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I.S. 10101 / ET101 Wiring Rules study materials
I.S. 10101 / ET101 Wiring Rules study resources

I.S. 10101 / ET101 Wiring Rules practice questions

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Question 1Scope, Fundamental Principles and Definitions

What is the primary purpose of the I.S. 10101 wiring rules?

  1. To specify the manufacturing tolerances for domestic appliances
  2. To set the requirements for the safe design and installation of electrical installations Correct answer.
  3. To regulate the retail price of electrical energy
  4. To describe the maintenance schedule for power stations

Explanation

The wiring rules establish fundamental safety requirements for designing, erecting and verifying electrical installations so that people, livestock and property are protected. They do not govern appliance manufacture, energy pricing or power station maintenance.

Question 2Scope, Fundamental Principles and Definitions

Under the fundamental principles, which characteristic must an electrical installation satisfy to be considered compliant?

  1. It must be inspected annually by the supply authority
  2. It must be safe for persons, livestock and property Correct answer.
  3. It must be capable of operating at double its rated load
  4. It must use only metallic conduit throughout

Explanation

The fundamental principles require protection against electric shock, thermal effects, overcurrent and other hazards so that the installation is safe for persons, livestock and property. Materials and inspection regimes are means to that end, not the principle itself.

Question 3Scope, Fundamental Principles and Definitions

In the definitions, what does the term 'live part' refer to?

  1. The metal enclosure of an appliance
  2. A part that is always at earth potential
  3. Only the protective earth conductor
  4. Any conductor or conductive part intended to be energised in normal use Correct answer.

Explanation

A live part is a conductor or conductive part intended to be energised in normal use, including a neutral conductor but by convention not a PEN conductor. Earthing and enclosure metalwork are not live parts in this sense.

Question 4Scope, Fundamental Principles and Definitions

What distinguishes an 'exposed conductive part' from an 'extraneous conductive part'?

  1. Exposed parts are outdoors, extraneous parts are indoors
  2. Exposed parts belong to the installation and can become live under fault, extraneous parts do not Correct answer.
  3. Exposed parts carry current in normal use, extraneous parts never do
  4. Exposed parts are always metallic, extraneous parts are always non-metallic

Explanation

An exposed conductive part is a conductive part of the installation that can become live under fault conditions, such as metal enclosure of equipment. An extraneous conductive part is not part of the installation but may introduce a potential, such as a metal water pipe.

Question 5Scope, Fundamental Principles and Definitions

Which fundamental principle addresses the risk of injury from contact with live parts under normal conditions?

  1. Protection against thermal effects
  2. Protection against electric shock Correct answer.
  3. Protection against overvoltage
  4. Protection against electromagnetic disturbance

Explanation

Protection against electric shock covers measures to prevent dangerous contact with live parts in normal use and under single-fault conditions. Thermal, overvoltage and EMC principles address different hazards.

Question 6Scope, Fundamental Principles and Definitions

What is meant by 'basic protection' in the fundamental principles?

  1. Protection against lightning surges
  2. Protection against contact under single-fault conditions
  3. Protection against contact with live parts under normal conditions Correct answer.
  4. Protection of equipment against mechanical damage

Explanation

Basic protection (formerly direct contact protection) prevents contact with live parts under normal, fault-free conditions, typically by insulation or barriers. Fault-condition protection is a separate principle.

Question 7Scope, Fundamental Principles and Definitions

Which of the following best describes 'fault protection'?

  1. Protection against overload currents in the neutral conductor
  2. Protection against contact with live parts during normal operation
  3. Protection against electric shock under single-fault conditions Correct answer.
  4. Protection against mechanical impact on cables

Explanation

Fault protection is designed to operate when a single fault occurs, such as failure of basic insulation, so that dangerous touch voltages do not persist. It works together with basic protection to give overall protection.

Question 8Scope, Fundamental Principles and Definitions

A circuit is designed so that automatic disconnection occurs within a specified time when a fault of negligible impedance connects a live conductor to an exposed conductive part. Which principle does this implement?

  1. Protection by limitation of discharge energy
  2. Protection by insulation
  3. Protection by extra-low voltage
  4. Protection by automatic disconnection of supply Correct answer.

Explanation

Automatic disconnection of supply removes the fault current quickly enough to prevent harmful physiological effects from touch voltage. Insulation, extra-low voltage and energy limitation are distinct protective measures.

Question 9Scope, Fundamental Principles and Definitions

According to the definitions, what is an 'earthed' conductive part?

  1. A part connected to a protective conductor that is connected to earth Correct answer.
  2. A part that carries fault current to the transformer
  3. A part connected only to the neutral conductor
  4. A part that is insulated from earth

Explanation

An earthed part is one connected to a protective conductor that provides a path to earth. Connection solely to neutral, or insulation from earth, does not make a part earthed.

Question 10Scope, Fundamental Principles and Definitions

What does the term 'protective conductor' (PE) designate?

  1. A conductor used to carry current in normal service
  2. A conductor that is always at line voltage
  3. A conductor used only for control signals
  4. A conductor provided for protection against electric shock Correct answer.

Explanation

A protective conductor is provided for safety purposes, such as earthing exposed conductive parts or bonding extraneous conductive parts. It is not intended to carry load current in normal service.

Question 11Scope, Fundamental Principles and Definitions

Which of the following is a fundamental principle regarding protection against thermal effects?

  1. Conductors must be sized so that operating temperature limits are not exceeded Correct answer.
  2. All equipment must be rated for 1000 V
  3. All cables must be installed in thermal insulation
  4. Every circuit must include a residual current device

Explanation

Protection against thermal effects requires that conductors and equipment are selected and installed so that temperature limits are not exceeded in normal operation or under fault. RCDs and voltage ratings are not the general thermal principle.

Question 12Scope, Fundamental Principles and Definitions

What is the meaning of the term 'origin of the installation'?

  1. The main earthing terminal
  2. The point where the supply is delivered to the installation Correct answer.
  3. The location of the electricity meter only
  4. The furthest point of the final circuit

Explanation

The origin of the installation is the point at which the supply is delivered, typically the incoming supply terminals. It is used as the reference point for many requirements such as main switchgear location.

Question 13Scope, Fundamental Principles and Definitions

A designer must ensure an installation provides protection against overcurrent. Which combination of protective measures is typically required?

  1. Protection against short-circuit only
  2. Protection against overload only
  3. Protection against both overload and short-circuit currents Correct answer.
  4. Protection against leakage current only

Explanation

Overcurrent protection must address both overload currents (excess current in a healthy circuit) and short-circuit currents (fault of negligible impedance). Leakage protection is a separate concern.

Question 14Scope, Fundamental Principles and Definitions

Which statement correctly describes the relationship between basic protection and fault protection?

  1. Basic protection replaces fault protection in TN systems
  2. Together they provide the required protection against electric shock Correct answer.
  3. Fault protection is only required in TT systems
  4. Either one alone provides complete protection against electric shock

Explanation

The fundamental principles require both basic protection and fault protection, working together, to achieve protection against electric shock. Neither alone is generally sufficient.

Question 15Scope, Fundamental Principles and Definitions

Which of the following is an extraneous conductive part?

  1. The live terminal of a socket outlet
  2. The protective conductor of a final circuit
  3. A metal water service pipe entering the building Correct answer.
  4. The metal casing of a washing machine

Explanation

An extraneous conductive part is not part of the electrical installation but can introduce a potential, such as a metallic water pipe. The washing machine casing and protective conductor are parts of the installation.

Question 16Scope, Fundamental Principles and Definitions

Which fundamental principle requires that an installation be arranged so that it can be isolated and switched off for maintenance?

  1. Protection against thermal effects
  2. Provision for safe isolation and switching Correct answer.
  3. Protection against electromagnetic disturbance
  4. Protection against overvoltage

Explanation

The principles require means for isolation and switching so that the installation or its parts can be safely de-energised for maintenance and emergency control. This is distinct from thermal, overvoltage and EMC principles.

Question 17Scope, Fundamental Principles and Definitions

What is designated by the term 'final circuit'?

  1. A circuit directly supplying current-using equipment or socket outlets Correct answer.
  2. The circuit between the transformer and the main switchboard
  3. The circuit supplying the main distribution board
  4. A circuit used only for protective bonding

Explanation

A final circuit is the circuit that directly supplies current-using equipment or accessories such as socket outlets. Distribution circuits upstream of the final circuit are not final circuits.

Question 18Scope, Fundamental Principles and Definitions

An installation is being assessed. The assessor notes that a metal handrail in a bathroom is bonded to the protective conductor. Into which classification does the handrail fall before bonding?

  1. Protective conductor
  2. Exposed conductive part
  3. Live part
  4. Extraneous conductive part Correct answer.

Explanation

A metal handrail is not part of the electrical installation but can introduce a potential, so it is an extraneous conductive part requiring protective bonding. It is not a live part, installation part or protective conductor.

Question 19Scope, Fundamental Principles and Definitions

Which of the following is a fundamental principle concerning protection against voltage disturbances?

  1. All cables must be screened
  2. Equipment must be protected against overvoltages and electromagnetic disturbance Correct answer.
  3. All circuits must be isolated from earth
  4. All equipment must be double insulated

Explanation

The principles require that persons and equipment be protected against the effects of overvoltages and electromagnetic disturbance. Screening, isolation and double insulation are particular techniques, not the general principle.

Question 20Scope, Fundamental Principles and Definitions

What does the definition of 'injury' in the context of electric shock refer to?

  1. Damage to equipment from overcurrent
  2. Only physical burns caused by arcing
  3. Financial loss from supply interruption
  4. Harm to persons from physiological effects of current passing through the body Correct answer.

Explanation

In the context of electric shock, injury refers to harm to persons from the physiological effects of current passing through the body, which is why touch voltage and disconnection times are limited. Equipment damage and financial loss are not the concern of shock protection.

Question 21General Characteristics and Assessment of General Characteristics

Under the wiring rules, what is the primary purpose of assessing the general characteristics of an electrical installation before design begins?

  1. To select the colour of the consumer unit enclosure
  2. To finalize the contractor's price for the whole project
  3. To decide the brand of accessories to be installed
  4. To establish the supply and load parameters that determine the design basis Correct answer.

Explanation

Assessment of general characteristics establishes the design inputs: maximum demand, supply characteristics, earthing arrangements, environmental conditions and load nature. These parameters drive every subsequent design decision, so they must be determined before selection of protective devices and conductors.

Question 22General Characteristics and Assessment of General Characteristics

Which quantity represents the maximum current the installation is expected to draw under normal operating conditions and is used to size the supply?

  1. Prospective short-circuit current
  2. Maximum demand Correct answer.
  3. Earth fault loop impedance
  4. Nominal voltage

Explanation

Maximum demand is the greatest load current expected in normal service, allowing for diversity. It is the figure used to determine the rating of the supply and the main protective device.

Question 23General Characteristics and Assessment of General Characteristics

What does the term 'diversity' mean when assessing the maximum demand of an installation?

  1. The spread of protective devices across the distribution board
  2. The difference between phase and neutral currents
  3. The allowance that not all loads operate simultaneously at full rating Correct answer.
  4. The variation in supply voltage throughout the day

Explanation

Diversity recognises that connected loads are rarely all energised at once, so the assumed maximum demand may be less than the arithmetic sum of connected loads. Applying diversity factors prevents gross oversizing of the supply.

Question 24General Characteristics and Assessment of General Characteristics

Which characteristic of the supply must be confirmed when assessing an installation because it affects the selection of protective devices and conductor insulation?

  1. The nominal frequency and voltage Correct answer.
  2. The colour of the meter box
  3. The length of the service cable trench
  4. The supplier's billing tariff

Explanation

Nominal voltage and frequency are fundamental supply characteristics that influence equipment selection, insulation coordination and protective device performance. They must be verified from the supply authority before design.

Question 25General Characteristics and Assessment of General Characteristics

When classifying environmental conditions for an installation, which factor is assessed to determine the required degree of protection of enclosures?

  1. The tariff category of the consumer
  2. The age of the building
  3. The presence of water, dust and corrosive substances Correct answer.
  4. The number of socket outlets per room

Explanation

Environmental assessment considers external influences such as water, dust, corrosion, mechanical impact and temperature. These determine the IP rating and material of enclosures and accessories.

Question 26General Characteristics and Assessment of General Characteristics

Which of the following is a characteristic of the load that must be assessed because it affects the choice of protective and switching devices?

  1. The nature of the load, such as motors or discharge lighting Correct answer.
  2. The colour coding of the final circuits
  3. The manufacturer of the luminaires
  4. The number of spare ways in the board

Explanation

The nature of the load, for example inductive motor loads or discharge lighting with high starting currents, influences protective device type and rating. Load characteristics are therefore part of the general assessment.

Question 27General Characteristics and Assessment of General Characteristics

An installation has connected loads totalling 60 A but, after applying a diversity factor of 0.6, what maximum demand should be used for supply sizing?

  1. 60 A
  2. 100 A
  3. 36 A Correct answer.
  4. 24 A

Explanation

Maximum demand after diversity is the connected load multiplied by the diversity factor: 60 A × 0.6 = 36 A. This reduced figure is used to size the supply and main protective device.

Question 28General Characteristics and Assessment of General Characteristics

A designer is assessing an installation in a swimming pool plant room. Which external influence most directly affects the selection of equipment?

  1. The presence of water and moisture Correct answer.
  2. The building's postal address
  3. The number of staff employed
  4. The supplier's meter reading schedule

Explanation

Water and moisture are external influences that demand increased degrees of protection and affect the choice of equipment and wiring systems. Wet locations require enclosures and accessories suited to that condition.

Question 29General Characteristics and Assessment of General Characteristics

Why must the prospective short-circuit current at the origin of an installation be determined during the assessment?

  1. To select the cable sheath colour
  2. To ensure protective devices have adequate breaking capacity Correct answer.
  3. To confirm the supply can be disconnected by the main switch
  4. To calculate the monthly energy consumption

Explanation

The prospective short-circuit current at the origin establishes the fault level the protective devices must safely interrupt. Devices must have a rated breaking capacity at least equal to this value.

Question 30General Characteristics and Assessment of General Characteristics

Which earthing arrangement characteristic must be established during the general assessment because it governs protective measure selection?

  1. The depth at which the service cable is buried
  2. The make of the energy meter
  3. The number of light fittings in the corridor
  4. Whether the system is TN, TT or IT Correct answer.

Explanation

The earthing arrangement (TN, TT or IT) determines the fault path and hence the protective measures and disconnection times that can be applied. It is a core supply characteristic to be confirmed.

Question 31General Characteristics and Assessment of General Characteristics

A building is divided into several separately metered occupancies. Why is each occupancy assessed separately for maximum demand?

  1. Because each has its own load profile and diversity Correct answer.
  2. Because the earthing system must change per room
  3. Because protective devices cannot be shared in a building
  4. Because the supply voltage differs in each occupancy

Explanation

Each occupancy has a distinct set of loads and operating patterns, so its maximum demand and diversity differ. Assessing them separately ensures each supply and protective device is correctly rated.

Question 32General Characteristics and Assessment of General Characteristics

When assessing an installation, why is the expected ambient temperature of a location recorded?

  1. To fix the number of socket outlets required
  2. To determine the tariff band applicable to the supply
  3. To allow correct correction factors for conductor current-carrying capacity Correct answer.
  4. To select the colour of the trunking

Explanation

Ambient temperature is an external influence that affects the current-carrying capacity of conductors. Correction factors must be applied to tabulated ratings when the ambient differs from the reference conditions.

Question 33General Characteristics and Assessment of General Characteristics

A designer records that a workshop contains flammable dust. Which aspect of the general assessment does this most directly influence?

  1. The number of emergency lights required
  2. The calculation of the electricity bill
  3. The selection of the meter reading route
  4. The choice of equipment and wiring suitable for a hazardous area Correct answer.

Explanation

Flammable dust creates a hazardous external influence requiring equipment and wiring systems appropriate to the zone classification. This is a direct input from the environmental assessment.

Question 34General Characteristics and Assessment of General Characteristics

Why is the nature of the supply, including whether it is single-phase or three-phase, relevant to the assessment of general characteristics?

  1. It sets the colour of the consumer unit
  2. It determines the supplier's billing period
  3. It determines the permissible methods of fault protection and load balance Correct answer.
  4. It fixes the number of light switches in each room

Explanation

Whether the supply is single-phase or three-phase affects load balancing, available fault protection methods and equipment selection. It is therefore a fundamental supply characteristic to record.

Question 35General Characteristics and Assessment of General Characteristics

Which statement best explains why the assessment of general characteristics must consider both present and reasonably foreseeable future loads?

  1. Because the supply authority requires annual re-assessment
  2. Because diversity factors only apply to future loads
  3. Because regulations forbid any later modification of an installation
  4. Because future loads may exceed the capacity of the selected supply and devices Correct answer.

Explanation

Good practice accounts for foreseeable growth so that the supply and protective devices retain adequate capacity. This avoids premature overloading or costly upgrading later.

Question 36General Characteristics and Assessment of General Characteristics

In assessing an installation, what is the purpose of identifying the points of supply and the means of isolation?

  1. To determine the cable manufacturer
  2. To confirm the supply can be safely isolated for maintenance and emergency Correct answer.
  3. To select the paint finish of the building
  4. To calculate the number of light fittings

Explanation

Identifying the origin and isolation points ensures the installation can be safely isolated for maintenance, testing and emergencies. This is part of establishing the supply characteristics and operational arrangements.

Question 37General Characteristics and Assessment of General Characteristics

A large installation has a connected load of 120 A with a diversity factor of 0.5 applied to 80 A of it and no diversity on the remaining 40 A. What maximum demand results?

  1. 80 A Correct answer.
  2. 120 A
  3. 60 A
  4. 100 A

Explanation

Diversity applies only to the 80 A portion: 80 A × 0.5 = 40 A, plus the 40 A with no diversity gives 80 A total. Diversity is applied selectively to loads that are not simultaneous.

Question 38General Characteristics and Assessment of General Characteristics

Why does the assessment of general characteristics need to record the type of overcurrent protective device at the origin when determining disconnection times?

  1. Because the device type sets the supplier's fuse rating only
  2. Because device type and rating affect the time-current characteristic used to verify disconnection Correct answer.
  3. Because the device type determines the building's fire rating
  4. Because the device type fixes the nominal supply voltage

Explanation

Verifying disconnection times requires the time-current characteristic of the protective device, which depends on its type and rating. This links the supply characteristics to the protective measure design.

Question 39General Characteristics and Assessment of General Characteristics

An installation is to be extended and the existing supply has little spare capacity. Which assessment conclusion is most appropriate before proceeding?

  1. The existing diversity factor can be ignored
  2. The prospective fault current need not be reconsidered
  3. The maximum demand must be recalculated and the supply capacity re-verified Correct answer.
  4. The earthing arrangement can be assumed unchanged without check

Explanation

An extension changes the load profile, so maximum demand must be recalculated and compared with the supply capacity. Fault level and earthing should also be re-verified as part of a proper reassessment.

Question 40General Characteristics and Assessment of General Characteristics

Two identical buildings are supplied from the same network, but one is in a coastal location. Which assessment difference most affects equipment selection?

  1. The coastal site has a higher diversity factor
  2. The coastal site must use a different earthing system by law
  3. The coastal site has a more corrosive environment requiring suitable protection Correct answer.
  4. The coastal site has a lower nominal voltage by regulation

Explanation

A coastal environment introduces salt-laden air, a corrosive external influence that demands enclosures and materials with appropriate corrosion resistance. Environmental assessment therefore differentiates otherwise identical buildings.

Question 41Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

What is the primary purpose of automatic disconnection of supply as a protective measure against electric shock?

  1. To cut off the supply to a circuit when a fault could cause a dangerous touch voltage Correct answer.
  2. To balance the load evenly across phases
  3. To reduce energy consumption during normal operation
  4. To limit the prospective fault current to a safe value

Explanation

Automatic disconnection removes the supply quickly enough that dangerous touch voltages cannot persist on accessible conductive parts. It relies on protective earthing combined with an overcurrent or residual current device.

Question 42Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

In a TN system, the protective device must disconnect the supply within the prescribed time when a fault of negligible impedance occurs between a line conductor and a protective conductor. What does this requirement ensure?

  1. That the supply voltage stays within tolerance
  2. That the protective conductor never carries current
  3. That the insulation resistance remains above 1 MΩ
  4. That the exposed conductive part does not remain at a dangerous potential for a prolonged period Correct answer.

Explanation

A line-to-protective-conductor fault raises the potential of exposed conductive parts. Fast disconnection limits the duration of the touch voltage to a level the body can tolerate.

Question 43Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

Which protective measure relies on separating a circuit from earth and from other circuits so that a single fault does not cause a dangerous current to flow?

  1. Automatic disconnection
  2. Protective earthing
  3. Electrical separation Correct answer.
  4. Equipotential bonding

Explanation

Electrical separation uses an isolating transformer or equivalent source so that touching one live part and earth does not complete a circuit through the body. It is typically limited to supplying a single item or a small group.

Question 44Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

What is the main function of a residual current device (RCD) in protecting against electric shock?

  1. It prevents overload by sensing conductor temperature
  2. It detects a difference between line and neutral currents and disconnects the supply Correct answer.
  3. It disconnects the supply when the supply voltage falls below a set level
  4. It limits the magnitude of prospective short-circuit current

Explanation

An RCD compares the current flowing out on the line conductor with that returning on the neutral. A residual current indicates leakage to earth, and the device trips at its rated residual operating current.

Question 45Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

For additional protection against electric shock, socket-outlets intended for general use by ordinary persons are commonly protected by an RCD having what rated residual operating current?

  1. 30 mA Correct answer.
  2. 300 mA
  3. 500 mA
  4. 100 mA

Explanation

Additional protection is provided by an RCD with a rated residual operating current not exceeding 30 mA. This value is chosen to reduce the risk of fatal shock from contact with live parts.

Question 46Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

Which of the following is a fundamental requirement for basic protection against direct contact with live parts?

  1. Insulation, barriers or enclosures preventing contact with live parts Correct answer.
  2. Automatic disconnection within 0.4 s
  3. Use of a residual current device on every circuit
  4. Equipotential bonding of all metalwork

Explanation

Basic protection prevents contact with live parts under normal conditions, achieved by insulation, barriers, enclosures or obstacles. Fault protection then addresses contact under fault conditions.

Question 47Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

What class of equipment provides protection against electric shock by relying on basic insulation only, with no provision for connection to a protective conductor?

  1. Class II
  2. Class I
  3. Class III
  4. Class 0 Correct answer.

Explanation

Class 0 equipment relies on basic insulation alone and has no means of connecting accessible metalwork to a protective conductor. It is generally not permitted in modern installations for general use.

Question 48Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

What is the purpose of a surge protective device (SPD) installed at the origin of an installation?

  1. To limit transient overvoltages and divert surge currents to earth Correct answer.
  2. To interrupt fault current from a short circuit
  3. To reduce the earth fault loop impedance
  4. To detect residual current to earth

Explanation

An SPD limits transient overvoltages by diverting surge current to earth or by clamping the voltage. This protects equipment and reduces the risk of fire and insulation breakdown from surges.

Question 49Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

A final circuit protected by a 32 A circuit breaker has a calculated earth fault loop impedance measured at 0.9 Ω. The maximum permitted value for the device to operate within the required disconnection time is 1.1 Ω. Which conclusion is correct?

  1. The circuit complies because the measured impedance is below the maximum Correct answer.
  2. The circuit fails because the impedance is too high for the RCD
  3. The circuit fails because the impedance must always be below 0.5 Ω
  4. Compliance cannot be judged without knowing the cable length

Explanation

Automatic disconnection is verified by comparing the measured earth fault loop impedance with the maximum value that allows the protective device to operate within the disconnection time. A measured value below that maximum satisfies the requirement.

Question 50Protection for Safety: Electric Shock, Thermal Effects, Overcurrent and Surge

Why does a TN-C-S (PME) system require particular care with equipotential bonding of metallic services entering the installation?

  1. Because the neutral and protective functions are combined, so potential differences can arise on metallic services Correct answer.
  2. Because the supply has no earth connection at the source
  3. Because the protective conductor must be double-insulated
  4. Because the fault current is always below the RCD threshold

Explanation

In a TN-C-S system the combined neutral and protective conductor carries load and fault current, so its potential can rise above true earth. Bonding metallic services limits dangerous potential differences within the installation.