City & Guilds Electrical Installation (Level 3) BS 7671 Wiring Regulations and Special Installations Syllabus

State the relationship the protective device rating must satisfy for overload protection (coordination rule).

$$I_b \leq I_n \leq I_z$$ and $$I_2 \leq 1.45\,I_z$$ where $I_b$ is design current, $I_n$ device rating, $I_z$ cable capacity, and $I_2$ the current ensuring effective operation of the device.

What is the general purpose of Chapter 51 (Common rules)?

It gives common rules for the selection and erection of equipment: compliance with standards, operational and external-influence conditions, accessibility, and identification/notices applicable to all installations.

How must conductors be identified by colour under the harmonised system in Chapter 51?

Protective conductor: green-and-yellow. Neutral: blue. Single-phase line: brown. Three-phase lines: brown ($L1$), black ($L2$), grey ($L3$). A PEN conductor is green-and-yellow with blue markings at terminations.

What does Chapter 51 require regarding equipment selected for an installation?

Every item of equipment must comply with the relevant standard, be suitable for the design voltage/current/frequency and power, and be appropriate for the external influences (environmental conditions) to which it will be exposed.

What is the scope of Chapter 52 (Selection and erection of wiring systems)?

Chapter 52 covers the selection and erection of wiring systems: types of cable and conductor, methods of installation, current-carrying capacity, cross-sectional area, voltage drop, electrical connections, and minimising the spread of fire.

What maximum voltage drop does BS 7671 (Chapter 52/Appendix 4) recommend for a low-voltage installation supplied directly from the public network?

$3\%$ of nominal voltage for lighting and $5\%$ for other uses. For $230\text{ V}$ this is about $6.9\text{ V}$ (lighting) and $11.5\text{ V}$ (other).

Give the formula for voltage drop in a cable using the tabulated $(\text{mV/A/m})$ value.

$$V_d = \frac{(\text{mV/A/m}) \times I_b \times L}{1000}$$ where $I_b$ is the design current in amperes and $L$ the cable length in metres.

Under Chapter 52, what must be considered where cables pass through holes in fire-rated barriers?

Any opening for a wiring system passing through a floor, wall, ceiling or partition must be sealed (firestopped) to maintain the fire resistance of that element, and supports must not allow premature collapse of cables in fire (e.g. metal clips on escape routes).

What is the purpose of Chapter 53 (Protection, isolation, switching, control and monitoring)?

It covers the selection and erection of devices that perform protection, isolation, switching and control functions, including circuit-breakers, RCDs, isolators, and functional switching, and where each is required.

Distinguish 'isolation', 'switching off for mechanical maintenance', 'emergency switching' and 'functional switching'.

Isolation cuts off all live supply for safety during work. Switching off for mechanical maintenance allows safe non-electrical work. Emergency switching rapidly removes danger (must be readily accessible). Functional switching is for normal operational control of equipment.

What does Chapter 46 deal with, and how does it relate to Chapter 53?

Chapter 46 (Devices for protection against electric shock - isolation and switching) sets out the requirements/duties for isolation and switching, while Chapter 53 covers the selection and erection of the actual devices that fulfil those functions.

What requirements apply to a means of isolation under BS 7671?

It must disconnect all live conductors (except a PEN/protective), have a secure off position or be lockable/removable, be clearly identified to the circuit it isolates, and prevent inadvertent reclosure. Provision should be made to discharge stored energy where needed.

What is the maximum disconnection time for a $230\text{ V}$ TN final circuit not exceeding $63\text{ A}$ (socket-outlet)?

$0.4\text{ s}$ for final circuits up to $63\text{ A}$ in a TN system at $230\text{ V}$. Distribution circuits and final circuits over $63\text{ A}$ are allowed up to $5\text{ s}$.

What is the maximum disconnection time for a $230\text{ V}$ final circuit up to $63\text{ A}$ in a TT system?

$0.2\text{ s}$. Distribution circuits in TT are allowed up to $1\text{ s}$.

State the earth fault loop impedance verification inequality used for automatic disconnection.

$$Z_s \leq \frac{U_0}{I_a}$$ where $Z_s$ is the measured/calculated earth fault loop impedance, $U_0$ the nominal line-to-earth voltage, and $I_a$ the current causing disconnection within the required time.

What is the relationship $Z_s = Z_e + (R_1 + R_2)$ used for?

It calculates the total earth fault loop impedance: $Z_e$ is the external loop impedance at the origin and $(R_1+R_2)$ is the resistance of the line conductor plus the circuit protective conductor of the final circuit.

What is the scope of Chapter 54 (Earthing arrangements and protective conductors)?

It covers earthing arrangements, protective conductors, protective bonding conductors, earthing of combined PEN conductors, and the earth electrode requirements that make automatic disconnection effective.

Give the adiabatic equation used to verify the cross-sectional area of a protective conductor.

$$S = \frac{\sqrt{I^2 t}}{k}$$ where $S$ is the minimum cross-sectional area $(\text{mm}^2)$, $I$ the fault current (A), $t$ the disconnection time (s), and $k$ a factor from material and insulation.

How is the minimum size of a protective conductor determined by the 'table method' in Chapter 54?

By relating to the line conductor size $S$: for $S \leq 16\text{ mm}^2$, CPC $= S$; for $16 < S \leq 35\text{ mm}^2$, CPC $= 16\text{ mm}^2$; for $S > 35\text{ mm}^2$, CPC $= S/2$ (same material).

What is the minimum cross-sectional area of a main protective bonding conductor in a TN-C-S (PME) supply?

It depends on the PEN conductor (neutral) size; commonly a minimum of $10\text{ mm}^2$ copper, increasing with supply size (e.g. $16$, $25$ or $50\text{ mm}^2$) per the PME table in Chapter 54.

What is the minimum cross-sectional area of a main protective bonding conductor for a non-PME (TN-S/TT) supply?

Half the cross-sectional area of the earthing conductor, with a minimum of $6\text{ mm}^2$ copper and need not exceed $25\text{ mm}^2$ copper equivalent.

What is supplementary equipotential bonding and when is it required?

It connects together simultaneously-accessible exposed-conductive-parts and extraneous-conductive-parts to limit touch voltage. It is required where disconnection times cannot be met and in certain special locations unless conditions for omission are satisfied.

Give the resistance condition for supplementary bonding between exposed and extraneous conductive parts.

$$R \leq \frac{50}{I_a}$$ where $R$ is the resistance between the parts $(\Omega)$ and $I_a$ is the operating current of the protective device (or RCD residual operating current) in amperes; $50\text{ V}$ is the touch-voltage limit.

What are the zones defined in Section 701 (locations containing a bath or shower)?

Zone 0 = the interior of the bath/shower basin; Zone 1 = above Zone 0 up to $2.25\text{ m}$ from the floor (around the tap outlet); Zone 2 = the $0.6\text{ m}$ horizontal extent beyond Zone 1.

What degree of protection (IP rating) is required for equipment in Zones 0, 1 and 2 of a bath/shower location?

Zone 0: at least $IPX7$ (immersion). Zone 1 and Zone 2: at least $IPX4$ (splashing); where water jets are used for cleaning, $IPX5$.

What additional protection by RCD is required in a location containing a bath or shower (Section 701)?

All low-voltage circuits serving the location must be protected by a $30\text{ mA}$ RCD providing additional protection.

What supplementary bonding requirement applies to a bath/shower location, and when may it be omitted?

Supplementary equipotential bonding is required between extraneous- and exposed-conductive-parts, but may be omitted where all circuits have $30\text{ mA}$ RCD protection, disconnection times are met, and main bonding is satisfactory throughout the premises.

What are the zone classifications for a swimming pool location (Section 702)?

Zone 0 = the interior of the basin; Zone 1 = the area $2\text{ m}$ horizontally from the rim and up to $2.5\text{ m}$ high; Zone 2 = a further $1.5\text{ m}$ horizontal extent beyond Zone 1 (Zone 2 is not always present for some basins).

What protective measure (ELV) is generally required for equipment within Zones 0 and 1 of a swimming pool?

SELV at a nominal voltage not exceeding $12\text{ V}$ AC or $30\text{ V}$ ripple-free DC, with the safety source installed outside Zones 0, 1 and 2.

What special considerations apply to saunas (Section 703)?

The sauna is divided into temperature zones; equipment must withstand high ambient temperature, have at least $IP24$ protection, wiring must be heat-resistant, and additional $30\text{ mA}$ RCD protection is required (except the sauna heater where appropriate).

What reduced disconnection requirement and RCD provision apply on construction sites (Section 704)?

Construction and demolition sites require reduced low-voltage systems (e.g. $110\text{ V}$ centre-tapped giving $55\text{ V}$ to earth) where practicable, and socket-outlet circuits must have $30\text{ mA}$ RCD additional protection; harsh external influences demand robust equipment (min IP rating, mechanical protection).

Why is reduced low voltage (e.g. $110\text{ V}$ CTE) used on construction sites, and what is the voltage to earth?

A centre-tapped-to-earth $110\text{ V}$ transformer limits the shock voltage to earth to $55\text{ V}$ on each leg, greatly reducing the severity of electric shock in the high-risk site environment.

What particular hazards and protective measures define agricultural and horticultural premises (Section 705)?

Presence of livestock, corrosion, dust and water demand: additional protection by $30\text{ mA}$ RCD for socket circuits (and $\leq 300\text{ mA}$ RCD for fire protection at the origin), supplementary bonding of all exposed/extraneous parts accessible to livestock, and equipment with suitable IP rating; livestock are more sensitive to touch voltages than humans.

Name several other special installations or locations covered in Part 7 besides baths, pools and sites.

Examples include 706 conducting locations with restricted movement, 708 caravan/camping parks, 709 marinas, 710 medical locations, 711 exhibitions/shows/stands, 712 solar PV, 717 mobile units, 721 caravans, 722 electric vehicle charging, and 753 floor/ceiling heating systems.

What additional protection (RCD) requirement is common to many Part 7 special locations and to general socket-outlets up to 32 A?

Additional protection by a residual current device with a rated residual operating current not exceeding $30\text{ mA}$ ($I_{\Delta n} \leq 30\text{ mA}$), giving rapid disconnection on contact with live parts.