Earthing & Bonding

What is a protective conductor in BS 7671 terminology?

• AAn insulated conductor carrying load current between the distribution board and a socket outlet, sized to match the line 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?

• ABlack, which was the colour used for the neutral conductor in pre-2004 single-phase cables
• BRed
• CBlue, which identifies the neutral conductor in cables made to the harmonised colour code
• DGreen/yellow stripes✓

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

What is main bonding in an electrical installation?

• AConnecting components within a circuit
• BA practice that UK installations no longer make use of
• CConnecting the supply only
• DBonding main metallic services (water, gas, structural steel) to earth✓

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 storage tank in the loft
• BAt any convenient point on the pipe run
• COn the supply side of the water meter
• DOn the consumer's side of the meter, as near as practicable to where the pipe enters the building✓

Answer: The main protective bonding connection is made as near as practicable to where the pipe enters the premises, on the consumer's side of any meter or isolation point and before any branch pipework (normally within 600mm).

What is supplementary bonding?

• ABonding that replaces main bonding wherever an RCD protects the circuits
• BA practice that was dropped from modern installations some years ago
• CBonding that is only required in TT systems
• DBonding nearby exposed and extraneous parts in higher-risk locations, such as some bathrooms✓

Answer: Supplementary bonding connects simultaneously accessible exposed-conductive and extraneous-conductive parts in locations of increased shock risk, such as swimming pools and agricultural premises. In a bathroom it may be omitted where every circuit has 30 mA RCD protection and main bonding is in place. A kitchen is not a special location.

In a TN-S system, where does the installation's means of earthing come from?

• AThe supply neutral, joined to earth at the cut-out
• BA separate protective conductor, usually the supply cable sheath✓
• CA local earth electrode, with no earth from the supply
• DThe incoming metal water service pipe

Answer: In a TN-S system the supplier provides a separate protective conductor, usually the metal sheath of the supply cable, as the means of earthing. In TN-C-S (PME) the combined neutral and earth provides it, and in a TT system a local earth electrode does.

What is an earth electrode?

• AA protection device only, mounted in the consumer unit to disconnect the circuit when earth current flows
• BA type of fuse
• CA conductor or conductive object in contact with earth to establish an earth connection✓
• DAn electronic device

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 1000 Ohms
• CThe target is unspecified
• DLess than 200 Ohms✓

Answer: BS 7671 Regulation 411.5.3 requires Ra x Ia <= 50V, where Ra is the earth electrode resistance and Ia is the current that causes automatic disconnection within the required time. With a 30mA RCD this permits an earth electrode resistance of up to around 1667 Ohms in theory, but UK guidance commonly recommends a practical design target of no more than 200 Ohms, since a lower and more stable resistance is less affected by soil and weather variation and gives a more reliable installation.

How is earth electrode resistance measured?

• ABy calculation, using the soil resistivity figure published for the local authority area
• BWith a standard multimeter, set to the 200 ohm range and connected between the rod and the main earthing terminal
• CWith specialised earth loop testing equipment✓
• DTaken from the network operator's declared Ze rather than measured

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

What is an earth rod?

• AA fuse holder
• BA lightning conductor fixed to the roof ridge, which BS 7671 classes as the installation's only earth electrode
• CA cylindrical conductor (typically copper or steel) driven into the ground as an earth electrode✓
• DA circuit breaker

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

What is the minimum size of a copper main protective bonding conductor in a TN-S installation?

• A2.5mm²
• BHalf the earthing conductor's cross-section (minimum 6mm²)✓
• C1mm²
• D16mm² in every case, whatever the supply

Answer: For a TN-S or TT supply, Regulation 544.1.1 requires main protective bonding conductors of at least half the earthing conductor's cross-section, and not less than 6 mm². A copper conductor need not exceed 25 mm². PME supplies are sized from Table 54.8 instead.

How should a circuit protective conductor be run when the circuit is wired in conduit?

• AIn a separate conduit, away from the live conductors
• BAny route, provided it is green-and-yellow
• CClipped to the outside of the conduit
• DIn the same conduit as the circuit's live conductors✓

Answer: Where overcurrent devices give fault protection, BS 7671 requires the protective conductor to be in the same wiring system as the live conductors, or close to them. Running it in the same conduit keeps the earth fault loop impedance low.

What is the purpose of equipotential bonding?

• ATo reduce current
• BTo increase voltage
• CTo reduce voltage drop along the final circuit by providing a second parallel path for the load current
• DTo maintain metallic parts at similar electrical potential to prevent shock hazards✓

Answer: Equipotential bonding reduces potential differences between metallic parts, preventing shock during faults.

In bathrooms, what metallic parts typically require supplementary bonding?

• AMetal service and waste pipes, heating pipes and accessible structural metalwork✓
• BMetal window frames and door architraves, whatever they are fixed to
• CWater pipes alone, as gas and heating pipes are covered by main bonding
• DRadiators and towel rails, but not the pipes that feed the taps

Answer: Regulation 701.415.2 requires local supplementary bonding connecting the protective conductor terminals of circuits supplying Class I and Class II equipment to the accessible extraneous-conductive-parts: metallic service and waste pipes (such as water and gas), metallic central heating and air-conditioning pipework, and accessible metallic structural parts of the building. Metal window frames and door architraves are not extraneous-conductive-parts unless they are connected to structural metalwork. The bonding may be omitted only where the building has main protective bonding, every final circuit in the room meets the disconnection times and has RCD additional protection, and all extraneous-conductive-parts are effectively connected to the main bonding.

What is a bonding conductor?

• AThe main earthing conductor to the electrode
• BA length of fuse wire used to link two terminals
• CA conductor used to establish equipotential bonding between metallic parts✓
• DAn insulated wire run between two separate accessories

Answer: Bonding conductors connect metallic parts to reduce potential differences during fault conditions.

What colour are supplementary bonding conductors?

• AGrey, the colour used for line 3
• BBrown, to match the line conductors
• CBlue, as used for neutral conductors
• DGreen-and-yellow, never plain green✓

Answer: A supplementary bonding conductor is a protective conductor. Table 51 of BS 7671 identifies protective conductors by the bi-colour green-and-yellow, and Regulation 514.4.2 reserves that combination exclusively for protective conductors. Plain green is found in some older installations but is not a current identification colour for a protective conductor.

How are earth rods typically driven into ground?

• ABy digging trenches at least 600mm deep and laying the rod horizontally along the bottom
• BBy hand, pushing the rod into undisturbed soil until only the connecting clamp remains above ground
• CThey are not driven at all, but laid loose on the surface
• DUsing specialised percussion driving equipment✓

Answer: Earth rods are typically driven into ground using a percussion tool to achieve proper depth and contact.

What is an earth test clamp?

• AA circuit breaker
• BA type of fuse
• CA temporary connection used during testing for safe earth loop measurements✓
• DA permanent earth connection between the main earthing terminal and the incoming water service pipe

Answer: Test clamps allow safe measurement of earth loop impedance without disconnecting the earth conductor.

In a PME (Protective Multiple Earthing) system, why are multiple earth points necessary?

• ATo limit earth conductor potential rise during fault currents✓
• BIt is simply custom, with no underlying reason
• CFor aesthetics
• DTo reduce cost

Answer: Multiple earth points in PME systems reduce the potential rise of the neutral during high fault currents.

What is the relationship between earth electrode resistance and RCD sensitivity?

• ALower electrode resistance permits an RCD with a higher rated residual current✓
• BInapplicable
• CThe two are unconnected
• DHigher resistance allows a lower RCD sensitivity, so a 500mA device may be used wherever the rod reads above 200 ohms

Answer: In a TT system Regulation 411.5.3 requires RA × IΔn to be no more than 50 V. The lower the electrode resistance RA, the higher the rated residual current of RCD that still meets this limit.

How should steel gas and water pipes be bonded to earth?

• AUsing an appropriately sized bonding conductor with a labelled clamp✓
• BWith self-amalgamating tape wrapped around the pipe and the bonding conductor where they meet
• CBy direct welding of the bonding conductor to the pipe, which BS 7671 requires for all gas services
• DThey cannot be bonded, because bonding a gas pipe would create a parallel path for the supply neutral

Answer: Service pipes bond to earth using bonding conductors with labelled clamps, avoiding corrosion.

What is the minimum recommended spacing between earth rods in a multi-rod electrode array?

• AAt least the driven depth of the rods✓
• B0.5m, measured between the centres of adjacent rods regardless of their driven depth
• CUnlimited, since rods connected in parallel behave independently of their spacing
• D1m

Answer: BS 7430 treats earth rods as outside each other's resistance areas when they are spaced at least their driven depth apart. Rods placed closer share the same soil and add little when connected in parallel.

In an IT system, how may the supply source be connected to Earth?

• AThrough the PEN conductor, as in PME
• BThrough bonding to the incoming metal water pipe
• CThrough a high impedance✓
• DDirectly to earth at the transformer star point, exactly as in a TN-S distribution system

Answer: In an IT system no live conductor is solidly earthed: the source is either isolated from Earth or, where it is connected, the connection is made through a deliberately high impedance. A first earth fault therefore draws only a very small current, so the supply can continue while the fault is found. A PEN conductor belongs to a TN-C-S (PME) supply, and a solid connection at the transformer star point is the TN-S arrangement.

What is the purpose of an earth loop impedance test?

• ATo test the continuity of the ring final circuit conductors
• BTo measure the total impedance of the earth fault loop, verifying protection device coordination✓
• CTo measure current, by clamping around the line conductor while the circuit carries its full design load
• DTo measure voltage, between the line conductor and the main earthing terminal with the circuit isolated

Answer: Earth loop impedance testing verifies that protective devices will disconnect faults quickly enough.

How is protective conductor continuity verified?

• ALeft unverified
• BVisually
• CBy voltage testing at each accessory
• DBy a low-resistance ohmmeter test (R1+R2 or R2), comparing the reading with the expected value✓

Answer: Protective conductor continuity is verified with a low-resistance ohmmeter during initial verification. The reading is compared with the expected value for the conductor length and size.