Selection & Erection of Equipment

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

• APrice alone
• BCurrent-carrying capacity (ampacity), voltage drop, and mechanical strength✓
• CColor preference
• DLength only

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

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

• AVoltage only
• BConductor cross-section, insulation material, installation method, and ambient temperature✓
• CColor only
• DManufacturer only

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?

• AIncreasing voltage
• BAccounting for ambient temperature, grouping, and cable laying conditions✓
• CReducing cost
• DNot used

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 electrical power
• BThe reduction in voltage between source and load due to conductor resistance✓
• CA type of protective device
• DLoss of voltage safety

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%✓
• C10%
• D1%

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

What is the formula for calculating voltage drop?

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

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

How are cables protected from mechanical damage in walls?

• ANo protection needed
• BIn conduit, trunking, or buried at sufficient depth✓
• COnly with paint
• DWith tape

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
• C500mm✓
• D1000mm

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 secure cables
• BTo protect cables from sharp edges when passing through walls or conduit✓
• CTo increase ampacity
• DTo reduce voltage drop

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?

• ANo support needed
• BAt regular intervals according to manufacturer guidance and installation code✓
• COnly at corners
• DEvery 10 meters only

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
• BTo create clean curves avoiding cable damage and excessive bending radius✓
• CTo test cables
• DTo measure voltage

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?

• A10mm
• B20mm✓
• C50mm
• D100mm

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