BS 7671 Run-Through: Part by Part Study Guide

BS 7671 is the national standard for electrical installations in the UK. Published by the IET (Institution of Engineering and Technology) and the BSI (British Standards Institution), it sets out the rules for the design, erection, and verification of electrical installations operating at voltages up to and including 1,000V AC.

The current edition is the 18th Edition: BS 7671:2018+A4:2026. The "+A4:2026" indicates that Amendment 4 has been incorporated. Every electrician working in the UK must understand BS 7671 — it is tested in the C&G 2382 exam, it governs every installation you design and build, and it is the standard against which your work is inspected and certified.

This run-through study guide walks you through every part of BS 7671, highlighting the key regulations, exam focus areas, and practical applications. It is designed as a companion to your study — read it alongside your copy of the brown book.

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Part 1 establishes what BS 7671 covers (and what it does not), its objectives, and the fundamental principles that underpin every regulation in the book.

Scope — BS 7671 applies to the design, erection, and verification of electrical installations operating at nominal voltages up to and including 1,000V AC or 1,500V DC. It covers fixed installations in buildings, temporary installations (construction sites, exhibitions), and certain types of external installations. It does not cover systems for distribution of electricity to the public (DNO networks), lightning protection systems (covered by BS EN 62305), electrical equipment on board ships, offshore installations, or mines and quarries (which have their own regulations).

Fundamental principles — the seven fundamental principles are: protection against electric shock, protection against thermal effects, protection against overcurrent, protection against fault current, protection against voltage disturbances, isolation and switching, and application of protective measures for safety. These principles flow through every chapter of the book and are the foundation for all the detailed regulations that follow.

Exam tip — Part 1 questions typically test whether you know the scope of BS 7671. Common questions include: "Does BS 7671 apply to [specific situation]?" and "Which fundamental principle relates to [specific protection]?" Know the scope boundaries and the seven principles.

Part 2 contains the definitions of all technical terms used in BS 7671. These definitions have specific, precise meanings — do not assume you know what a term means from everyday usage.

Key definitions to know for the exam include: basic protection (protection against electric shock under fault-free conditions), fault protection (protection against electric shock under single fault conditions), automatic disconnection of supply (ADS), protective conductor, circuit protective conductor (cpc), exposed-conductive-part, extraneous-conductive-part, and the difference between isolation and switching.

Exam tip — definition questions appear regularly. The examiners test whether you understand the precise BS 7671 definition, not a general understanding. For example, knowing the exact difference between an "exposed-conductive-part" (a conductive part of equipment which can be touched and which is not normally live but which can become live under fault conditions) and an "extraneous-conductive-part" (a conductive part not forming part of the electrical installation but liable to introduce a potential) is critical.

Spend time reading Part 2 carefully. Mark definitions you find confusing and use the Elec-Mate 18th Edition course flashcards to practise them until they are second nature.

Part 3 covers the information you need to gather before designing an electrical installation. This is the assessment stage — understanding the supply characteristics, the building use, and the external influences that affect your design decisions.

Chapter 31 — Supply characteristics. You must determine the nature of the supply (single-phase, three-phase), the supply voltage and frequency, the prospective fault current at the origin, the type of earthing arrangement (TN-S, TN-C-S, TT, IT), and the external earth fault loop impedance (Ze).

Chapter 32 — Classification of external influences. External influences are coded using a three-letter system: the first letter indicates the category (A = environment, B = utilisation, C = building construction), the second letter indicates the nature of the influence, and the number indicates the class. For example, AD4 indicates "presence of water — splashes." These codes determine the minimum IP rating for equipment installed in that environment.

Chapter 33 — Compatibility. The installation must be compatible with other installations and equipment connected to the same supply. This includes consideration of harmonics, voltage fluctuations, and electromagnetic compatibility.

Exam tip — expect questions on earthing arrangements (TN-S, TN-C-S, TT) and their characteristics, external influence codes, and the information you need to obtain before starting a design. Tab the external influences table for quick reference.

Part 4 is the most heavily examined part of BS 7671. It contains the regulations for protecting people and property from the hazards created by electrical installations. Focus your study time here.

Chapter 41 — Protection against electric shock. This is the most important chapter for the exam. Key topics include: automatic disconnection of supply (ADS) as the primary protective measure, maximum disconnection times (Table 41.1), earth fault loop impedance requirements (Appendix 3 tables), additional protection by RCDs (30mA for socket outlets not exceeding 32A), and the requirements for different earthing systems.

Chapter 43 — Protection against overcurrent. Understanding the relationship between cable current-carrying capacity (Iz), nominal protective device rating (In), design current (Ib), and tabulated current-carrying capacity (It) is essential. The fundamental rule is: Ib ≤ In ≤ Iz.

Chapter 44 — Protection against voltage disturbances. This covers overvoltage protection and undervoltage protection. The Amendment 4 addition on bidirectional devices (Section 530, A4:2026 addition) is relevant here.

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Part 5 covers the practical rules for selecting and installing equipment — cables, switchgear, accessories, and other components. This part translates the protection requirements of Part 4 into specific installation rules.

Chapter 52 — Selection and erection of wiring systems. This is the cable selection chapter. Key topics include: current-carrying capacity (using Appendix 3 tables), correction factors for ambient temperature, grouping, and thermal insulation, voltage drop calculations (maximum 3% for lighting, 5% for other circuits from the origin), and cable installation methods (reference methods).

Chapter 53 — Switchgear. Covers the selection of protective devices, isolation devices, and switching devices. The A4:2026 addition on bidirectional devices sits in this chapter.

Exam tip — cable sizing questions are almost guaranteed. Practise the full calculation process: determine design current, select protective device rating, apply correction factors, look up the minimum cable size from the Appendix 3 tables, and verify voltage drop. Use the cable sizing calculator to check your manual calculations during revision.

Part 6 covers the verification of electrical installations — both initial verification (before a new installation is put into service) and periodic inspection and testing.

Chapter 61 — Initial verification. Every new installation, addition, or alteration must be inspected and tested before being put into service. The inspection verifies that the installation complies with BS 7671, while the testing confirms that the installation is safe. The tests must be carried out in a specific sequence: continuity of protective conductors, continuity of ring final circuit conductors, insulation resistance, polarity, earth fault loop impedance, prospective fault current, and RCD operation.

Chapter 62 — Periodic inspection and testing. Existing installations must be periodically inspected and tested to confirm they remain safe for continued use. The EICR (Electrical Installation Condition Report) documents the findings, using observation codes C1 (Danger Present), C2 (Potentially Dangerous), C3 (Improvement Recommended), and FI (Further Investigation).

Exam tip — know the testing sequence and the minimum acceptable values for each test. Insulation resistance must be at least 1 megohm for circuits up to 500V. RCDs rated at 30mA must trip within 200ms at rated current and 40ms at 5 times rated current. These values appear in exam questions regularly.

Part 7 contains additional or modified requirements for installations in specific locations where the risks are higher than normal. Each section in Part 7 supplements (and sometimes modifies) the general requirements in Parts 1 to 6.

Other sections that appear in exams include Section 702 (Swimming Pools), Section 705 (Agricultural Premises), Section 708 (Caravan Parks), Section 711 (Exhibitions), and Section 753 (Floor and Ceiling Heating). Focus on the sections most relevant to your work, but be prepared for questions on any Part 7 section.

The C&G 2382 exam is 2 hours long with 60 multiple-choice questions. You need 36 correct answers (60%) to pass. The exam is open book — you can bring your copy of BS 7671 but nothing else.

The most common reason for failure is running out of time because of slow book navigation. The second most common reason is not practising enough mock exams. Both are completely avoidable with preparation. Use Elec-Mate mock exams and flashcards consistently in the weeks before your exam.