What Parts 1-2 actually cover

BS 7671 is split into Parts. Part 1 sets the scope - what the regulations apply to. Part 2 gives the definitions used throughout the rest of the book. Get these wrong in the exam and you will misread every later question, because terms like 'circuit', 'skilled person' and 'exposed-conductive-part' all have precise legal meanings, not everyday ones.

Scope (Part 1)

• BS 7671 applies to the design, erection and verification of electrical installations, plus additions and alterations to existing ones.
• Covers installations up to 1000V AC (1500V DC) between conductors, or 600V AC (900V DC) to earth, at standard frequencies.
• Includes fixed wiring, but also things people forget: caravans, marinas, construction sites, agricultural premises, solar PV, and low voltage generating sets.
• Does NOT cover systems for the distribution of electricity to the public (that is the Distribution Network Operator's job), railway traction, equipment aboard ships/aircraft, or mines and quarries specific equipment - these have their own standards.
• Existing installations only need to comply with the edition in force when they were installed - BS 7671 is not retrospective, but any new work must meet the current edition.

Key definitions to know cold

• Circuit: an assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s).
• Skilled person: someone with technical knowledge and experience relevant to the work (an electrician).
• Instructed person: adequately advised or supervised by a skilled person, so they can avoid dangers.
• Ordinary person: neither skilled nor instructed - the general public.
• Exposed-conductive-part: a conductive part of equipment that can be touched and is not live in normal use, but could become live under fault conditions (e.g. a metal appliance casing).
• Extraneous-conductive-part: a conductive part NOT forming part of the electrical installation, liable to introduce a potential, usually earth potential (e.g. metal gas pipes, structural steel).
• Origin of an installation: the point at which electrical energy is delivered - usually the supply intake position/meter.

Fundamental principles (Part 1, Chapter 13)

• Every installation must be designed and constructed to protect against electric shock, thermal effects (fire/burns), overcurrent, fault current, and overvoltage.
• Design must allow for the maximum demand, the number of circuits needed, and future alterations.
• All equipment must be suitable for its environment and correctly identified.

Common mistakes

• Confusing exposed-conductive-part (part OF the installation) with extraneous-conductive-part (NOT part of the installation).
• Assuming BS 7671 covers public distribution networks - it does not.
• Mixing up skilled and instructed persons in questions about who can carry out what task.

Protection for safety — the big picture

Part 4 of BS 7671 covers three separate hazards: electric shock, thermal effects (fire and burns), and overcurrent (overload and fault current). Each has its own chapter and its own set of rules, but they interact — a device sized to clear a fault fast enough also limits the energy that could start a fire.

Shock protection (Chapter 41)

Basic protection stops contact with live parts (insulation, barriers, enclosures to at least IP2X or IPXXB, or IP4X/IPXXD on horizontal top surfaces). Fault protection stops a fault from becoming dangerous, mainly via automatic disconnection of supply (ADS).

• Disconnection times for TN systems: 0.4s for final circuits up to 32A, 5s for distribution circuits.
• For TT systems: 0.2s for final circuits up to 32A, 1s for distribution circuits — much faster because earth fault loop impedance is higher and less predictable.
• These times assume a nominal voltage Uo of 230V. Different Uo values shift the permitted times (see Table 41.1).
• RCDs are usually needed to hit TT disconnection times, since TT earth electrodes rarely give a low enough loop impedance for overcurrent devices alone.
• 30mA RCD protection is required for socket outlets up to 32A rated in general use, for cables buried less than 50mm in a wall or partition (with some exceptions), and for mobile equipment used outdoors — this is additional protection, separate from ADS.
• Common mistake: thinking RCDs replace earthing. They don't — RCDs are additional protection against shock when basic and fault protection have already failed; they're not a substitute for a low-impedance earth path.

Thermal protection (Chapter 42 / 43)

Protection against fire is about keeping fault energy and surface temperatures away from combustible material and skin. Devices, enclosures and cables must be selected so normal operation doesn't create excessive heat, and arc fault detection devices (AFDDs) are increasingly recommended (and sometimes required) in higher-risk locations like HMOs and buildings with sleeping accommodation.

Overcurrent protection (Chapter 43 / 433)

Overcurrent is split into overload (too much current in a healthy circuit) and fault current (short circuit or earth fault). The classic design rule is the three-part inequality: Ib is less than or equal to In is less than or equal to Iz, and I2 is less than or equal to 1.45 times Iz.

• Ib = design current, In = rating of protective device, Iz = current-carrying capacity of the cable, I2 = current causing effective operation of the device.
• Common mistake: forgetting the I2 check — examiners love testing this second condition, not just Ib/In/Iz.
• Fault current protection uses breaking capacity and let-through energy (I squared t) checks against cable withstand (the adiabatic equation), especially for smaller cables where fault energy could damage insulation before the device trips.

Exam tips

Learn the disconnection-time table (41.1) cold — TN vs TT, final vs distribution circuit. Know which situations mandate 30mA RCDs regardless of calculation. And always check both halves of the overload equation, not just the first.

Selection and erection - the big picture

Part 5 of BS 7671 covers how you choose and install equipment so it survives its environment and doesn't become a hazard. Every piece of kit must suit the voltage, current, frequency, power, and the external influences it will face (damp, dust, impact, corrosion, vibration).

External influences (Section 522 and Appendix 5)

• Coded using the AA/BB/CC style from Appendix 5, eg AD4 for splashes of water.
• Cables and equipment in a location must carry an IP rating suited to that influence - IPX4 for splashproof, IPX7 for temporary immersion.
• Common exam trap: mixing up IP first digit (solid objects/dust) and second digit (liquids). First digit 0-6, second digit 0-9 (0-8 for older tables, but 9 now exists for high pressure jet washing).

Cable support and containment

• Cables must be supported so their own weight doesn't strain terminations - use the spacing tables in the On-Site Guide or manufacturer data, not guesswork.
• Non-sheathed cables (singles) must always be enclosed in conduit, trunking, or ducting - never run bare.
• Segregation matters: keep Band I (ELV, eg bell wiring) and Band II (mains) apart, or use earthed metal barriers/separate compartments.
• Zones for cables in walls: within 150 mm of the top of a wall or a corner, or within 150 mm of an accessory, so cables don't need an earthed conduit or 50 mm depth/RCD protection outside those zones (Regulation 522.6.202).

Thermal effects and proximity to other services

• Keep cables clear of heat sources like hot pipes, flues, or insulation that will push conductor temperature past its rating.
• Where cables cross other services (gas, water), maintain separation or provide mechanical protection - shared trenches need barriers.

Isolation and switching (Section 537)

• Every circuit must have a means of isolation, and it must be clearly identified as to what it isolates.
• Devices for isolation must break all live conductors (not usually neutral in TN-C-S unless specifically required) and be lockable off where needed for safety.
• Emergency switching must be readily accessible and act directly, not through a timer or programmable control.

Common mistakes to avoid

• Forgetting that accessories in bathrooms need the right IP rating for their zone (Zone 1 typically IPX4 minimum).
• Assuming any conduit run is fine for segregation - it isn't, unless it meets the separation or barrier requirement.
• Ignoring manufacturer's instructions - BS 7671 explicitly requires equipment to be installed per the manufacturer's data as well as the Regulations.
• Ignoring correction factors for grouping, ambient temperature and thermal insulation when selecting cable size for the equipment being supplied.

Revise the IP code table, the wall cable zones, and the isolation rules first - these come up again and again in exam questions on Part 5.

Why inspection and testing matters

Part 6 of BS 7671 covers initial verification (new work) and periodic inspection (existing installations). Every installation must be inspected and tested before being put into service, and a certificate issued confirming it meets the requirements of BS 7671 as amended.

The three certificate types

• Electrical Installation Certificate (EIC) - for new installations, or major alterations/additions creating a new circuit. Needs a designer, installer and inspector/tester (can be the same person for domestic work).
• Minor Electrical Installation Works Certificate (MEIWC) - for work not extending to a new circuit, eg adding a socket to an existing circuit. No new circuit involved.
• Electrical Installation Condition Report (EICR) - for periodic inspection of an existing installation. Gives an overall assessment as satisfactory or unsatisfactory, and codes any defects.

Sequence: inspection before testing

Always inspect before testing, and with the installation dead where practicable. Inspection checks things testing cannot, such as correct cable selection, connections, and protection against fire spread, before any power is applied.

The dead test sequence (in order)

1. Continuity of protective conductors (including main and supplementary bonding).

2. Continuity of ring final circuit conductors.

3. Insulation resistance.

4. Polarity.

5. Earth electrode resistance (where applicable, eg TT systems).

Live tests come last

Once dead tests pass, live tests follow: earth fault loop impedance (Zs), prospective fault current, and RCD operation (using an RCD tester, checking trip times at rated and 5x rated current). Functional testing of switchgear, RCDs (test button) and interlocks completes the process.

Key figures to remember

• Insulation resistance test voltage: 500V DC for circuits up to 500V (most domestic/commercial work).
• Minimum acceptable insulation resistance: 1 megohm (MΩ) for most circuits.
• SELV/PELV circuits use a 250V DC test and require a minimum of 0.5 MΩ.
• RCD trip time at rated residual current (IΔn): must operate within 300ms for general RCDs (200ms for time-delayed/S-type); at 5x IΔn, within 40ms.

Reporting periods and coding

EICR recommended intervals: domestic 10 years (or change of occupancy), commercial 5 years, rented domestic property 5 years (or change of tenancy), swimming pools/agricultural 1-3 years, construction sites 3 months. Observations are coded C1 (danger present, immediate action), C2 (potentially dangerous, urgent remediation), C3 (improvement recommended), or FI (further investigation required).

Common exam mistakes

• Confusing EIC with MEIWC - remember MEIWC is for work NOT creating a new circuit.
• Forgetting the order of dead tests, especially continuity before insulation resistance.
• Mixing up 500V (LV circuits) and 250V (SELV/PELV) insulation test voltages.
• Coding a genuinely dangerous fault as C2 instead of C1.
• Forgetting that a satisfactory EICR still lists C3 items as recommendations, not failures.

Why Part 7 exists

Ordinary rules assume dry, low-risk surroundings. Part 7 of BS 7671 bolts on extra protection for locations where water, confined space, high body-current risk, or unusual hazards make a standard installation unsafe. Each section (601 to 753 and beyond) covers one location type with its own zone diagram and rules.

Bathrooms (Section 701)

• Zones 0, 1 and 2 are defined around the bath or shower tray, each with its own IP rating and equipment restrictions.
• Zone 0 (inside the bath/tub itself) needs equipment rated at least IPX7, and must be SELV supplied, safety source outside the zones.
• Zone 1 needs at least IPX4 (IPX5 if water jets are likely, eg power showers).
• Zone 2 needs at least IPX4 too.
• All circuits in a bathroom need 30 mA RCD protection, whatever the zone.
• Supplementary bonding between zones can be omitted only if every circuit is RCD protected AND main protective bonding is in place and verified - a common exam trap.
• Socket-outlets must be at least 3 m horizontally from the boundary of zone 1 (many candidates misquote this as 2 m, which was an old rule).

Swimming pools (Section 702)

• Similar zone concept (0, 1, 2) but based on the pool and surrounding wet area, with tighter distances because of prolonged water contact.
• Zone 0 equipment must be SELV 12 V AC / 30 V DC max, IPX8.
• Zone 1 needs SELV or RCD-protected circuits, IPX4 minimum (IPX5 for jets).

Agricultural and horticultural premises (Section 705)

• Livestock produces lower body resistance, so touch voltage limits are stricter (25 V AC / 60 V DC in areas with livestock, versus 50 V/120 V normally).
• RCDs of 30 mA required for socket-outlets and, in areas housing livestock, all final circuits generally.
• Fire risk from dust/vermin means mechanical protection of cables is emphasised.

Construction site installations (Section 704)

• Reduced low voltage systems (110 V centre-tapped-to-earth, giving 55 V to earth) are the standard for hand tools.
• All socket-outlets need 30 mA RCD protection regardless of voltage.

Caravans, motor caravans and caravan parks (Sections 721/708)

• Each pitch supply normally protected by an individual 30 mA RCD and, commonly, 16 A socket-outlet per caravan.

Common mistakes

• Mixing up zone dimensions between bathrooms and pools - they are NOT the same.
• Forgetting that Zone 0 always means SELV.
• Assuming supplementary bonding is always needed in bathrooms - it can be omitted under the stated conditions.
• Quoting the old 2 m socket rule instead of the current 3 m from zone 1.

Why the Appendices matter

BS 7671 Appendices turn the regulations into usable design data. For the exam you need to know which appendix holds what, and be able to apply the tables under time pressure.

Appendix 4 - cable sizing

This is the big one. It gives current-carrying capacity (Iz) tables for different installation methods, reference methods (like Method C clipped direct, Method 100/101 for cables in insulation), and correction factors:

• Ca - ambient temperature correction (higher temp = lower rating)
• Cg - grouping correction (more circuits bunched together = derate)
• Ci - thermal insulation correction (cable in or touching insulation derates hard - a single cable surrounded by insulation for over 0.5m can use Ci as low as 0.5)
• Cf - semi-enclosed (rewireable) fuse correction, 0.725, only applies to BS 3036 fuses

The design current Ib must be less than or equal to In (the protective device rating), which must be less than or equal to Iz (the tabulated rating times all correction factors). This is Regulation 433.1.1, often written Ib <= In <= Iz.

After sizing for current, you must always check voltage drop (Appendix 4 tables give mV/A/m values) against the limits in Appendix 12: 3% for lighting, 5% for other uses, from the origin of the installation.

Appendix 3 - time/current for devices

Works with Appendix 4 for disconnection time verification, especially useful for checking maximum earth fault loop impedance against device type and rating.

Appendix 1 - diversity

Appendix 1 (previously often referenced as the diversity guidance, formerly IEE On-Site Guide material) allows you to apply diversity factors when calculating the total assumed current demand of an installation - you rarely add up every circuit at 100% because not everything runs at once. Typical examples: 100% of largest cooker circuit plus 30% of the rest, or 66% for socket outlet circuits in some premises. Diversity is about assessing realistic maximum demand, not a safety margin you can invent - always use the recognised published percentages.

Appendix 7 - symbols

Appendix 7 gives the standard graphical and letter symbols used on electrical drawings and schedules (switches, sockets, distribution boards, cable types). Exam questions often show a symbol and ask you to identify it, or ask which appendix a symbol comes from.

Common mistakes

• Forgetting to apply BOTH grouping and thermal insulation factors together when both apply
• Checking current rating but skipping the voltage drop check entirely
• Confusing Ib, In and Iz and getting the inequality the wrong way round
• Applying diversity to fault current calculations (you never should - diversity is for demand assessment only)
• Mixing up Appendix 4 (cable data) with Appendix 3 (device time/current data) in exam references