⏱️ ADS, Disconnection Times and Earth Fault Loop Impedance (Part 4, Chapter 41)

1. In a TN system at a nominal line voltage to earth U0 of 230 V AC, what is the maximum disconnection time for a final circuit not exceeding 63 A with one or more socket-outlets?

1. 0.4 seconds
2. 5 seconds
3. 0.2 seconds
4. 1 second

For TN systems at 120 V < U0 ≤ 230 V AC, Table 41.1 requires final circuits covered by 411.3.2.2 to disconnect within 0.4 seconds. (BS 7671:2018, Table 41.1 (TN, 120 V < U0 ≤ 230 V AC))

2. In a TN system, what is the maximum permitted disconnection time for a distribution circuit?

1. 5 seconds
2. 0.4 seconds
3. 0.2 seconds
4. 1 second

Regulation 411.3.2.3 permits a maximum disconnection time of 5 seconds for distribution circuits in a TN system. (BS 7671:2018, Regulation 411.3.2.3 (TN))

3. A 230 V TN final circuit supplies a ring of 13 A socket-outlets rated at 32 A. Which maximum disconnection time must it achieve to comply with Table 41.1?

1. 0.4 seconds
2. 5 seconds
3. 1 second
4. 0.2 seconds

A 32 A socket-outlet final circuit is within the 63 A limit of 411.3.2.2, so the 0.4 s time of Table 41.1 applies for TN at 230 V. (BS 7671:2018, Regulation 411.3.2.2 and Table 41.1)

4. A 230 V TN final circuit supplies only fixed connected current-using equipment and is rated at 40 A. Which maximum disconnection time is permitted?

1. 5 seconds
2. 0.4 seconds
3. 0.2 seconds
4. 1 second

Final circuits supplying only fixed equipment fall under the 0.4 s rule only up to 32 A; at 40 A the circuit exceeds that limit, so the 5 s time of 411.3.2.3 applies. (BS 7671:2018, Regulations 411.3.2.2 and 411.3.2.3 (TN))

5. Table 41.1 maximum disconnection times for final circuits apply to circuits not exceeding which rating where socket-outlets are present?

1. 63 A
2. 32 A
3. 100 A
4. 45 A

Regulation 411.3.2.2 applies the Table 41.1 times to final circuits not exceeding 63 A with one or more socket-outlets. (BS 7671:2018, Regulation 411.3.2.2)

6. For a final circuit supplying only fixed connected current-using equipment, up to what rating does Regulation 411.3.2.2 apply the Table 41.1 disconnection times?

1. 32 A
2. 63 A
3. 20 A
4. 16 A

For fixed connected current-using equipment the Table 41.1 times apply to final circuits not exceeding 32 A. (BS 7671:2018, Regulation 411.3.2.2)

7. In a TN AC system, which voltage band gives a maximum disconnection time of 0.2 seconds in Table 41.1?

1. 230 V < U0 ≤ 400 V
2. 120 V < U0 ≤ 230 V
3. 50 V < U0 ≤ 120 V
4. U0 > 400 V

In Table 41.1 for TN AC systems, the band 230 V < U0 ≤ 400 V requires disconnection within 0.2 seconds. (BS 7671:2018, Table 41.1 (TN systems, AC))

8. In a TN AC system, what is the maximum disconnection time in Table 41.1 for a final circuit where 50 V < U0 ≤ 120 V?

1. 0.8 seconds
2. 0.4 seconds
3. 0.2 seconds
4. 0.1 seconds

Table 41.1 gives 0.8 seconds for TN final circuits in the band 50 V < U0 ≤ 120 V AC. (BS 7671:2018, Table 41.1 (TN systems, AC))

9. Which protective measure relies on protective earthing, protective equipotential bonding and automatic disconnection in case of a fault for its fault protection?

1. Automatic Disconnection of Supply (ADS)
2. Electrical separation
3. Double or reinforced insulation
4. Extra-low voltage (SELV)

ADS provides fault protection by protective earthing, protective equipotential bonding and automatic disconnection, with basic protection by insulation, barriers or enclosures. (BS 7671:2018, Regulation 411.1 / Section 411)

10. A submain (distribution circuit) and a 32 A socket-outlet final circuit both supply parts of a 230 V TN installation. Which statement correctly pairs each with its maximum disconnection time?

1. Distribution circuit 5 s; final circuit 0.4 s
2. Distribution circuit 0.4 s; final circuit 5 s
3. Both 0.4 s
4. Both 5 s

In a TN system distribution circuits may disconnect within 5 s, while final circuits covered by 411.3.2.2 must disconnect within 0.4 s at 230 V. (BS 7671:2018, Regulations 411.3.2.2 and 411.3.2.3, Table 41.1)

11. Approximately what AC rms value is the conventional touch-voltage limit underpinning the Table 41.1 disconnection-time philosophy?

1. 50 V
2. 25 V
3. 120 V
4. 230 V

50 V AC rms is the conventional touch-voltage limit; ADS disconnects quickly enough that a higher fault touch voltage does not persist dangerously. (BS 7671:2018, Section 411 / IEC 60364-4-41 basis for Table 41.1)

12. For a TN system, the condition for effective automatic disconnection is expressed by which relationship between earth fault loop impedance Zs and disconnecting current Ia?

1. Zs x Ia ≤ U0 x Cmin
2. Zs x Ia ≥ U0 x Cmin
3. Zs / Ia ≤ U0
4. Zs x U0 ≤ Ia x Cmin

Regulation 411.4.5 requires Zs x Ia ≤ U0 x Cmin, which rearranges to maximum Zs = (U0 x Cmin) / Ia. (BS 7671:2018, Regulation 411.4.5 (TN))

13. In the TN disconnection condition Zs x Ia ≤ U0 x Cmin, what value is given to the voltage factor Cmin?

1. 0.95
2. 1.0
3. 0.8
4. 1.05

Cmin accounts for supply voltage tolerances and is given the value 0.95. (BS 7671:2018, Regulation 411.4.5 and IET guidance)

14. Using Zs = (U0 x Cmin) / Ia with U0 = 230 V and Cmin = 0.95, what is the maximum Zs for a device whose Ia for the required time is 160 A?

1. 1.37 Ω
2. 1.44 Ω
3. 2.30 Ω
4. 0.87 Ω

(230 x 0.95) / 160 = 218.5 / 160 ≈ 1.37 Ω, matching the tabulated value for a 32 A Type B MCB at 0.4 s. (BS 7671:2018, Regulation 411.4.5 and Table 41.3 (32 A Type B, 0.4 s))

15. For instantaneous operation, what multiple of the rated current In is taken as the disconnecting current Ia for a Type B MCB to BS EN 60898?

1. 5 x In
2. 10 x In
3. 20 x In
4. 3 x In

A Type B MCB is taken to operate instantaneously at 5 x In, whereas Type C is 10 x In and Type D is 20 x In. (BS 7671:2018, Appendix 3 / Table 41.3)

16. For a Type C MCB to BS EN 60898, what disconnecting current Ia is taken for instantaneous operation?

1. 10 x In
2. 5 x In
3. 20 x In
4. 8 x In

A Type C MCB is taken to operate instantaneously at 10 x In; Type B is 5 x In and Type D is 20 x In. (BS 7671:2018, Appendix 3 / Table 41.3)

17. A tabulated maximum Zs from Table 41.3 assumes conductors at normal operating temperature. When checking a measured Zs taken at around 10 °C ambient, what should be done to the tabulated value before comparison?

1. Multiply it by 0.8 (the 80% rule)
2. Multiply it by 1.2
3. Divide it by 0.95
4. Use it unchanged

Because conductor resistance rises with temperature under fault, the tabulated Zs is multiplied by 0.8 to give a rule-of-thumb limit for measurements at ambient temperature. (IET On-Site Guide (BS 7671:2018) Appendix B / Guidance Note 3)

18. On a 230 V TN final circuit the measured Zs is too high for the protective MCB to disconnect within 0.4 s. Which measure is most appropriate to achieve the required disconnection time?

1. Add an RCD to provide automatic disconnection
2. Increase the circuit overcurrent rating
3. Remove the circuit protective conductor
4. Accept the high Zs as compliant

Where Zs cannot satisfy the overcurrent device, an RCD can be used to provide automatic disconnection within the required time. (BS 7671:2018, Section 411 (use of RCDs for ADS where Zs cannot be achieved))

19. In a TT system protected by an RCD, fault protection is satisfied when which condition is met?

1. RA x IΔn ≤ 50 V
2. RA x IΔn ≥ 50 V
3. RA / IΔn ≤ 50 V
4. RA x IΔn ≤ 230 V

For TT systems with an RCD, Regulation 411.5.3 requires RA x IΔn ≤ 50 V, where RA is the earth electrode plus protective conductor resistance. (BS 7671:2018, Regulation 411.5.3 (TT))

20. Using RA ≤ 50 V / IΔn, what is the limiting earth electrode resistance for a 30 mA RCD in a TT system?

1. 1667 Ω
2. 500 Ω
3. 200 Ω
4. 50 Ω

RA ≤ 50 / 0.030 = 1667 Ω; for a 100 mA RCD the limit would be 500 Ω. (BS 7671:2018, Regulation 411.5.3 (TT))

21. In the TT fault-protection condition RA x IΔn ≤ 50 V, what does the term RA represent?

1. The resistance of the earth electrode plus the protective conductor
2. The rated residual operating current of the RCD
3. The earth fault loop impedance of the supply
4. The nominal line voltage to earth

RA is the sum of the resistances of the earth electrode and the protective conductor connecting it to exposed-conductive-parts. (BS 7671:2018, Regulation 411.5.3 (TT))

22. In a TT system at 230 V, what is the maximum disconnection time in Table 41.1 for a final circuit covered by 411.3.2.2?

1. 0.2 seconds
2. 0.4 seconds
3. 1 second
4. 5 seconds

For TT systems at 120 V < U0 ≤ 230 V AC, Table 41.1 requires final circuits to disconnect within 0.2 seconds. (BS 7671:2018, Table 41.1 (TT, 120 V < U0 ≤ 230 V AC))

23. Where a fault-protection device cannot meet the required disconnection time, what is the correct status of supplementary equipotential bonding as a measure?

1. It limits touch voltage but does not by itself remove the need for ADS compliance
2. It fully replaces the need for automatic disconnection
3. It increases the permitted disconnection time to 5 s
4. It is prohibited in such circumstances

Supplementary equipotential bonding limits touch voltage as an additional measure, but ADS compliance must still be achieved by other means. (BS 7671:2018, Regulation 411.3.2.6)

24. In a TT system at a nominal line voltage to earth (U0) of 230 V AC, what is the maximum disconnection time for a final circuit covered by Regulation 411.3.2.2 (e.g. a 32 A circuit supplying fixed equipment or a socket-outlet circuit)?

1. 0.2 seconds
2. 0.4 seconds
3. 1 second
4. 5 seconds

Table 41.1 specifies a maximum disconnection time of 0.2 s for TT final circuits in the band 120 V < U0 ≤ 230 V AC. (BS 7671:2018, Table 41.1 (TT, 120 V < U0 ≤ 230 V AC))

25. A TT-supplied installation has a final circuit of a 32 A socket-outlet rated below 63 A. Which maximum disconnection time at 230 V applies to this circuit?

1. 0.1 seconds
2. 0.2 seconds
3. 1 second
4. 5 seconds

Socket-outlet final circuits not exceeding 63 A fall under 411.3.2.2, so the TT value of 0.2 s from Table 41.1 applies. (BS 7671:2018, Table 41.1 and Regulation 411.3.2.2 (TT))

26. An 80 A submain (distribution circuit) feeds a sub-distribution board in a TT installation. What is the maximum disconnection time that may be permitted for this circuit at 230 V?

1. 0.2 seconds
2. 0.4 seconds
3. 1 second
4. 5 seconds

Distribution circuits in a TT system are permitted a maximum disconnection time of 1 second under Regulation 411.3.2.4. (BS 7671:2018, Regulation 411.3.2.4 (TT))

27. How does the maximum disconnection time for a TT final circuit at 230 V compare with the equivalent TN final circuit?

1. TT is 0.2 s, which is shorter than the 0.4 s required for TN
2. TT is 0.4 s, the same as TN
3. TT is 1 s, which is longer than the 0.4 s required for TN
4. TT is 5 s, which is longer than TN

Table 41.1 requires faster disconnection in TT systems (0.2 s) than in TN systems (0.4 s) for final circuits at 230 V. (BS 7671:2018, Table 41.1 (TN and TT, 120 V < U0 ≤ 230 V AC))

28. A TT installation has a final circuit supplying only fixed connected current-using equipment rated at 50 A. Because it exceeds the 32 A limit of 411.3.2.2 for fixed equipment, which maximum disconnection time may be applied?

1. 0.2 seconds
2. 0.4 seconds
3. 1 second
4. 5 seconds

A final circuit exceeding the 411.3.2.2 ratings is treated like a distribution circuit, so the TT 1 second limit of 411.3.2.4 applies. (BS 7671:2018, Regulation 411.3.2.4 (TT) and 411.3.2.2)

29. Which two maximum disconnection times appear for TT systems at 230 V in BS 7671 for final circuits within 411.3.2.2 and for distribution circuits respectively?

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

TT final circuits within 411.3.2.2 require 0.2 s, while TT distribution circuits and larger final circuits are permitted 1 s. (BS 7671:2018, Table 41.1 and Regulations 411.3.2.2 / 411.3.2.4 (TT))

30. A TT installation is protected throughout by a 30 mA RCD. For a 230 V final circuit of a 13 A socket-outlet, which disconnection-time requirement must the RCD satisfy?

1. It must disconnect within 5 seconds
2. It must disconnect within 1 second
3. It must disconnect within 0.4 second
4. It must disconnect within 0.2 second

The socket-outlet circuit is within 411.3.2.2, so the TT final-circuit time of 0.2 s from Table 41.1 applies; RCDs readily meet this. (BS 7671:2018, Table 41.1 and Regulation 411.3.2.2 (TT))

31. Which regulation gives the 1 second maximum disconnection time permitted for distribution circuits in a TT system?

1. Regulation 411.3.2.2
2. Regulation 411.3.2.3
3. Regulation 411.3.2.4
4. Regulation 411.5.3

Regulation 411.3.2.4 sets the 1 second limit for TT distribution circuits and over-rated final circuits. (BS 7671:2018, Regulation 411.3.2.4 (TT))

32. Table 41.1 maximum disconnection times apply to final circuits with one or more socket-outlets not exceeding what rating?

1. 20 A
2. 32 A
3. 63 A
4. 100 A

Regulation 411.3.2.2 applies the Table 41.1 times to socket-outlet final circuits not exceeding 63 A. (BS 7671:2018, Regulation 411.3.2.2)

33. For final circuits supplying only fixed connected current-using equipment, Table 41.1 disconnection times apply where the circuit does not exceed which rating?

1. 16 A
2. 32 A
3. 63 A
4. 80 A

Regulation 411.3.2.2 applies Table 41.1 to fixed-equipment final circuits not exceeding 32 A. (BS 7671:2018, Regulation 411.3.2.2)

34. A TT installation uses RCDs for fault protection. A designer claims that, because RCDs are inherently fast, the TT 230 V final-circuit disconnection time may be relaxed to 0.4 s like a TN circuit. Which statement is correct?

1. The designer is correct; TT and TN both require 0.4 s for 230 V final circuits
2. The designer is wrong; the TT final-circuit requirement at 230 V is 0.2 s
3. The designer is wrong; the TT final-circuit requirement at 230 V is 5 s
4. The designer is correct only if a 100 mA RCD is used

Table 41.1 sets 0.2 s for TT final circuits at 230 V, which is faster than the TN value of 0.4 s; it cannot be relaxed. (BS 7671:2018, Table 41.1 (TT, 120 V < U0 ≤ 230 V AC))

35. The conventional touch-voltage limit of 50 V AC rms underpins the disconnection-time philosophy of Table 41.1. What is the purpose of fast automatic disconnection?

1. To prevent a dangerous touch voltage above 50 V persisting long enough to be harmful
2. To keep the supply voltage below 50 V at all times
3. To ensure fuses never operate above 50 A
4. To limit the earth electrode resistance below 50 ohms

ADS disconnects fast enough that a fault touch voltage exceeding 50 V AC does not persist long enough to be dangerous. (BS 7671:2018, Section 411 / IEC 60364-4-41 basis for Table 41.1)