Earthing & Bonding

What is a protective conductor in BS 7671 terminology?

• AAn insulated conductor
• BA conductor that provides protection against electric shock by connecting exposed parts to earth✓
• CA fuse
• DA circuit breaker only

Answer: Protective conductors (earth/ground wires) connect exposed conductive parts to the earthing system.

What is the typical color of protective conductors in UK installations?

• ARed
• BBlack
• CGreen/yellow stripes✓
• DBlue

Answer: Protective conductors are identified by green-and-yellow striped insulation.

What is main bonding in an electrical installation?

• AConnecting components within a circuit
• BBonding main metallic services (water, gas, structural steel) to earth✓
• CConnecting the supply only
• DNot used in UK installations

Answer: Main bonding connects metallic services (water pipes, gas pipes, structural steel) to the earthing system.

Where should the main earth clamp be positioned on water pipes?

• AAt the water meter
• BWithin 600mm of the water entry to the building✓
• CAt the water tank
• DAny location on the pipe

Answer: The main earth bonding clamp on water pipes must be within 600mm of where water enters the building.

What is supplementary bonding?

• AAdditional protection beyond main bonding
• BBonding extraneous conductive parts and exposed parts in high-risk areas (bathrooms, kitchens)✓
• COnly required in TT systems
• DNot used in modern installations

Answer: Supplementary bonding provides additional equipotential bonding in areas with shock risk (bathrooms, kitchens).

In a TN-S system, what is the earth electrode for?

• ANot present in TN-S
• BProvides earthing for external installations✓
• CUsed for lightning protection
• DReduces voltage drop

Answer: TN-S systems may have an earth electrode for external applications, but the main earth comes from the supply neutral.

What is an earth electrode?

• AAn electronic device
• BA conductor or conductive object in contact with earth to establish an earth connection✓
• CA type of fuse
• DA protection device only

Answer: Earth electrodes are conductive objects (rods, plates, strips) buried in soil to establish an earth connection.

What is the typical earth electrode resistance target in TT systems?

• ALess than 50 Ohms✓
• BLess than 200 Ohms
• CLess than 1000 Ohms
• DNo target specified

Answer: TT system earth electrodes should have resistance less than 50 Ohms to ensure effective RCD operation.

How is earth electrode resistance measured?

• AWith a multimeter only
• BWith specialized earth loop testing equipment✓
• CBy calculation only
• DNot measured

Answer: Earth electrode resistance is measured using a dedicated earth loop impedance tester.

What is an earth rod?

• AA lightning conductor
• BA cylindrical conductor (typically copper or steel) driven into the ground as an earth electrode✓
• CA fuse holder
• DA circuit breaker

Answer: Earth rods are cylindrical conductors, commonly 16mm diameter and 1-3m long, driven into soil.

What is the minimum conductor size for earthing conductors in TN-S systems?

• A1mm²
• B2.5mm²
• CHalf the main incoming cable cross-section (minimum 6mm²)✓
• D16mm²

Answer: Main earthing conductors should be at least half the main cable size with a 6mm² minimum.

How should protective conductors be installed in conduit?

• AAny way is acceptable
• BSeparate from live conductors where practical✓
• CMixed with live conductors
• DNot in conduit

Answer: Protective conductors should be installed separately from live conductors to prevent thermal transfer.