BS 7671 Chapter 43 — Overcurrent Protection (Exam Prep)

The overload rule in Regulation 433.1.1 is Ib ≤ In ≤ Iz: the device rating In must be at least the design current Ib and no more than the cable’s current-carrying capacity Iz after rating factors, and the current I2 that operates the device must not exceed 1.45 Iz. Circuit-breakers to BS EN 60898 and BS 88 fuses meet the I2 condition automatically; a BS 3036 rewirable fuse needs an extra factor of 0.725. Type B suits resistive and general loads, Type C moderate inrush, and Type D high inrush such as transformers.

Critical when you have downstream devices feeding circuits. The upstream device must NOT operate when a downstream device clears a fault. Discrimination is achieved by: time-current characteristics (slower upstream device), current rating (substantially higher upstream), or specific manufacturer charts. The exam often shows a one-line diagram and asks "is discrimination achieved between the 32A Type B downstream and the 100A Type B upstream?" Look up the time-current curves.

For fault current, the conductor must withstand the energy let through before the device clears: S ≥ √(I²t) ÷ k, where I is the fault current, t the operating time and k a factor for the conductor and insulation (115 for copper with 70 °C thermoplastic insulation; protective conductor k values are in Tables 54.2 to 54.5). For circuit-breakers the manufacturer’s I²t let-through is usually far lower than I² × t using a time read off the curve.

BS 88-2 fuses: industrial and commercial distribution, high breaking capacity. BS 88-3 fuses: the cartridge fuses in older domestic and small commercial boards (formerly BS 1361). BS EN 60898 circuit-breakers: resettable, typically 6 kA or 10 kA. BS EN 61009 RCBOs: circuit-breaker plus RCD in one device. A typical answer for a 32 A kitchen radial in a new board is a 32 A Type B RCBO, Type A for the RCD part.

Chapter 43 is not among the main changes listed in the A4:2026 introduction, so expect the same core principles. The A4 changes you are more likely to be asked about are elsewhere: PNB in Regulation 312.2.1.1, Table 52.1 for cables in walls, Section 545 and Chapter 57.

"A circuit has a prospective fault current of 1.5 kA and the device clears in 0.1 s. What is the minimum protective conductor size, with k = 115?" S = √(1,500² × 0.1) ÷ 115 = 4.12 mm², so 4 mm² is too small and the next standard size, 6 mm², is needed. Using the circuit-breaker’s published I²t let-through instead usually gives a much smaller answer. "Type B or Type C for a motor with a high starting current?" Type C, checking that Zs still meets Table 41.3 for Type C.

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