📊 Appendices: Time/Current Curves and Cable Ratings (Appendices 3 and 4)

1. In BS 7671, which Appendix contains the time/current characteristics of overcurrent protective devices such as fuses and circuit-breakers?

1. Appendix 3
2. Appendix 4
3. Appendix 5
4. Appendix 7

Appendix 3 presents the time/current characteristics of fuses, circuit-breakers, RCBOs and RCDs as graphs of time against prospective fault current. (BS 7671:2018+A2 Appendix 3)

2. On the time/current characteristic graphs in Appendix 3, what is plotted on the two axes?

1. Time in seconds against prospective fault current in amperes
2. Voltage in volts against resistance in ohms
3. Power in watts against time in minutes
4. Temperature in degrees Celsius against current in amperes

The Appendix 3 curves plot disconnection time (in seconds) on the vertical axis against prospective fault current (in amperes) on the horizontal axis. (BS 7671:2018+A2 Appendix 3)

3. In the 18th Edition, the earth fault loop impedance content formerly held in Appendix 14 of the 17th Edition was relocated into which Appendix?

1. Appendix 3
2. Appendix 4
3. Appendix 6
4. Appendix 12

The former Appendix 14 material on earth fault loop impedance was incorporated into Appendix 3, which now also gives maximum Zs values associated with the device curves. (BS 7671:2018 Appendix 3; IET Wiring Matters (18th Edition changes))

4. For a Type B circuit-breaker to BS EN 60898, instantaneous (magnetic) tripping occurs within which range of rated current (In)?

1. 3 to 5 times In
2. 5 to 10 times In
3. 10 to 20 times In
4. 20 to 40 times In

A Type B device trips instantaneously between 3 and 5 times In; the upper figure of 5 In is used to derive the maximum Zs. (BS EN 60898-1; BS 7671 Appendix 3 (Type B characteristic))

5. For a Type C circuit-breaker to BS EN 60898, instantaneous tripping occurs within which range of rated current (In)?

1. 5 to 10 times In
2. 3 to 5 times In
3. 10 to 20 times In
4. 2 to 4 times In

A Type C device trips instantaneously between 5 and 10 times In; the figure of 10 In is used for the maximum Zs derivation. (BS EN 60898-1; BS 7671 Appendix 3 (Type C characteristic))

6. For a Type D circuit-breaker to BS EN 60898, which multiple of rated current (In) is used to derive the maximum earth fault loop impedance?

1. 20 times In
2. 10 times In
3. 5 times In
4. 40 times In

A Type D device trips instantaneously between 10 and 20 times In, and the upper figure of 20 In is used for the maximum Zs derivation. (BS EN 60898-1; BS 7671 Appendix 3 (Type D characteristic))

7. A 32 A Type B circuit-breaker protects a final circuit. Using the upper instantaneous-trip limit from Appendix 3, what fault current is taken to ensure rapid magnetic operation?

1. 160 A
2. 96 A
3. 320 A
4. 640 A

For a Type B device the maximum instantaneous trip current is 5 In, so 5 x 32 A = 160 A is used to derive the maximum Zs. (BS EN 60898-1; BS 7671 Appendix 3 (Type B, 5 In))

8. A 20 A Type C circuit-breaker is installed. Based on the Appendix 3 derivation multiple, what minimum fault current must flow to guarantee operation in the instantaneous region?

1. 200 A
2. 100 A
3. 60 A
4. 400 A

A Type C device uses 10 In for the maximum Zs derivation, so 10 x 20 A = 200 A is the current taken for instantaneous operation. (BS EN 60898-1; BS 7671 Appendix 3 (Type C, 10 In))

9. To read the fault current Ia required for disconnection within a given time from an Appendix 3 graph, what is the correct procedure?

1. Draw a horizontal line at the required time to the device curve, then drop a vertical line to read the current
2. Draw a vertical line at the rated current and read the time at the top of the graph
3. Read the current directly from where the curve meets the time axis
4. Multiply the rated current by the supply voltage

A horizontal line is drawn at the required disconnection time until it meets the device curve, then a vertical line is dropped to read the corresponding current. (BS 7671 Appendix 3 (method of reading the curves))

10. The maximum Zs values in Chapter 41 Tables 41.2 to 41.4 are derived from the Appendix 3 curves at which two disconnection times for TN systems?

1. 0.4 s for final circuits up to 63 A and 5 s for distribution circuits
2. 0.2 s for final circuits and 1 s for distribution circuits
3. 0.1 s for all circuits
4. 5 s for final circuits and 0.4 s for distribution circuits

In TN systems the tabulated maximum Zs values correspond to 0.4 s for final circuits up to 63 A and 5 s for distribution circuits. (BS 7671:2018 Chapter 41 (Tables 41.2-41.4) and Appendix 3)

11. When designing a circuit, what does Appendix 3 advise regarding the maximum earth fault loop impedance values to use?

1. Use the protective device manufacturer's specific data wherever possible, as values vary between types and makers
2. Always use the lowest tabulated value regardless of the device fitted
3. Use a single fixed value of 1 ohm for all devices
4. Ignore manufacturer data and use only generic graphs

Appendix 3 notes that Zs values vary between device types and manufacturers, so the designer should use the manufacturer's specific data where possible. (BS 7671:2018 Appendix 3 (introductory notes))

12. What does the rating factor Ca in Appendix 4 account for?

1. Ambient temperature differing from the reference value
2. Grouping of circuits
3. Thermal insulation surrounding the cable
4. Depth of burial in the ground

Ca is the correction factor applied when the ambient temperature differs from the tabulated reference value. (BS 7671:2018 Appendix 4 (rating factors, Tables 4B1/4B2))

13. What does the rating factor Cg in Appendix 4 take into account?

1. Grouping of more than one circuit
2. Ambient temperature
3. Conductor surrounded by thermal insulation
4. Soil thermal resistivity

Cg is the grouping factor applied when more than one circuit is installed together, reducing the current-carrying capacity. (BS 7671:2018 Appendix 4 (grouping factors, Table 4C1))

14. Which rating factor is applied when a cable is wholly surrounded by thermal insulation?

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

Ci is the factor applied when a conductor is surrounded by thermal insulation, which impedes heat dissipation. (BS 7671:2018 Appendix 4 (rating factor Ci))

15. A circuit must carry a design current Ib of 24 A. The applicable factors are Ca = 0.94, Cg = 0.80 and Ci = 1. What is the minimum tabulated current-carrying capacity It required (assuming the protective device rating equals Ib)?

1. 31.9 A
2. 18.0 A
3. 24.0 A
4. 28.6 A

It must be at least In divided by the product of factors: 24 / (0.94 x 0.80 x 1) = 24 / 0.752 = 31.9 A. (BS 7671:2018 Appendix 4 (It = In / (Ca x Cg x Ci)))

16. Where a circuit is protected against overload by a semi-enclosed (rewireable) fuse to BS 3036, what rating factor must be applied to the tabulated current-carrying capacity?

1. 0.725
2. 0.9
3. 0.8
4. 1.45

A factor of 0.725 (derived from 1.45 / 2) is applied because the fusing factor of a BS 3036 fuse is higher than that of cartridge fuses. (BS 7671:2018 Appendix 4 (semi-enclosed fuse rating factor 0.725))

17. From which calculation is the BS 3036 semi-enclosed fuse rating factor of 0.725 derived?

1. 1.45 divided by 2
2. 0.94 multiplied by 0.77
3. 1 minus 0.275
4. 2 divided by 1.45

The factor 0.725 is obtained from 1.45 / 2, reflecting the higher fusing factor of a rewireable fuse compared with a cartridge fuse. (BS 7671:2018 Appendix 4; IET Wiring Matters 'Semi-enclosed fuses')

18. For a cable buried direct in the ground or installed in a duct in the ground, what value of the rating factor Cc is applied to the buried tabulated values?

1. 0.9
2. 0.725
3. 1.0
4. 0.8

A rating factor Cc of 0.9 is applied to the buried tabulated current-carrying capacity values for cables in ducts or buried direct. (BS 7671:2018 Appendix 4 (buried-cable factor Cc = 0.9))

19. What reference ambient temperatures are the Appendix 4 current-carrying capacities based on?

1. 30 degrees Celsius for cables in air and 20 degrees Celsius for cables buried in the ground
2. 20 degrees Celsius in air and 30 degrees Celsius in the ground
3. 25 degrees Celsius in air and 15 degrees Celsius in the ground
4. 40 degrees Celsius in air and 25 degrees Celsius in the ground

The tables assume a reference ambient temperature of 30 degrees Celsius in air and a ground temperature of 20 degrees Celsius for buried cables. (BS 7671:2018 Appendix 4, Section 2 (reference conditions))

20. What is the maximum permitted conductor operating temperature on which the ratings for thermoplastic (PVC) insulated cables in Appendix 4 are based?

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

Thermoplastic (PVC) cable ratings are based on a maximum conductor operating temperature of 70 degrees Celsius, whereas thermosetting cables use 90 degrees Celsius. (BS 7671:2018 Appendix 4 (4D series thermoplastic 70 C))

21. In Appendix 4, Table 4A2 assigns each recognised installation method to a lettered Reference Method. Which letters are used?

1. A, B, C, D, E, F and G
2. X, Y and Z
3. 1, 2 and 3
4. P, Q, R and S

Table 4A2 assigns installation methods to Reference Methods lettered A to G, which determine the applicable current-rating table. (BS 7671:2018 Appendix 4, Table 4A2)

22. Appendix 3 of BS 7671 presents the time/current characteristics of protective devices as graphs. What two quantities are plotted on these graphs?

1. Time in seconds against prospective fault current in amperes
2. Voltage in volts against resistance in ohms
3. Power in watts against current in amperes
4. Temperature in degrees Celsius against time in seconds

The Appendix 3 graphs plot disconnection time (in seconds) on the vertical axis against prospective fault current (in amperes) on the horizontal axis. (BS 7671:2018+A2 Appendix 3)

23. To find the fault current required to disconnect a device within a given time using an Appendix 3 time/current graph, what is the correct method?

1. Draw a horizontal line at the required disconnection time to the device curve, then drop a vertical line to read the current
2. Draw a vertical line at rated current up to the curve, then read the time
3. Multiply the rated current by the power factor and read the result
4. Add the prospective fault current to the load current and locate the sum on the curve

A horizontal line is drawn at the required disconnection time until it intersects the device curve, then a vertical line is dropped to read the corresponding current on the horizontal axis. (BS 7671 Appendix 3 (method of reading time/current curves))

24. In the 18th Edition, the content formerly contained in Appendix 14 (relating to earth fault loop impedance and converting tabulated Zs values) was incorporated into which appendix?

1. Appendix 3
2. Appendix 4
3. Appendix 6
4. Appendix 12

From the 18th Edition, the former Appendix 14 content was merged into Appendix 3, which now also gives maximum earth fault loop impedance values associated with the device curves. (BS 7671:2018 Appendix 3; IET Wiring Matters (changes in the 18th Edition))

25. Within what multiple of rated current (In) does the instantaneous magnetic tripping of a Type B circuit breaker to BS EN 60898 occur?

1. Between 3 and 5 times In
2. Between 5 and 10 times In
3. Between 10 and 20 times In
4. Between 1 and 2 times In

A Type B circuit breaker trips instantaneously at between 3 and 5 times rated current, with the upper limit of 5 In used to derive the maximum Zs. (BS EN 60898-1; BS 7671 Appendix 3 (Type B characteristic))

26. Within what multiple of rated current (In) does the instantaneous magnetic tripping of a Type C circuit breaker to BS EN 60898 occur?

1. Between 5 and 10 times In
2. Between 3 and 5 times In
3. Between 10 and 20 times In
4. Between 2 and 4 times In

A Type C circuit breaker trips instantaneously at between 5 and 10 times rated current, with 10 In used for the maximum Zs derivation. (BS EN 60898-1; BS 7671 Appendix 3 (Type C characteristic))

27. Within what multiple of rated current (In) does the instantaneous magnetic tripping of a Type D circuit breaker to BS EN 60898 occur?

1. Between 10 and 20 times In
2. Between 5 and 10 times In
3. Between 3 and 5 times In
4. Between 20 and 40 times In

A Type D circuit breaker trips instantaneously at between 10 and 20 times rated current, with 20 In used for the maximum Zs derivation. (BS EN 60898-1; BS 7671 Appendix 3 (Type D characteristic))

28. A 32 A Type B circuit breaker protects a circuit. Using the upper limit of the magnetic tripping range to determine the fault current that guarantees instantaneous disconnection, what current value should be used?

1. 160 A
2. 96 A
3. 320 A
4. 640 A

For a Type B device the upper limit is 5 In, so 5 multiplied by 32 A gives 160 A, the current that ensures disconnection within 0.1 to 5 s. (BS EN 60898-1; BS 7671 Appendix 3 (Type B, 5 In))

29. A 20 A Type C circuit breaker is used. Using the upper limit of its magnetic tripping range to derive the minimum fault current for instantaneous operation, what current is required?

1. 200 A
2. 100 A
3. 60 A
4. 400 A

For a Type C device, 10 In is used, so 10 multiplied by 20 A gives 200 A as the current ensuring instantaneous disconnection. (BS EN 60898-1; BS 7671 Appendix 3 (Type C, 10 In))

30. Appendix 3 advises that maximum earth fault loop impedance values vary between device types and manufacturers. What does it recommend the designer use wherever possible?

1. The protective device manufacturer's specific data
2. The generic values from Chapter 41 only
3. The mean of all manufacturers' values
4. An assumed worst-case value of 0.1 ohms

Appendix 3 advises designers to use the protective device manufacturer's specific data rather than the generic tabulated values wherever possible. (BS 7671:2018 Appendix 3 (introductory notes))

31. The maximum earth fault loop impedance (Zs) values tabulated in Chapter 41 (Tables 41.2 to 41.4) are derived directly from which source?

1. The time/current graphs in Appendix 3
2. The voltage drop columns of Appendix 4
3. The installation methods in Table 4A2
4. The diversity figures in Appendix 1

The maximum Zs values in Chapter 41 are derived directly from the Appendix 3 time/current graphs at the relevant disconnection times. (BS 7671:2018 Chapter 41 (Tables 41.2-41.4) and Appendix 3)

32. For a final circuit not exceeding 63 A in a TN system, what maximum disconnection time is used when deriving the tabulated maximum Zs values?

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

For final circuits up to 63 A in TN systems, the maximum disconnection time used is 0.4 s; distribution circuits use 5 s. (BS 7671:2018 Chapter 41 (Tables 41.2-41.4) and Appendix 3)

33. What maximum disconnection time is used when deriving the tabulated maximum Zs values for distribution circuits in a TN system?

1. 5 s
2. 0.4 s
3. 0.2 s
4. 10 s

Distribution circuits use a 5 s maximum disconnection time, whereas final circuits up to 63 A use 0.4 s. (BS 7671:2018 Chapter 41 (Tables 41.2-41.4) and Appendix 3)

34. In what units does Appendix 4 of BS 7671 express its voltage drop data alongside the current ratings?

1. Millivolts per ampere per metre (mV/A/m)
2. Volts per kilometre (V/km)
3. Ohms per metre (ohm/m)
4. Watts per metre (W/m)

Appendix 4 provides voltage drop data in millivolts per ampere per metre (mV/A/m) for use in voltage drop calculations. (BS 7671:2018 Appendix 4 (voltage drop columns))

35. For a lighting circuit supplied at low voltage from a public distribution network, what is the maximum permitted voltage drop as a percentage of the nominal voltage?

1. 3%
2. 5%
3. 2.5%
4. 6%

BS 7671 limits voltage drop to 3% of nominal voltage for lighting circuits and 5% for other circuits. (BS 7671:2018 Appendix 4 / Appendix 12 / Regulation 525)