🔌 Assessment of General Characteristics: Supplies and Earthing Systems (Part 3)

1. In the letter code used to classify system earthing arrangements in BS 7671, what does the FIRST letter describe?

1. The relationship of the supply source (power system) to earth
2. The relationship of the installation's exposed-conductive-parts to earth
3. Whether the neutral and protective functions are separate or combined
4. The nominal voltage of the supply

The first letter denotes how the source is connected to earth: 'T' for one point directly earthed, 'I' for isolated or high-impedance earthed. (BS 7671:2018, Regulation 312.2)

2. In a system earthing letter code, what does the SECOND letter describe?

1. The arrangement of neutral and protective conductors
2. The relationship of the installation's exposed-conductive-parts to earth
3. The supply frequency
4. The prospective short-circuit current

The second letter describes how the installation's exposed-conductive-parts relate to earth: 'T' for an independent earth, 'N' for a connection to the source's earthed point. (BS 7671:2018, Regulation 312.2)

3. In the system earthing classification, what do the subsequent letters 'S' and 'C' indicate?

1. S = single-phase supply; C = combined (three-phase) supply
2. S = separate neutral and protective conductors; C = neutral and protective functions combined in a single (PEN) conductor
3. S = source earthed; C = consumer earthed
4. S = sheathed cable; C = conduit wiring

'S' means the neutral and protective functions are provided by separate conductors, while 'C' means they are combined in one conductor (a PEN conductor). (BS 7671:2018, Regulation 312.2)

4. Which earthing arrangement keeps the neutral and protective conductors separate throughout the entire system, with the earth traditionally provided by the metallic sheath or armour of the supply cable?

1. TN-C-S
2. TT
3. TN-S
4. IT

In a TN-S system the neutral and protective conductors remain separate throughout, with the supply providing the earth, traditionally via the cable sheath or armour. (BS 7671:2018, Regulation 312.2; IET On-Site Guide)

5. Which earthing system is also commonly known as Protective Multiple Earthing (PME)?

1. TN-S
2. TN-C-S
3. TT
4. IT

TN-C-S, in which the supply neutral and protective functions are combined as a PEN conductor and separated at the installation, is the PME supply arrangement. (BS 7671:2018, Regulation 312.2; IET On-Site Guide)

6. A property has no supplier-provided earth terminal, and the installation's exposed-conductive-parts are connected to a local earth electrode that is electrically independent of the source earth. Which system is this?

1. TN-S
2. TN-C-S (PME)
3. TT
4. IT

A TT system has the source earthed but the installation relies on its own independent earth electrode, with no earth terminal supplied by the distributor. (BS 7671:2018, Regulation 312.2; IET On-Site Guide)

7. Which statement best describes an IT system?

1. The source neutral and protective functions are combined as a PEN conductor up to the installation
2. The source has no direct connection to earth (isolated or earthed through high impedance), while the installation's exposed-conductive-parts are earthed
3. Both the source and the installation share a common directly earthed point throughout
4. The source is directly earthed and the installation uses an independent earth electrode

In an IT system the source is isolated from earth or earthed through high impedance, while the installation's exposed-conductive-parts are earthed, so a first fault need not interrupt supply. (BS 7671:2018, Regulation 312.2; IEC 60364-1, Clause 312)

8. In a TN-C-S (PME) supply, where does the change from a combined to a separated arrangement occur?

1. The neutral and protective functions are combined (PEN) for part of the supply and are separated at the installation
2. The neutral and protective functions are separate at the source and combined at the installation
3. The neutral and protective functions are combined throughout, including the installation
4. The neutral and protective functions are separate throughout the entire system

In TN-C-S the supply uses a combined PEN conductor for part of the system, which is then split into separate neutral and protective conductors at the installation. (BS 7671:2018, Regulation 312.2; IET On-Site Guide)

9. In the first-letter code, what does the letter 'I' signify about the source?

1. One point of the source is directly earthed
2. The source is isolated from earth or connected to earth through a high impedance
3. The installation has an independent earth electrode
4. The neutral and protective functions are combined

'I' (Isolated) means the source has no direct earth connection, being either isolated or earthed via a high impedance, as in an IT system. (BS 7671:2018, Regulation 312.2; IEC 60364-1, Clause 312)

10. Why is an IT system selected for certain medical locations such as operating theatres?

1. It provides the lowest possible earth fault loop impedance
2. A single (first) earth fault does not cause an interruption of the supply, preserving continuity for critical loads
3. It removes the need for any earthing of exposed-conductive-parts
4. It allows the supply voltage to exceed 1000 V AC safely

Because the source is isolated or high-impedance earthed, a first earth fault does not require disconnection, so IT is used where continuity of supply is critical, such as operating theatres. (BS 7671:2018, Section 411.6 (IT systems); IEC 60364-1)

11. Which combination of letters correctly describes a system where the source has one point directly earthed and the installation's exposed-conductive-parts are connected to the earthed point of the source by a conductor separate from the neutral throughout?

1. TT
2. IT
3. TN-S
4. TN-C

First letter T = source directly earthed; second letter N = parts connected to the source's earthed point; final letter S = separate neutral and protective conductors, giving TN-S. (BS 7671:2018, Regulation 312.2)

12. What is the standard UK low-voltage nominal supply voltage for a single-phase supply assessed under Part 3?

1. 110 V
2. 230 V
3. 240 V
4. 400 V

The declared UK nominal single-phase low-voltage supply is 230 V, with three-phase being 400 V line-to-line. (BS 7671:2018, Chapter 31; ESQCR 2002 (230 V +10%/-6%))

13. What is the standard UK nominal three-phase (line-to-line) supply voltage and frequency?

1. 230 V at 60 Hz
2. 400 V at 50 Hz
3. 415 V at 60 Hz
4. 690 V at 50 Hz

The standard UK three-phase low-voltage supply is 400 V line-to-line at a frequency of 50 Hz. (BS 7671:2018, Chapter 31 (Regulation 313))

14. Under Regulation 313.1, which of the following must the designer ascertain about a new supply before designing the installation?

1. Only the nominal voltage and frequency
2. The nominal voltage, nature of current and frequency, prospective short-circuit current, external earth fault loop impedance, the distributor's protective device and the earthing arrangement
3. Only the colour of the supply cable and the meter type
4. Only the prospective short-circuit current and the cable length

Regulation 313.1 requires the designer to determine a range of supply characteristics including nominal voltage, nature/frequency of current, prospective short-circuit current, external earth fault loop impedance, the distributor's protective device, and the type of earthing system. (BS 7671:2018, Regulation 313.1; IET On-Site Guide)

15. The external earth fault loop impedance (Ze) of a supply is best described as a value that is:

1. Fully within the installer's control and set by the final circuit design
2. Outside the installer's control, determined by the distributor's network and earthing system
3. Always identical regardless of the earthing system in use
4. Measured only after the consumer unit has been installed and energised

Ze is the supply-side (external) portion of the earth fault loop and depends on the distributor's network and earthing system, so it lies outside the installer's control. (BS 7671:2018, Regulation 313.1; IET On-Site Guide)

16. Which set of typical maximum design values for the external earth fault loop impedance (Ze) correctly matches the earthing systems?

1. TN-S 21 ohm; TN-C-S 0.8 ohm; TT 0.35 ohm
2. TN-S 0.8 ohm; TN-C-S 0.35 ohm; TT 21 ohm
3. TN-S 0.35 ohm; TN-C-S 21 ohm; TT 0.8 ohm
4. TN-S 0.35 ohm; TN-C-S 0.8 ohm; TT 0.8 ohm

Typical UK DNO maximum design Ze values are about 0.8 ohm for TN-S, 0.35 ohm for TN-C-S (PME) and 21 ohm for TT. (ENA Engineering Recommendation; IET On-Site Guide)

17. BS 7671 (Part 1) applies to electrical installations operating at a nominal supply voltage not exceeding which values?

1. 230 V AC or 110 V DC
2. 1000 V AC or 1500 V DC
3. 400 V AC or 600 V DC
4. 11 kV AC or 1500 V DC

The scope of BS 7671 covers installations with a nominal voltage not exceeding 1000 V AC or 1500 V DC. (BS 7671:2018, Regulation 110.1 (Scope))

18. For a 230 V (U0) TN system, what is the maximum permitted disconnection time for a final circuit not exceeding 63 A with socket-outlets, and for a distribution circuit respectively?

1. 0.2 s for the final circuit and 1 s for the distribution circuit
2. 0.4 s for the final circuit and 5 s for the distribution circuit
3. 5 s for the final circuit and 0.4 s for the distribution circuit
4. 1 s for the final circuit and 0.2 s for the distribution circuit

For TN systems at U0 = 230 V, Table 41.1 gives 0.4 s for final circuits up to 63 A (socket-outlets) or 32 A (fixed equipment) and 5 s for distribution circuits. (BS 7671:2018, Table 41.1 and Regulation 411.3.2)

19. For a 230 V (U0) TT system, what are the maximum disconnection times for a final circuit and a distribution circuit respectively?

1. 0.4 s and 5 s
2. 0.2 s and 1 s
3. 0.1 s and 0.4 s
4. 1 s and 5 s

For TT systems at U0 = 230 V, the maximum disconnection time is 0.2 s for final circuits and 1 s for distribution circuits. (BS 7671:2018, Table 41.1 and Regulation 411.3.2)

20. Why does fault protection by automatic disconnection of supply in a TT installation normally rely on a residual current device (RCD) rather than overcurrent devices alone?

1. Because TT supplies are always single-phase and overcurrent devices cannot be used on them
2. Because the earth electrode gives a characteristically high earth fault loop impedance, so overcurrent devices alone may not disconnect quickly enough
3. Because RCDs are cheaper than overcurrent protective devices
4. Because the distributor prohibits the use of fuses on TT systems

A TT installation depends on an earth electrode with a high earth fault loop impedance, so the fault current may be too low for overcurrent devices to operate quickly, making an RCD the normal means of achieving ADS. (BS 7671:2018, Regulation 411.5 (TT systems); IET On-Site Guide)

21. Under Part 3, which regulation specifically deals with the assessment of supplies for safety services and standby systems?

1. Regulation 311
2. Regulation 313.2
3. Regulation 411.3.2
4. Regulation 110.1

Regulation 313.2 covers the assessment of supplies provided for safety services and for standby purposes. (BS 7671:2018, Regulation 313.2)

22. When an installation includes a supply for safety services or standby purposes, what must be assessed for that source under Part 3?

1. Only the colour of its enclosure
2. Its characteristics, such as voltage, nature of current and frequency, in the same way as the main supply
3. Only the manufacturer's brand name
4. Nothing, because standby supplies are outside the scope of BS 7671

Where a supply for safety services or standby systems is provided, its characteristics must be determined and assessed under Regulation 313.2, just as for the normal supply. (BS 7671:2018, Regulation 313.2)

23. Which of the following is a typical example of a source used to provide a supply for safety services or standby purposes?

1. The distributor's normal mains supply only
2. A standby generator or battery (e.g. a UPS) supplying essential loads on loss of the main supply
3. The earth electrode of a TT system
4. The metallic sheath of the supply cable

Safety service and standby supplies are commonly provided by independent sources such as standby generators or batteries (UPS) that energise essential loads when the main supply fails. (BS 7671:2018, Regulation 313.2)

24. An installation has a main DNO supply plus a standby generator that automatically takes over essential circuits if the mains fails. In relation to Part 3, the correct approach is to:

1. Assess the characteristics of the main supply only, ignoring the generator
2. Assess the characteristics of both the normal supply and the standby/safety services source so the design suits each
3. Assume the generator has the same Ze as the DNO supply without checking
4. Treat the generator as exempt from any assessment under BS 7671

Regulation 313.2 requires the characteristics of supplies for safety services and standby systems to be assessed in addition to the normal supply, since their parameters (and earthing) may differ and affect protection. (BS 7671:2018, Regulation 313.2 and 313.1)

25. In the BS 7671 letter code for system earthing, what does the FIRST letter describe?

1. The relationship of the supply source (power system) to earth
2. The relationship of the installation's exposed-conductive-parts to earth
3. Whether the neutral and protective conductors are separate or combined
4. The number of phases provided by the distributor

The first letter denotes the relationship of the supply source to earth: 'T' for a directly earthed point and 'I' for isolated or high-impedance earthing. (BS 7671:2018, Regulation 312.2)

26. In the system earthing letter code, what does the SECOND letter describe?

1. The relationship of the installation's exposed-conductive-parts to earth
2. The relationship of the supply source to earth
3. Whether protective and neutral functions are combined
4. The nominal voltage of the supply

The second letter describes how the installation's exposed-conductive-parts relate to earth: 'T' for an independent earth and 'N' for connection to the source's earthed point. (BS 7671:2018, Regulation 312.2)

27. In a system designation such as TN-S or TN-C, what does the subsequent letter 'S' indicate?

1. Neutral and protective functions are provided by separate conductors
2. Neutral and protective functions are combined in a single conductor
3. The supply is single-phase
4. The source is isolated from earth

'S' stands for Separate: the neutral and protective functions are provided by separate conductors throughout. (BS 7671:2018, Regulation 312.2)

28. In a system designation, what does the subsequent letter 'C' indicate?

1. Neutral and protective functions are combined in a single conductor
2. Neutral and protective functions are kept separate
3. The installation has a continuous earth electrode
4. The supply is connected via a high impedance

'C' stands for Combined: the neutral and protective functions are combined in a single PEN conductor. (BS 7671:2018, Regulation 312.2)

29. Which system has the neutral and protective conductors kept separate throughout, with the earth traditionally provided by the metallic sheath of the supply cable?

1. TN-S
2. TN-C-S
3. TT
4. IT

In a TN-S system the neutral and protective conductors are separate throughout, with the protective earth provided by the supply (traditionally the cable sheath/armour). (BS 7671:2018, Regulation 312.2; IET On-Site Guide)

30. A supply enters an installation with the neutral and protective functions combined in a single PEN conductor up to the origin, then separated into neutral and earth within the installation. Which system is this, and what is its common UK name?

1. TN-C-S, known as Protective Multiple Earthing (PME)
2. TN-S, known as a cable-sheath earth
3. TT, known as an earth-electrode system
4. IT, known as an isolated system

A combined PEN conductor that is separated at the installation defines a TN-C-S system, commonly provided in the UK as PME. (BS 7671:2018, Regulation 312.2; IET On-Site Guide)

31. During an assessment you find the distributor provides no earth terminal, and the installation's exposed-conductive-parts are connected to a local earth electrode independent of the source earth. Which system type is this?

1. TT
2. TN-S
3. TN-C-S
4. IT

Where the supplier provides no earth terminal and the installation uses its own independent earth electrode, the system is TT. (BS 7671:2018, Regulation 312.2; IET On-Site Guide)

32. Which system has the source with no direct connection to earth (isolated or earthed through a high impedance), while the installation's exposed-conductive-parts are earthed?

1. IT
2. TN-S
3. TN-C-S
4. TT

In an IT system the source is isolated from earth or connected through a high impedance, while the installation's exposed-conductive-parts are earthed. (BS 7671:2018, Regulation 312.2)

33. In which system does a first earth fault NOT cause an interruption of the supply, making it suitable where continuity is critical such as operating theatres?

1. IT
2. TN-C-S
3. TT
4. TN-S

Because the source is isolated or high-impedance earthed, a single earth fault in an IT system need not interrupt supply, so it suits continuity-critical loads such as medical locations. (BS 7671:2018, Section 411.6; IEC 60364-1)

34. In the earthing-system letter code, what does the letter 'I' as the first letter signify about the source?

1. The source is isolated from earth or connected via a high impedance
2. The source has one point directly earthed
3. The installation uses an independent earth electrode
4. The protective and neutral conductors are combined

As the first letter, 'I' (Isolated) means the source has no direct earth connection, being isolated or earthed through a high impedance. (BS 7671:2018, Regulation 312.2; IEC 60364-1, Clause 312)

35. In a TN system, the second letter 'N' indicates that the installation's exposed-conductive-parts are connected to which point?

1. The earthed point of the source, usually via the neutral
2. An independent local earth electrode
3. A high-impedance earthing resistor
4. A separate functional earth bar only

In a TN system the 'N' means exposed-conductive-parts are connected directly to the earthed point of the source, usually through the supply neutral. (BS 7671:2018, Regulation 312.2)