Chapter 41 is the single most-tested chapter in the 18th Edition exam — Automatic Disconnection of Supply (ADS), maximum Zs values, RCD requirements, and the protective measures hierarchy. This guide is for electricians studying for the 18th Edition exam — Chapter 41 deep-dive.
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The short answer
Protection against electric shock: automatic disconnection of supply, maximum disconnection times (0.4 s TN and 0.2 s TT for final circuits up to 63 A with sockets), the maximum Zs Tables 41.2 to 41.5, RCD additional protection under Regulations 411.3.3 and 411.3.4, and the other protective measures such as double insulation, electrical separation and SELV.
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01 · Exam Prep
Chapter 41 establishes a protective hierarchy: (1) Basic protection — preventing contact with live parts (insulation, barriers). (2) Fault protection via ADS — disconnecting fault current within the required time. (3) Additional protection — 30mA RCD as a final safety net. Most exam questions test the ADS calculations. Memorise: Ohm’s Law applied to the fault circuit: I_fault = U_o / Z_fault, where Z_fault = R_phase + R_neutral + R_cpc (or Z_e + R_1 + R_2 in measurement terms).
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02 · Exam Prep
Table 41.1 (Regulation 411.3.2.2) applies to final circuits rated up to 63 A with one or more socket-outlets, and up to 32 A supplying only fixed equipment: 0.4 s in a 230 V TN system and 0.2 s in a 230 V TT system. Distribution circuits and larger final circuits are allowed 5 s in TN systems (Regulation 411.3.2.3) and 1 s in TT systems (Regulation 411.3.2.4). Where an RCD provides fault protection, the Zs limits in Table 41.5 can be used (Regulation 411.4.204).
03 · Exam Prep
Table 41.2: fuses (BS 88-2, BS 88-3, BS 3036, BS 1362) for 0.4 s. Table 41.3: circuit-breakers to BS EN 60898 and the overcurrent part of RCBOs, Types B, C and D. Table 41.4: fuses for 5 s. Table 41.5: maximum Zs where an RCD provides fault protection, for example 1,667 Ω for a 30 mA RCD (50 V ÷ IΔn). Learn the Type B values: 6 A 7.28 Ω, 16 A 2.73 Ω, 32 A 1.37 Ω.
04 · Exam Prep
Additional protection by a 30 mA RCD (Regulation 415.1.1) is required for socket-outlets up to 32 A (Regulation 411.3.3, with a documented risk-assessment exception outside dwellings), mobile equipment up to 32 A used outdoors, AC final circuits supplying luminaires in dwellings (Regulation 411.3.4), and in many Part 7 special locations. Separately, Regulation 421.1.7 in Chapter 42 has required AFDDs on single-phase socket-outlet circuits up to 32 A in HMOs, care homes, purpose-built student accommodation and high rise residential buildings since A2:2022.
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05 · Exam Prep
Chapter 41 protection requirements extend through the Section 700-series (Section 701 bathrooms, 702 swimming pools, 705 agricultural, 706 caravan parks, 710 medical, 717 mobile units, 722 EV charging, 740 amusement parks, 753 floor and ceiling heating). The exam regularly asks "what additional protection is required in Section 70X?" — typically supplementary bonding + 30mA RCD + reduced Zs.
06 · Exam Prep
"A 32 A Type B circuit-breaker protects a radial with a measured Zs of 1.2 Ω at room temperature. Does it comply?" Table 41.3 gives 1.37 Ω at operating temperature; a cold reading is compared with 80% of that, 1.10 Ω, so 1.2 Ω fails the check. "What is the maximum disconnection time for a 32 A socket circuit on a 230 V TT system?" 0.2 s. "Which socket-outlets need 30 mA additional protection?" Those rated up to 32 A, under Regulation 411.3.3.
07 · Exam Prep
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