🛡️ Protection Against Overcurrent and Discrimination (Part 4, Chapter 43)

1. In the first condition of Regulation 433.1.1 for coordination between a conductor and its overload protective device, what does the symbol Ib represent?

1. The design current of the circuit
2. The rated current or setting of the protective device
3. The lowest current-carrying capacity of the conductors
4. The current ensuring effective operation of the device

In the coordination conditions, Ib is the design (load) current of the circuit. It must be the smallest value in the relationship Ib ≤ In ≤ Iz. (BS 7671, Regulation 433.1.1; Part 2 definitions)

2. What does the symbol In stand for in the overload coordination condition Ib ≤ In ≤ Iz?

1. The design current of the circuit
2. The rated current or current setting of the protective device
3. The prospective fault current at the origin
4. The maximum permitted operating current of the conductor

In is the rated current, or the current setting, of the protective device, sitting between the design current Ib and the conductor capacity Iz. (BS 7671, Regulation 433.1.1; Part 2 definitions)

3. In Regulation 433.1.1, what does Iz represent?

1. The lowest continuous current-carrying capacity of the conductors of the circuit
2. The current ensuring effective operation of the protective device
3. The design current of the circuit
4. The rated breaking capacity of the protective device

Iz is the lowest continuous current-carrying capacity of any conductor in the circuit, after applying relevant correction factors. (BS 7671, Regulation 433.1.1; Part 2 definitions)

4. Which inequality correctly states the first condition of Regulation 433.1.1 for overload protection coordination?

1. Ib ≤ In ≤ Iz
2. Iz ≤ In ≤ Ib
3. In ≤ Ib ≤ Iz
4. Ib ≤ Iz ≤ In

The first condition requires that the design current does not exceed the device rating, which in turn does not exceed the conductor's current-carrying capacity: Ib ≤ In ≤ Iz. (BS 7671, Regulation 433.1.1)

5. What is the second condition of Regulation 433.1.1 for coordination of overload protection?

1. I2 ≤ 1.45 × Iz
2. I2 ≤ 1.45 × In
3. I2 ≥ 1.45 × Iz
4. Iz ≤ 1.45 × I2

The second condition requires that the current ensuring effective operation of the device (I2) does not exceed 1.45 times the conductor's current-carrying capacity Iz. (BS 7671, Regulation 433.1.1)

6. In the second condition of Regulation 433.1.1, what does the symbol I2 represent?

1. The current ensuring effective operation of the protective device
2. The design current of the circuit
3. The rated current of the protective device
4. The conductor's continuous current-carrying capacity

I2 is the current ensuring effective operation of the protective device (its conventional operating/tripping current). (BS 7671, Regulation 433.1.1; Part 2 definitions)

7. What is the purpose of the factor 1.45 applied to Iz in Regulation 433.1.1?

1. It limits the conductor operating current I2, giving a margin that prevents conductors overheating beyond their thermal limits during sustained overload
2. It is the standard correction factor for ambient temperature above 30 degrees Celsius
3. It increases the permitted design current of the circuit
4. It is the ratio between the device rated current and its breaking capacity

The 1.45 multiplier bounds the conductor operating current I2 relative to Iz, providing a thermal safety margin so sustained overload does not damage the conductor. (BS 7671, Regulation 433.1.1)

8. A circuit has a conductor with Iz of 40 A. What is the maximum value of I2 permitted by the second condition of Regulation 433.1.1?

1. 58 A
2. 40 A
3. 27.6 A
4. 46 A

I2 must not exceed 1.45 × Iz, so 1.45 × 40 A = 58 A is the maximum permitted operating current. (BS 7671, Regulation 433.1.1)

9. A circuit has a design current Ib of 26 A and a conductor with Iz of 32 A. Which standard circuit-breaker rating satisfies the first condition of Regulation 433.1.1?

1. A 32 A device
2. A 25 A device
3. A 40 A device
4. A 20 A device

In must satisfy Ib ≤ In ≤ Iz, i.e. 26 A ≤ In ≤ 32 A; a 32 A device meets this, whereas 25 A is below Ib and 40 A exceeds Iz. (BS 7671, Regulation 433.1.1)

10. A circuit has a design current Ib of 22 A. A 25 A protective device is proposed. What is the minimum conductor current-carrying capacity Iz needed to satisfy the first condition of Regulation 433.1.1?

1. 25 A
2. 22 A
3. 20 A
4. 32 A

The first condition requires In ≤ Iz, so with In = 25 A the conductor must have Iz of at least 25 A. (BS 7671, Regulation 433.1.1)

11. For a modern overcurrent device complying with a relevant product standard such as BS EN 60898, the manufacturer's I2 is typically 1.45 × In. Given this, why does satisfying In ≤ Iz also satisfy the condition I2 ≤ 1.45 × Iz?

1. Because if In ≤ Iz then 1.45 × In ≤ 1.45 × Iz, so I2 (= 1.45 × In) is automatically within 1.45 × Iz
2. Because I2 is independent of In for such devices
3. Because Iz is always defined as 1.45 × In
4. Because the 1.45 factor cancels with the device rated current

When I2 equals 1.45 × In, multiplying In ≤ Iz by 1.45 gives 1.45 × In ≤ 1.45 × Iz, so the second condition is met automatically once the first condition holds. (BS 7671, Regulation 433.1.1)

12. A protective device has a quoted I2 (conventional operating current) of 63 A. To satisfy the second condition of Regulation 433.1.1, what is the minimum conductor current-carrying capacity Iz required?

1. At least 43.4 A
2. At least 63 A
3. At least 91.4 A
4. At least 31.5 A

The condition is I2 ≤ 1.45 × Iz, so Iz ≥ I2 / 1.45 = 63 / 1.45 = 43.4 A (approximately). (BS 7671, Regulation 433.1.1)

13. A designer selects a protective device whose rated current In is greater than the conductor's current-carrying capacity Iz. According to Regulation 433.1.1, what is the consequence?

1. The first coordination condition is not satisfied and the conductor is not properly protected against overload
2. It is acceptable provided Ib is small
3. It is acceptable because the 1.45 factor provides enough margin
4. It only matters if the circuit is a ring final circuit

The first condition requires In ≤ Iz; if In exceeds Iz the conductor could carry a sustained overload above its capacity without the device operating, so overload protection is inadequate. (BS 7671, Regulation 433.1.1)

14. A lighting circuit has a design current Ib of 8 A and is wired in cable with Iz of 17.5 A. Which device rating gives valid overload coordination while being closest to the load?

1. A 10 A device
2. A 6 A device
3. A 20 A device
4. A 25 A device

In must satisfy 8 A ≤ In ≤ 17.5 A; a 10 A device meets this, while 6 A is below Ib and 20 A or 25 A exceed Iz. (BS 7671, Regulation 433.1.1)

15. Where an overload protective device's stated I2 is greater than 1.45 × In (for example certain fuses or older devices), what additional check must the designer carry out under Regulation 433.1.1?

1. Verify directly that the device's I2 does not exceed 1.45 × Iz, because the In ≤ Iz check alone is not sufficient
2. No further check is needed because In ≤ Iz always covers I2
3. Apply a 0.725 factor to Ib instead
4. Increase the device breaking capacity to match I2

If I2 exceeds 1.45 × In, satisfying In ≤ Iz does not guarantee I2 ≤ 1.45 × Iz, so the second condition must be verified explicitly against Iz. (BS 7671, Regulation 433.1.1)

16. Which two conditions together form the requirements of Regulation 433.1.1 for coordination of a conductor and its overload protective device?

1. Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz
2. Ib ≤ In ≤ Iz and I2 ≥ 1.45 × Iz
3. In ≤ Ib ≤ Iz and Iz ≤ 1.45 × I2
4. Ib ≤ Iz ≤ In and I2 ≤ 0.725 × Iz

Regulation 433.1.1 sets two conditions: Ib ≤ In ≤ Iz, and I2 ≤ 1.45 × Iz; both must be satisfied for proper overload coordination. (BS 7671, Regulation 433.1.1)

17. Regulation 433.1.204 deems a ring final circuit using BS 1363 socket-outlets to comply with overload coordination. Which conductor and protection arrangement is specified?

1. A 30 A or 32 A protective device with 2.5 mm² copper conductors (1.5 mm² for mineral-insulated cable) having an Iz of not less than 20 A
2. A 40 A protective device with 4 mm² copper conductors having an Iz of not less than 30 A
3. A 20 A protective device with 1.5 mm² copper conductors having an Iz of not less than 15 A
4. A 32 A protective device with 6 mm² copper conductors having an Iz of not less than 40 A

Regulation 433.1.204 deems compliance for a ring final circuit protected at 30 A or 32 A using 2.5 mm² copper (1.5 mm² mineral-insulated) conductors with Iz of at least 20 A. (BS 7671, Regulation 433.1.204)

18. Under which broad circumstance does Regulation 433.3 permit overload protection to be omitted?

1. Where the circuit design makes it unlikely that the load current will exceed the conductor's current-carrying capacity (Iz) for long periods
2. Whenever the circuit supplies socket-outlets
3. Whenever the cable cross-sectional area is at least 6 mm²
4. Whenever a residual current device is fitted

Regulation 433.3 permits omission where the circuit is unlikely to carry overload for long periods, so the conductor will not be overloaded beyond Iz. (BS 7671, Regulations 433.3 / 433.3.1)

19. Where overload protection is omitted in accordance with Regulation 433.3 because overload is not expected, which type of overcurrent protection must still generally be provided for the circuit?

1. Fault (short-circuit) protection
2. No overcurrent protection at all is required
3. Only residual current protection
4. Only overload protection at the load end

Omission under 433.3 concerns overload protection only; protection against fault (short-circuit) current must still be provided where required by the Regulations. (BS 7671, Regulation 433.3 and Section 434)

20. According to Section 432, how may protection against overcurrent be provided?

1. By a single device protecting against both overload and fault current, or by separate devices for overload and for fault protection
2. Only by a single device that handles both overload and fault current
3. Only by separate devices, one for overload and one for fault
4. Only by a residual current device combined with a fuse

Section 432 allows either a single device covering both overload and fault current, or separate devices each dedicated to overload or to fault protection. (BS 7671, Section 432 (Regulations 432.1, 432.2, 432.3))

21. Under Regulation 432.2, a device providing protection against overload current only is installed. What is permitted regarding its short-circuit breaking capacity?

1. Its breaking capacity may be below the prospective fault current at its point of installation, provided it is coordinated with the conductor in accordance with Section 433
2. Its breaking capacity must always equal or exceed the prospective fault current at its point of installation
3. It must have the highest breaking capacity available in the range
4. It need not be coordinated with the conductor at all

Regulation 432.2 allows an overload-only device to have a breaking capacity below the prospective fault current, on condition that separate fault protection exists and it is coordinated with the conductor per Section 433. (BS 7671, Regulation 432.2)

22. A single device is intended to protect against both overload and fault current. Under Regulation 432.1, what must this device be capable of doing?

1. Breaking (and for a circuit-breaker, making) any overcurrent up to and including the maximum prospective fault current at its point of installation, except where the back-up provision of Regulation 434.5.1 applies
2. Breaking only currents up to its rated current In
3. Breaking overload currents but not fault currents
4. Operating only when a residual current is present

Under Regulation 432.1 a combined device must be able to break (and make, if a circuit-breaker) any overcurrent up to the maximum prospective fault current at its location, unless back-up protection per 434.5.1 is provided. (BS 7671, Regulation 432.1; Regulation 434.5.1)

23. The provisions for omitting overload protection in defined situations are found in which group of regulations within BS 7671?

1. Regulation group 433.3
2. Regulation 434.5.2
3. Regulation 433.1.1
4. Section 536

Regulation group 433.3 (including 433.3.1) sets out the situations in which overload protection may be omitted. (BS 7671, Regulations 433.3 / 433.3.1)

24. According to Regulation 434.5.1, the rated short-circuit breaking capacity of a protective device shall normally be not less than which value at the point where the device is installed?

1. The design current of the circuit
2. The maximum prospective fault current at that point
3. The rated current setting of the device
4. The current-carrying capacity of the conductor

Regulation 434.5.1 requires the device's breaking capacity to be not less than the maximum prospective fault current at the point of installation, so it can safely interrupt the worst-case fault. (BS 7671, Regulation 434.5.1)

25. Which Chapter of BS 7671 (18th Edition) covers protection against overcurrent, encompassing both overload and fault current?

1. Chapter 41
2. Chapter 42
3. Chapter 43
4. Chapter 44

Chapter 43 in Part 4 of BS 7671 deals with protection against overcurrent, where overcurrent comprises overload current and fault (short-circuit or earth fault) current. (BS 7671:2018+A2:2022, Chapter 43, Part 4)

26. Regulation 434.5.1 permits the use of a device with a breaking capacity lower than the prospective fault current at its point of installation. What is the essential condition for this to be acceptable?

1. A residual current device is fitted on the load side
2. Another device with the necessary breaking capacity is installed on the supply side and the characteristics are coordinated
3. The circuit is protected by an additional earth electrode
4. The device is rated for a higher ambient temperature

This is back-up protection: a supply-side device with adequate breaking capacity must limit the energy let-through (I²t) so the load-side device and conductors are not damaged. (BS 7671, Regulation 434.5.1)

27. In the back-up protection arrangement permitted by Regulation 434.5.1, what quantity must be coordinated so the load-side device and conductors are not damaged?

1. The energy let-through (I²t) of the supply-side device
2. The rated voltage of the load-side device
3. The earth fault loop impedance
4. The ambient temperature of the enclosure

The let-through energy (I²t) of the upstream device must not exceed the value the downstream device and conductors can withstand without damage. (BS 7671, Regulation 434.5.1)

28. The adiabatic equation in Regulation 434.5.2 is used to verify what?

1. That the conductor can carry the design current without overheating
2. That the conductor will withstand the thermal effects of short-circuit current
3. That the earth fault loop impedance is low enough for disconnection
4. That the voltage drop along the conductor is within limits

Regulation 434.5.2 gives t = k²S²/I², used to confirm a conductor can withstand the thermal stress of a short-circuit during the disconnection time. (BS 7671, Regulation 434.5.2)

29. In the adiabatic equation of Regulation 434.5.2, t = k²S²/I², what does the symbol 't' represent?

1. The conductor temperature in degrees Celsius
2. The disconnection time in seconds
3. The time constant of the circuit
4. The thickness of the insulation in mm

In the adiabatic equation, t is the disconnection time in seconds, S the cross-sectional area in mm², I the short-circuit current in amperes, and k a material/insulation factor. (BS 7671, Regulation 434.5.2)

30. In the adiabatic equation t = k²S²/I² (Regulation 434.5.2), the factor 'k' depends primarily on which characteristics?

1. The conductor material and its insulation
2. The ambient temperature and installation method
3. The rated current of the protective device
4. The system voltage and frequency

The k factor depends on the conductor material and the type of insulation, with values given in BS 7671 tables such as Table 43.1. (BS 7671, Regulation 434.5.2 and Table 43.1)

31. The adiabatic check is often expressed in the form k²S² ≥ I²t. What does satisfying this inequality demonstrate?

1. The protective device has adequate breaking capacity
2. The conductor can withstand the let-through energy of the fault without exceeding its thermal limit
3. The circuit complies with the overload coordination condition Ib ≤ In ≤ Iz
4. The disconnection time meets the requirements for shock protection

Rearranging the adiabatic equation, k²S² ≥ I²t confirms the conductor's permitted thermal withstand (k²S²) is at least the let-through energy (I²t) of the fault. (BS 7671, Regulation 434.5.2)

32. Section 432 of BS 7671 permits overcurrent protection to be arranged in which of the following ways?

1. Only by a single device protecting against both overload and fault current
2. Only by separate devices for overload and fault current
3. By a single combined device, or by separate devices for overload and for fault current
4. Only by a residual current device covering both conditions

Section 432 allows either one device protecting against both overload and fault current, or separate devices each dedicated to overload or to fault protection. (BS 7671, Section 432 (Regulations 432.1, 432.2, 432.3))

33. Under Regulation 432.1, a device protecting against both overload and fault current must be capable of breaking any overcurrent up to and including which value at its point of installation?

1. The rated current of the device
2. 1.45 times the conductor current-carrying capacity
3. The maximum prospective fault current (except as permitted by the back-up provision)
4. Twice the design current of the circuit

Regulation 432.1 requires the combined device to break overcurrent up to the maximum prospective fault current at its point of installation, except where the back-up provision of 434.5.1 applies. (BS 7671, Regulation 432.1)

34. Under Regulation 432.2, a device providing protection against overload current only may have a rated short-circuit breaking capacity below the prospective fault current at its point of installation. What condition must be satisfied?

1. It must be coordinated with the conductor in accordance with Section 433
2. It must be a Type B circuit-breaker
3. It must be supplemented by an RCD rated at 30 mA
4. It must be installed within 0.5 m of the origin

Regulation 432.2 permits an overload-only device to have a lower breaking capacity provided it is coordinated with the conductor in accordance with Section 433. (BS 7671, Regulation 432.2)

35. A consumer unit has a measured prospective fault current at its incomer of 4.5 kA. Which protective device would, on its own breaking capacity alone, comply with Regulation 434.5.1 at that point?

1. A device with a rated breaking capacity of 3 kA
2. A device with a rated breaking capacity of 4 kA
3. A device with a rated breaking capacity of 6 kA
4. A device with a rated breaking capacity of 4 kA increased by tolerance

The breaking capacity must be not less than the maximum prospective fault current (4.5 kA), so a 6 kA device complies whereas 3 kA and 4 kA do not. (BS 7671, Regulation 434.5.1)