City & Guilds Electrical Installation (Level 3) Electrical Installations: Design, Erection and Wiring Systems Syllabus

What is the minimum cross-sectional area for a main protective bonding conductor in a TN-C-S (PME) supply with a 25 mm² tails?

Typically $10\,\mathrm{mm^{2}}$ copper for many domestic PME supplies; the size is determined by the PEN conductor cross-sectional area per BS 7671 Table 54.8 (minimum $10\,\mathrm{mm^{2}}$ up to a 35 mm² supply neutral).

Define earth fault loop impedance $Z_{s}$ and write its component equation.

$Z_{s}$ is the total impedance of the earth fault current loop. $$Z_{s} = Z_{e} + (R_{1} + R_{2})$$ where $Z_{e}$ is the external loop impedance and $R_{1}+R_{2}$ are the resistances of the line conductor and CPC of the circuit.

Why must $Z_{s}$ be kept below a maximum value for each protective device?

So that under an earth fault a large enough fault current flows to operate the protective device within the required disconnection time. $$I_{f} = \frac{U_{0}}{Z_{s}}$$ must exceed the device's tripping/fusing current.

State the required maximum disconnection times for final circuits up to 63 A in a TN system at $230\,\mathrm{V}$.

$0.4\,\mathrm{s}$ for final circuits not exceeding 63 A (socket-outlets and fixed equipment up to 32 A historically). $5\,\mathrm{s}$ is permitted for distribution circuits and fixed equipment final circuits over 32 A.

Why is supplementary equipotential bonding required in a 'special location' such as a bathroom?

To limit the touch voltage between simultaneously accessible conductive parts where the risk of electric shock is increased by body resistance being lowered (wet conditions). It may be omitted only when specific conditions on RCD protection, disconnection times, and main bonding are met.

State the condition under which supplementary bonding may be omitted in a bathroom (BS 7671).

It may be omitted if all circuits are RCD protected ($\leq 30\,\mathrm{mA}$), all circuits satisfy the required disconnection times, and main protective bonding is present and effective for the installation.

Distinguish 'basic protection' from 'fault protection'.

Basic protection guards against contact with parts that are live in normal service (e.g. insulation, barriers, enclosures). Fault protection guards against contact with parts made live by a fault (e.g. automatic disconnection of supply, double insulation).

Name the most common method of fault protection used in BS 7671 installations.

Automatic Disconnection of Supply (ADS) — combining protective earthing, protective equipotential bonding, and a device that disconnects the supply within the required time when an earth fault occurs.

What is the difference between a Type B, Type C, and Type D MCB?

They differ in magnetic trip range: Type B trips at $3$–$5\times I_{n}$, Type C at $5$–$10\times I_{n}$, Type D at $10$–$20\times I_{n}$. B for resistive/domestic loads, C for small inductive/motor loads, D for high inrush loads (transformers, X-ray).

What does an RCD detect and at what residual current does a $30\,\mathrm{mA}$ RCD provide additional protection against shock?

It detects an imbalance between line and neutral currents (residual/leakage current to earth). A $30\,\mathrm{mA}$ RCD provides additional protection against electric shock for direct contact and is required for most socket-outlets and cables in walls.

Distinguish overload current from short-circuit (fault) current.

Overload is an overcurrent occurring in a circuit that is electrically sound (e.g. too many appliances). Short-circuit/fault current is an overcurrent resulting from a fault of negligible impedance between live conductors or live and earth.

State the two coordination requirements between a conductor and its overload protective device.

$$I_{b} \leq I_{n} \leq I_{z}$$ and $$I_{2} \leq 1.45 \times I_{z}$$ where $I_{b}$=design current, $I_{n}$=device rating, $I_{z}$=cable capacity, $I_{2}$=current ensuring effective operation of the device.

Define the breaking capacity (Icn or Ics) of a protective device.

The maximum prospective fault current the device can safely interrupt without being damaged or causing danger. It must equal or exceed the prospective fault current ($I_{pf}$) at the point of installation.

What is an SPD and what does it protect against?

A Surge Protective Device. It protects equipment against transient overvoltages (surges) from lightning (atmospheric origin) or switching events, by diverting/limiting the surge energy to earth.

Name the SPD type classes and where each is installed.

Type 1 at the origin (handles direct lightning/partial lightning currents), Type 2 at distribution boards (switching/induced surges), Type 3 close to sensitive equipment (fine protection). They are often combined (Type 1+2).

Define design current $I_{b}$ and give the single-phase formula for a resistive load.

$I_{b}$ is the current intended to be carried by the circuit in normal service. For a single-phase resistive load: $$I_{b} = \frac{P}{U}$$ For a load with power factor: $$I_{b} = \frac{P}{U \cos\phi}$$

After determining $I_{b}$, how is the nominal protective device rating $I_{n}$ chosen?

Choose the next standard device rating equal to or greater than $I_{b}$, ensuring $I_{b} \leq I_{n}$, while not exceeding the cable's eventual current-carrying capacity ($I_{n} \leq I_{z}$).

State the formula for the minimum tabulated current-carrying capacity $I_{t}$ in the cable sizing process.

$$I_{t} \geq \frac{I_{n}}{C_{a} \times C_{g} \times C_{i} \times C_{c} \times C_{f}}$$ where the correction factors are ambient temperature, grouping, thermal insulation, BS 3036 fuse, and other applicable factors.

What does the rating factor $C_{a}$ (ambient temperature) correct for, and what is its reference temperature?

It corrects current-carrying capacity for ambient temperatures other than the reference $30\,^{\circ}\mathrm{C}$ (air). Higher ambient temperature gives $C_{a}<1$, reducing capacity; the factor comes from BS 7671 tables for the insulation type.

What correction factor $C_{c}$ value is applied when a BS 3036 rewirable fuse protects the circuit, and why?

$C_{c}=0.725$. Because the fusing factor of a rewirable fuse is high (it may carry well above its rated current before blowing), the cable must be effectively oversized to protect against overload.

Write the general formula for voltage drop along a circuit using tabulated mV/A/m values.

$$U_{d} = \frac{(mV/A/m) \times I_{b} \times L}{1000}$$ where $L$ is the route length in metres and the mV/A/m value comes from BS 7671 for the cable size and type.

State the maximum permitted voltage drop for lighting and for other (power) circuits supplied from a public LV network.

Lighting: $3\%$ of nominal voltage ($6.9\,\mathrm{V}$ at $230\,\mathrm{V}$). Other uses (power): $5\%$ ($11.5\,\mathrm{V}$ at $230\,\mathrm{V}$).

Why is voltage drop verification a necessary final check even when the cable passes the current-carrying capacity test?

A cable adequate for thermal/current rating may still cause excessive voltage drop on long runs, leading to poor equipment performance (dim lights, undervolt motors). If it fails, a larger conductor must be chosen.

State the adiabatic equation used to verify the minimum CPC cross-sectional area.

$$S = \frac{\sqrt{I^{2} t}}{k}$$ where $S$ is the minimum CSA (mm²), $I$ is the fault current (A), $t$ is the disconnection time (s), and $k$ is a material/insulation constant.

In the adiabatic equation, what does the factor $k$ represent and what affects its value?

$k$ accounts for the conductor material's resistivity, temperature coefficient, heat capacity, and the initial and final (limit) temperatures of the insulation. It is taken from BS 7671 tables (e.g. $k=115$ for $70\,^{\circ}\mathrm{C}$ PVC copper CPC).

How is the fault current $I$ for the adiabatic calculation obtained?

From $$I = \frac{U_{0}}{Z_{s}}$$ using the actual earth fault loop impedance, and the disconnection time $t$ is read from the device's time/current characteristic for that fault current.

Define 'maximum demand' and 'diversity' in installation design.

Maximum demand is the greatest load likely to be drawn simultaneously by an installation. Diversity is the allowance made because not all connected loads operate at full power at the same time, reducing the assessed maximum demand below the connected total.

Give a typical diversity allowance for a domestic cooker circuit.

First $10\,\mathrm{A}$ of the total rated current at $100\%$, plus $30\%$ of the remainder, plus $5\,\mathrm{A}$ if the cooker control unit incorporates a socket-outlet (per IET On-Site Guide).

What is the recommended torque consideration and key risk when terminating conductors?

Terminations must be tightened to the manufacturer's specified torque using a torque screwdriver/wrench. Under-tightening causes high-resistance joints and overheating; over-tightening can crush/damage the conductor. Loose terminations are a major fire risk.

Why must a stranded conductor not be 'tinned' (solder-dipped) before insertion into a screw terminal?

Solder cold-flows under the pressure of the screw, the joint loosens over time, creating a high-resistance connection that overheats. Use a ferrule (bootlace crimp) instead to consolidate the strands.

What does the standard 'ring final circuit' arrangement consist of and its typical protection?

A ring of cable starting and finishing at the same protective device, so each socket is fed from two directions. Typically $2.5\,\mathrm{mm^{2}}$ live conductors with a $1.5\,\mathrm{mm^{2}}$ CPC, protected by a $32\,\mathrm{A}$ device, serving an unlimited number of sockets within a floor area of up to $100\,\mathrm{m^{2}}$.

Compare a ring final circuit with a radial final circuit for socket outlets.

Ring: cable returns to the board, fed both ends, $32\,\mathrm{A}$ device, $2.5\,\mathrm{mm^{2}}$, up to $100\,\mathrm{m^{2}}$. Radial: single cable run terminating at last socket; a $20\,\mathrm{A}$ radial uses $2.5\,\mathrm{mm^{2}}$ up to $50\,\mathrm{m^{2}}$, a $32\,\mathrm{A}$ radial uses $4\,\mathrm{mm^{2}}$ up to $75\,\mathrm{m^{2}}$.

What are the standard mounting heights for socket-outlets and switches under Part M / good practice for accessibility?

Socket-outlets, switches and consumer controls should be mounted between $450\,\mathrm{mm}$ and $1200\,\mathrm{mm}$ above finished floor level so they are reachable by people with limited mobility (light switches typically around $1200\,\mathrm{mm}$, sockets around $450\,\mathrm{mm}$).

What are the 'safe zones' for concealed cables in walls under BS 7671?

Cables run vertically or horizontally within $150\,\mathrm{mm}$ of the top of the wall or of an angle/corner, or horizontally/vertically in line with an accessory, at a depth less than $50\,\mathrm{mm}$. Cables outside these zones (or any zone if <50 mm deep without mechanical protection) need RCD protection or earthed metallic covering.

Why should equipment such as isolators and distribution boards be positioned with ergonomics in mind?

So they are readily accessible for safe operation, isolation, and maintenance without strain or hazard — at a sensible height, with adequate working space, clear of obstructions, and visible, enabling emergency isolation and reducing risk to the operator.

What is the purpose of an earth electrode and how is its resistance verified in a TT system?

It provides the connection to the general mass of earth for the installation's earthing. Its resistance $R_{A}$ is measured (e.g. earth electrode resistance test) and must satisfy $$R_{A} \times I_{\Delta n} \leq 50\,\mathrm{V}$$ so the RCD limits touch voltage to a safe level.

Which BS 7671 rating factor accounts for cables grouped together and how does increasing the number of circuits affect it?

The grouping factor $C_{g}$. As the number of grouped loaded circuits increases, $C_{g}$ decreases (below 1), because mutual heating reduces each cable's ability to dissipate heat, lowering its current-carrying capacity.

What is the difference between an 'exposed-conductive-part' and an 'extraneous-conductive-part'?

An exposed-conductive-part is a conductive part of equipment that can be touched and may become live under fault (e.g. metal appliance casing). An extraneous-conductive-part is a conductive part not forming part of the installation but liable to introduce a potential, generally earth (e.g. metal water/gas pipe, structural steel).