Selection & Erection of Equipment

What is the primary factor in selecting cable for an installation?

• ALength only, with any run over 20m taken to the next conductor size up regardless of the load
• BPrice alone, since all cable made to BS 6004 carries the same current whatever the conductor size
• CCurrent-carrying capacity (ampacity), voltage drop, and mechanical strength✓
• DColour preference, so that circuits can be traced without reference to the distribution board schedule

Answer: Cable selection must consider ampacity, voltage drop, mechanical protection, and environmental factors.

What is cable current-carrying capacity (ampacity) dependent on?

• AThe manufacturer only, because each maker publishes its own ratings that override those in Appendix 4
• BConductor cross-section, insulation material, installation method, and ambient temperature✓
• CColour only, with grey sheathed cable rated higher than white sheathed cable of the same conductor size
• DVoltage only, so a cable used on a 230V circuit may carry twice the current allowed on a 400V circuit

Answer: Ampacity depends on conductor size, insulation type, installation method (e.g., buried, on surface), and ambient temperature.

What are correction factors applied to cable ampacity used for?

• AAccounting for ambient temperature, grouping, and cable laying conditions✓
• BIncreasing voltage
• CReducing cost
• DA factor that is no longer applied

Answer: Correction factors reduce ampacity when cables operate in high temperatures or when grouped closely together.

What is voltage drop in electrical installation?

• ALoss of voltage safety, meaning the point at which touch voltage exceeds the 50V limit in Chapter 41
• BLoss of electrical power, measured in watts and equal to the difference between supply and load ratings
• CA type of protective device fitted at the origin to limit the supply voltage reaching final circuits
• DThe reduction in voltage between source and load due to conductor resistance✓

Answer: Voltage drop is the potential difference lost due to conductor resistance, limiting maximum cable length.

What is the maximum permissible voltage drop in BS 7671 for final circuits?

• A5%
• B3%✓
• C1%
• D10%

Answer: BS 7671 recommends maximum 3% voltage drop on final circuits from main switchboard.

What is the formula for calculating voltage drop?

• AV = R / I
• BV = I × R × L × 2 / S × 1000 (for single-phase)✓
• CV = I / R
• DV = I × R

Answer: Voltage drop = (Current × Resistance per unit length × Cable length × 2) / Conductor cross-section.

How are cables protected from mechanical damage in walls?

• AWith tape
• BPVC-sheathed cable is rated to resist a 20 joule impact when buried in plaster, so protection is unnecessary
• CIn conduit, trunking, or buried at sufficient depth✓
• DOnly with paint

Answer: Cables in walls must be in conduit/trunking or buried at depth of at least 50mm from surface.

What is the typical minimum burial depth for cables under ground?

• A100mm
• B300mm
• C1000mm
• D500mm✓

Answer: Underground cables should be buried at least 500mm deep, or 300mm under paving.

What is the purpose of a grommet in cable installation?

• ATo protect cables from sharp edges when passing through walls or conduit✓
• BTo reduce voltage drop by increasing the effective cross-sectional area of the conductor at the entry point
• CTo increase ampacity
• DTo secure cables

Answer: Grommets protect cables from abrasion and cuts when passing through conduit ends or sharp-edged openings.

How should cables be supported in conduit runs?

• AOnly at corners
• BAt regular intervals according to manufacturer guidance and installation code✓
• CEvery 10 meters only
• DSupport is unnecessary

Answer: Cables in conduit must be supported at regular intervals (typically 1.5m) to prevent sagging.

What is the function of conduit bending equipment?

• ATo store cables neatly on the drum before they are drawn into the completed conduit run
• BTo test cables for continuity once they have been drawn through the finished conduit run
• CTo measure voltage between the conduit and the main earthing terminal before the circuit is energised
• DTo create clean curves avoiding cable damage and excessive bending radius✓

Answer: Proper bending equipment maintains minimum bending radius to prevent cable insulation damage.

What is the minimum bending radius for a 4mm² cable in conduit?

• A20mm✓
• B100mm
• C10mm
• D50mm

Answer: Minimum bending radius is typically 6× the cable diameter for this conductor size.

How should cables be routed to avoid electromagnetic interference?

• ARoute anywhere
• BKeep separate from high-current and RF sources using shielding or distance✓
• CUse the shortest path only, since interference falls away with the square of the cable length
• DInterference is not a concern in fixed wiring, as steel conduit amplifies rather than attenuates stray fields

Answer: Low-signal cables should be separated from high-current and RF sources to minimize interference.

What is cross-sectional area denoted by?

• AS (in mm²)✓
• BR
• CL
• DD

Answer: Conductor cross-sectional area is denoted by S and measured in square millimeters (mm²).

What cable size would typically be selected for a 20A final circuit?

• A6mm²
• B1mm²
• C2.5mm²✓
• D10mm²

Answer: A 2.5mm² cable rated at approximately 24A is suitable for a 20A circuit at typical installation conditions.

In trunking installations, what is the purpose of ventilation holes?

• ATo allow heat dissipation and prevent temperature buildup in grouped cables✓
• BTo reduce cost
• CThey serve no purpose
• DTo access cables

Answer: Ventilation in trunking allows heat dissipation, preventing excessive temperature rises in grouped cables.

What is the difference between armored and unarmored cables?

• AArmored cables have steel protection for mechanical damage resistance✓
• BArmoured cables are for outdoor use only and may not be installed inside a building in any circumstances
• CThe two are much the same in construction and in the protection they give
• DOnly a colour difference, with armoured cable always black and unarmoured cable always grey in the UK

Answer: Armored (SWA) cables have steel wire or tape armor for protection against mechanical damage.

How should multi-core cables be handled in high-temperature environments?

• ADerate ampacity and use temperature-rated cables✓
• BThe 70°C rating of PVC refers to the ambient air rather than the conductor, so no special handling is needed
• CUse smaller cables
• DTemperature does not affect cables, since the current rating tables already include a 30% safety margin

Answer: High temperatures reduce insulation capability, requiring derated ampacity ratings.

What is the maximum voltage drop permitted on the main distribution system?

• A1%
• B5%
• C3%
• D2%✓

Answer: BS 7671 recommends maximum 2% voltage drop from source to main switchboard, 3% to final circuit.

How are large cables (>35mm²) typically joined?

• ATwisted together and taped, a method BS 7671 permits for conductors above 35mm² in dry locations
• BCompression connectors or busbar terminations✓
• CSoldered joints, which BS 7671 requires for every conductor larger than 35mm² in a fixed installation
• DWire nuts, screwed onto the conductor ends and then enclosed in a purpose-made insulating shroud

Answer: Large cables use compression connectors or busbars for safe, reliable terminations.

What is the purpose of cable glands in equipment enclosures?

• ATo provide mechanical and environmental protection where cables enter enclosures✓
• BTo increase the current-carrying capacity of the cable by conducting heat away into the enclosure body
• CTo measure voltage
• DTo store cables

Answer: Cable glands provide strain relief and environmental sealing where cables enter equipment.

How should cables be labeled in an installation?

• AUnnecessary
• BLabeling is optional
• COnly at one end
• DWith circuit identification, voltage, and protection device details✓

Answer: Cables must be labeled clearly for safe identification during maintenance and troubleshooting.

What is a significant disadvantage of single-core cables in wet locations?

• AThey are harder to install
• BThey carry no disadvantage
• CCorrosion risk without proper sheathing✓
• DThey are more expensive

Answer: Single-core cables in wet locations risk corrosion and are typically not recommended without multi-core protection.

How should cables be routed past heat sources?

• AClipping the cable directly to the wall surface removes the need for any special consideration
• BHeat does not affect cables
• CClose as possible
• DAt least 200mm away or with thermal barrier✓

Answer: Cables must be routed away from heat sources or protected with thermal barriers.

What is the maximum permissible temperature for PVC cable insulation under normal operation?

• A100°C
• B120°C
• C70°C✓
• D40°C

Answer: PVC-insulated cables have a maximum operating temperature of 70°C under normal conditions.

In BS 7671, what is the key structural difference between a ring final circuit and a radial final circuit?

• AA ring final circuit is only permitted for lighting; a radial circuit is only permitted for socket outlets
• BA radial circuit always requires a larger cable size than a ring final circuit protected by the same rated device, so a 32A radial must be wired in 6mm² cable while a 32A ring may be wired in 2.5mm²; apart from that the conductors are connected at the distribution board in exactly the same way
• CThe two names are interchangeable terms for the same wiring method, with no structural difference
• DA ring final circuit's conductors leave and return to the same terminals at the distribution board, forming a closed loop through every outlet; a radial circuit runs one-way from the board to the last outlet with no return path✓

Answer: A ring final circuit's line, neutral and cpc conductors loop out from the distribution board, through every outlet in turn, and back to the same set of terminals at the board, so the load at any outlet is shared between two parallel paths around the ring. A radial final circuit runs in one direction only, from the distribution board to the last outlet, so the full circuit current passes through a single conductor run rather than being shared.