 414)

25. What is the key distinction between a PELV system and a SELV system?

1. A PELV system is not electrically separated from Earth, whereas a SELV system is
2. A PELV system uses a higher voltage than a SELV system
3. A PELV system has no protective separation from other circuits
4. A PELV system is always supplied from the mains without a transformer

A PELV system satisfies the requirements for SELV except that it is not electrically separated from Earth; SELV is separated from Earth, PELV is not. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'PELV system'; Section 414)

26. Which statement best describes a PELV system?

1. An extra-low voltage system that is not electrically separated from Earth but otherwise meets the SELV requirements
2. An extra-low voltage system that is fully separated from Earth and from all other systems
3. A low voltage system that relies on a fuse for protection against electric shock
4. A system where the voltage is too low to require any protective measures

PELV (protective extra-low voltage) meets the requirements for SELV except that it is connected to Earth, i.e. not electrically separated from Earth. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'PELV system'; Section 414)

27. In a FELV (functional extra-low voltage) system, why is the extra-low voltage used?

1. For functional reasons, and it is not relied upon for protection against electric shock
2. To provide full protective separation from Earth
3. Specifically to provide both basic and fault protection by the value of the voltage
4. To meet all the requirements of SELV and PELV simultaneously

In FELV the extra-low voltage is used for functional reasons only and is not relied upon for protection against electric shock; it does not satisfy all SELV or PELV requirements. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'FELV'; Regulation 411.7)

28. An extra-low voltage circuit is provided simply because a control device requires a low operating voltage, but it does not meet all the requirements for SELV or PELV. How should this system be classified?

1. FELV (functional extra-low voltage)
2. SELV (separated extra-low voltage)
3. PELV (protective extra-low voltage)
4. Reduced low voltage

Where extra-low voltage is used for functional reasons and does not meet all SELV or PELV requirements, the system is FELV. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'FELV'; Regulation 411.7)

29. Extra-low voltage (ELV) is defined in Part 2 of BS 7671 as a voltage normally not exceeding which values?

1. 50 V AC (rms) or 120 V ripple-free DC
2. 120 V AC (rms) or 50 V ripple-free DC
3. 230 V AC (rms) or 400 V ripple-free DC
4. 25 V AC (rms) or 60 V ripple-free DC

ELV is defined as not exceeding 50 V AC (rms) or 120 V ripple-free DC, whether between conductors or to Earth. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Extra-low voltage')

30. Which of the following is a correct consequence of a SELV source being electrically separated from Earth?

1. Live parts of the SELV circuit are not connected to Earth or to the protective conductor of another system
2. The SELV circuit must be bonded to the main earthing terminal
3. The SELV neutral must be connected to the PEN conductor of the supply
4. The SELV source must always derive its supply directly from the 230 V mains without separation

Because SELV is electrically separated from Earth and from other systems, its live parts are not connected to Earth or to the protective conductors of other systems. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'SELV system'; Section 414)

31. A 24 V control circuit is connected to Earth at one point for functional purposes but otherwise satisfies all the requirements applicable to SELV. Which classification applies?

1. PELV
2. SELV
3. FELV
4. Reduced low voltage

A system meeting the SELV requirements except that it is connected to Earth (not separated from Earth) is classified as PELV. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'PELV system'; Section 414)

32. Which extra-low voltage system is a recognised protective measure that is relied upon against electric shock and is electrically separated from Earth, in contrast to FELV?

1. SELV
2. FELV
3. PEN conductor system
4. TN-C system

SELV is a recognised protective measure relied upon against electric shock and is separated from Earth, in contrast to FELV which is not relied upon for shock protection. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'SELV system' / 'FELV'; Section 414, Reg 411.7)

33. In BS 7671, what is the abbreviation 'cpc' used for?

1. Circuit-protective conductor
2. Combined protective conductor
3. Current protective core
4. Continuous protective channel

cpc is the standard abbreviation for circuit-protective conductor in BS 7671. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Protective conductor'/'cpc')

34. Which of the following is the best description of a 'live part' as used in BS 7671?

1. A conductor or conductive part intended to be energised in normal use, including the neutral conductor
2. Any metal part that can be touched and may become live only under fault conditions
3. A conductive part that introduces Earth potential and is not part of the installation
4. The protective conductor connecting exposed-conductive-parts to the main earthing terminal

A live part is a conductor or conductive part intended to be energised in normal use, and this includes the neutral conductor. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Live part')

35. Is the neutral conductor of a normal a.c. installation regarded as a live part under BS 7671?

1. Yes, because the neutral is intended to be energised in normal use
2. No, because it is a protective conductor
3. No, because it is at Earth potential and never carries current
4. Only when the protective conductor is disconnected

The neutral conductor is intended to be energised in normal use, so it is a live part; it is not a protective conductor. (BS 7671:2018+A2:2022, Part 2 (Definitions) — 'Live part' / 'Neutral conductor')