Chapter 43 covers overcurrent protection — protective device selection, discrimination, cable adiabatic. The second-most-tested chapter after 41. This guide is for electricians studying for the 18th Edition exam — Chapter 43 deep-dive.
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The fundamental rule: I_n ≤ I_b ≤ I_z, where I_n is the device rating, I_b is the design current, I_z is the cable’s current-carrying capacity at the installation method, ambient, grouping etc. Plus: I_2 ≤ 1.45 × I_z (the device must operate within the cable’s short-time overcurrent capability). Type B for resistive loads (lighting, sockets), Type C for inductive loads (motors, fluorescent), Type D for very high inrush (transformers, large motors).
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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 short-circuit protection of cable: S ≥ √(I²t) / k, where I is the short-circuit current, t is the device clearing time, k is the cable insulation constant (115 for thermoplastic copper, 76 for thermoplastic aluminium). The exam will give you I_pf and ask "what is the minimum cpc cross-section under fault conditions?" Compute S, compare against the actual cable cpc, confirm compliance. Practice 20+ of these.
BS 88-3 fuses (still common on industrial supplies) — non-resettable, high breaking capacity. BS EN 60898 MCBs (most domestic / commercial) — resettable, lower breaking capacity. BS EN 61009 RCBOs — MCB + 30mA RCD combined. Exam questions often test which protective device is appropriate for a given application (e.g. "what device protects a 32A radial in a domestic kitchen?" — typical answer 32A RCBO Type A or B).
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Most Chapter 43 content is unchanged. A4:2026 clarifications: improved guidance on discrimination calculations, integration with Chapter 443 SPD coordination, and updated examples in the Appendices. The exam tests the same core principles as A3:2024.
"A 32A circuit has measured I_pf of 1.5kA. With a Type B 32A MCB (clearing time 0.1s at fault current), what is the minimum cpc cross-section?" Calculate S = √(1500² × 0.1) / 115 = 4.13mm² — so minimum 4.0mm² cpc. "Type B vs Type C for a 3kW motor circuit — which is correct?" (Type C — motor inrush 5-7× In.) Practice these systematically.
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