What is the difference between earthing and bonding?
The terms "earthing" and "bonding" get used interchangeably by people who don't actually understand what either one does. On forums, in casual conversation, sometimes even by electricians who should know better, you'll hear "earth bonding" as if it's a single concept rather than two distinct protective measures with completely different purposes.
- Technical review: Thomas Jevons (Head of Training, 20+ years)
- Employability review: Joshua Jarvis (Placement Manager)
- Editorial review: Jessica Gilbert (Marketing Editorial Team)
- Last reviewed:
- Changes: Updated for BS 7671:2018+A3:2024 Amendment 3, clarified supplementary bonding omission criteria under Regulation 701.415.2, expanded extraneous-conductive-parts testing guidance (23 kΩ threshold)
The terms “earthing” and “bonding” get used interchangeably by people who don’t actually understand what either one does. On forums, in casual conversation, sometimes even by electricians who should know better, you’ll hear “earth bonding” as if it’s a single concept rather than two distinct protective measures with completely different purposes.
Here’s the fundamental distinction that every Level 2 learner needs to grasp before touching an installation: Earthing provides a low-impedance path for fault currents to operate protective devices. Bonding equalises potentials between simultaneously accessible conductive parts to prevent dangerous touch voltages. They’re not the same thing. They’re not interchangeable. You cannot bond an appliance casing (it must be earthed). You do not earth a water pipe (it must be bonded).
This confusion isn’t just academic—it causes real assessment failures. NVQ candidates who write “bonding carries fault current to trip the breaker” in their portfolios demonstrate they don’t understand protective measures under BS 7671. AM2 candidates who can’t distinguish exposed-conductive-parts from extraneous-conductive-parts make incorrect installation decisions that assessors spot immediately. Improvers on site who over-bond non-extraneous parts or miss genuinely extraneous metalwork create installations that fail EICRs.
The problem stems partly from how earthing and bonding are taught—often as procedures to follow rather than principles to understand. You learn “connect the green-and-yellow to the casing” without grasping why that casing needs earthing (to provide fault current path) versus why the nearby radiator needs bonding (to prevent it rising to a different potential during that fault). Both contribute to automatic disconnection of supply (ADS) under Chapter 41 of BS 7671, but they achieve safety through different mechanisms.
This article breaks down what earthing actually does, what bonding actually does, why BS 7671 treats them separately under different regulations, how they work together during faults, and where learners consistently get confused. No vague explanations about “safety” without specifics. No claims that one replaces the other. Just clear technical distinction supported by regulation references, so you can explain to an assessor—or a client—exactly why that motor casing gets earthed while that gas pipe gets bonded.
What Earthing Actually Does
Earthing exists to provide a deliberate, low-resistance path for fault currents to flow from exposed-conductive-parts back to the supply source, enabling protective devices (MCBs, fuses, RCDs) to operate quickly and disconnect the faulty circuit.
The mechanism:
When a fault occurs—let’s say a loose live conductor inside a washing machine touches the metal casing—current needs somewhere to flow. Without earthing, that casing sits at 230V waiting for someone to touch it and become the path to earth through their body. With proper earthing via the circuit protective conductor (CPC), that fault current flows through the low-impedance green-and-yellow conductor back to the main earthing terminal, through the earthing conductor to the means of earthing (earth electrode, supplier’s earth terminal), and ultimately back to the transformer.
The safety outcome:
The magnitude of fault current—typically hundreds of amperes in a properly earthed TN system—causes the protective device to operate within the disconnection times specified in BS 7671 Table 41.1. For final circuits not exceeding 32A, that’s 0.4 seconds maximum for TN systems. The circuit disconnects, removing the hazard before sustained contact causes injury.
What earthing does NOT do:
Earthing doesn’t prevent the fault happening. It doesn’t keep the casing at 0V during the fault (there’s always some voltage rise during fault current flow due to conductor resistance). It doesn’t equalise potentials between different metalwork—that’s bonding’s job. And critically, earthing alone doesn’t protect you if you simultaneously touch the faulty appliance and a piece of extraneous metalwork at a different potential.
BS 7671 context:
Chapter 54 covers earthing arrangements and protective conductors. The regulations specify sizing requirements for circuit protective conductors (often using the adiabatic equation for short-circuit conditions), termination standards at the main earthing terminal, and earthing conductor requirements between the MET and the means of earthing.
Regulation 411.3.1.1 establishes that exposed-conductive-parts must be connected to the main earthing terminal via protective conductors—this is earthing. The intent is facilitating automatic disconnection, not potential equalisation.
Common misconception:
“Earthing sucks electricity away to the ground and that’s why you’re safe.” Not quite. In TN systems (the most common in the UK), fault current returns to the transformer via the earthing system, creating a circuit that allows high current to flow and trip the protective device. The “safety” comes from rapid disconnection, not from dumping current into the dirt.
What Bonding Actually Does
Bonding exists to create equipotential zones by connecting extraneous-conductive-parts to the main earthing terminal, ensuring these parts remain at substantially the same potential and preventing dangerous touch voltage differences between simultaneously accessible metalwork.
The mechanism:
Extraneous-conductive-parts—metal gas pipes, water services, structural steel, central heating pipework—can introduce earth potential into the building. During a fault elsewhere in the installation, or even under normal conditions with neutral-to-earth voltage in PME systems, these parts could sit at significantly different voltages than exposed-conductive-parts of the electrical installation.
Main protective bonding conductors connect these services to the main earthing terminal, typically where they enter the building. Supplementary bonding conductors create additional local equipotential zones in high-risk areas like bathrooms (Regulation 701.415.2) by connecting simultaneously accessible conductive parts—towel rails, metal baths, copper pipework, electric shower casings.
The safety outcome:
If you simultaneously touch a faulty appliance and a radiator, both connected through bonding to the same MET, they rise to the same voltage during the fault. No potential difference means no current flow through your body. The touch voltage—the voltage you experience—is minimized because bonding keeps everything at the same potential while earthing works to disconnect the circuit.
What bonding does NOT do:
Bonding doesn’t provide the primary fault current path. A bonding conductor might carry some current if there’s a potential difference between bonded parts, but it’s not designed or intended to carry the high fault currents that earthing conductors handle. Bonding alone cannot trip a protective device. And bonding doesn’t earth non-electrical metalwork—it connects it to the earthing system, which is different.
BS 7671 context:
Regulation 411.3.1.2 covers main protective bonding conductors connecting extraneous-conductive-parts to the main earthing terminal. Minimum sizing for TN-C-S (PME) systems is half the earthing conductor size, minimum 6mm², maximum 25mm² (unless calculation proves otherwise).
Regulation 701.415.2 addresses supplementary bonding in locations containing baths or showers. Critically, supplementary bonding may be omitted if all circuits are RCD-protected (30mA, rated to BS EN 61008 or BS EN 61009) and main protective bonding meets 411.3.1.2 requirements—but you must verify both conditions, not assume.
Common misconception:
“Bonding carries fault current to trip the MCB.” This confuses bonding’s purpose with earthing’s purpose. Bonding equalises potentials. During a fault, the earthing conductor carries the fault current. The bonding conductor might see some current if bonded parts are at different potentials, but that’s incidental to its primary function of potential equalisation, not its design purpose.
Exposed Parts vs Extraneous Parts: The Critical Distinction
The confusion between earthing and bonding stems largely from not understanding which parts get which treatment. The distinction is straightforward but absolutely fundamental.
Exposed-conductive-parts (these get EARTHED):
These are conductive parts of electrical equipment that you can touch, which aren’t normally live but could become live under fault conditions. Metal appliance casings, consumer unit enclosures, metal light fittings, cable glands, conduit and trunking systems that form part of the electrical installation.
They’re part of the electrical system. When a fault occurs, they need a path for fault current to return to the source so protective devices operate. Hence: earthing via circuit protective conductors.
BS 7671 Part 2 defines them precisely: “Conductive part of equipment which can be touched and which is not normally live, but which can become live when basic insulation fails.”
Extraneous-conductive-parts (these get BONDED):
These are conductive parts NOT forming part of the electrical installation, but capable of introducing an earth potential. Incoming metal gas pipes, water services, oil supply pipes, structural steel, central heating pipework connected to a boiler with earthed metalwork.
They’re not part of the electrical system, but they can introduce potentials from outside or provide alternative earth paths. They need connecting to the MET to keep them at the same potential as the electrical installation. Hence: bonding via main protective bonding conductors.
BS 7671 Part 2 defines them: “Conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation.”
The test for extraneous parts:
Not every piece of metal in a building is extraneous. Metal window frames, isolated radiators, metal sinks fed by plastic pipes—these might not actually be extraneous if they have no effective connection to earth.
The test: Measure resistance between the metal part and the main earthing terminal. If it exceeds 23 kΩ (derived from 230V ÷ 10mA, the threshold for perception), the part cannot introduce a dangerous potential and doesn’t require bonding. This is particularly relevant with modern plastic plumbing—just because there’s a metal tap doesn’t mean the installation is extraneous if it’s fed entirely by plastic pipe with no earth path.
Common errors:
Over-bonding: Connecting every radiator individually when they’re all fed from a central heating system already bonded at the boiler. Bonding metal window frames that have no earth connection. Creating unnecessary parallel paths that can actually create problems in fault conditions.
Under-bonding: Missing supplementary bonding where required in older bathrooms without RCD protection. Failing to bond metal gas pipes because “they’re not electrical.” Ignoring structural steel that absolutely can introduce potentials. Electricians who demonstrate clear understanding of exposed versus extraneous parts, and can explain bonding decisions with regulation references during site surveys, find that local courses teach bonding principles systematically rather than just teaching procedures—building competence that translates to higher client confidence and better inspection outcomes.
Assessment implications:
AM2 candidates must demonstrate they can identify which parts require earthing versus bonding. Assessors specifically watch for incorrect terminology—calling bonding conductors “earth wires” or claiming bonding “provides the earth” for services. These language mistakes reveal conceptual confusion that fails the assessment even if the physical work is correct.
How Earthing and Bonding Work Together During Faults
Neither earthing nor bonding works effectively alone. They’re the two halves of the protective measure known as automatic disconnection of supply (ADS) under BS 7671 Chapter 41.
Fault scenario: Live conductor touches washing machine casing
Phase 1 (fault occurs, 0-0.4 seconds):
Live conductor contacts the metal casing. Without earthing, the casing sits at 230V. With proper earthing via the circuit protective conductor, fault current immediately begins flowing: live → fault point → casing → CPC → MET → earthing conductor → means of earthing → transformer. Current magnitude: potentially several hundred amperes in a TN-S system, thousands in a TN-C-S system depending on external earth fault loop impedance.
During these milliseconds, the casing voltage rises above earth potential due to voltage drop across the CPC resistance. This is where bonding becomes critical.
Phase 2 (bonding prevents side-flash during disconnection time):
The nearby radiator, connected to the central heating system fed from a gas boiler with earthed metalwork, is bonded via main protective bonding to the same MET. As the faulty washing machine casing rises to (say) 50V above true earth potential during fault current flow, the bonding conductor ensures the radiator rises to a similar potential.
Person touching both: Experiences minimal voltage difference because both parts are at substantially the same potential. Touch voltage is low. Current through body is negligible. Person waits (unknowingly) for the protective device to operate.
Phase 3 (protective device operates):
The fault current causes the MCB to trip magnetically or the fuse to blow, typically within 0.1-0.4 seconds for final circuits. Circuit disconnects. Hazard is removed. Person touching the faulty appliance and bonded radiator simultaneously experienced minimal shock because bonding kept both at the same potential during the fault.
Without earthing:
Fault current has no low-impedance path. Current magnitude is limited by contact with true earth through other paths, often insufficient to operate protective devices. Faulty appliance remains live indefinitely. Person touching it completes the circuit through their body to earth—potentially fatal.
Without bonding:
Fault current flows through earthing conductor, protective device operates, but during the 0.1-0.4 second disconnection time, the faulty appliance casing might be at 50V above earth while the radiator remains at true earth potential. Person touching both simultaneously experiences 50V potential difference—dangerous shock current through body even though the fault clears quickly.
The integrated protection:
Earthing ensures disconnection. Bonding ensures safety during the disconnection time. Both are required. Neither is optional in installations using automatic disconnection as the protective measure, which is essentially all standard UK installations under Regulation 411.3.
Supplementary Bonding: When It's Required and When It's Not
Supplementary bonding causes significant confusion, particularly in bathroom installations where regulations have evolved and many electricians learned outdated requirements.
What supplementary bonding is:
Additional local bonding connecting simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts within special locations (primarily bathrooms and shower rooms under Section 701) to reduce touch voltage below main bonding alone.
It typically connects: metal baths, metal shower trays, central heating pipes and radiators, metal waste pipes, electric shower casings, heated towel rails, any structural metalwork within arm’s reach of bathing areas.
"Regulation 701.415.2 allows omitting supplementary bonding in bathrooms if all circuits are RCD-protected and main bonding meets 411.3.1.2 requirements. But you must verify both conditions—not just assume. AM2 candidates who skip supplementary without checking RCD compliance fail because they've not demonstrated competent application of the exception criteria."
The modern omission criteria (Regulation 701.415.2):
Supplementary bonding in locations containing baths or showers may be omitted where:
All circuits supplying the location have additional protection by RCD (rated residual operating current not exceeding 30mA and operating within 40ms at 150mA), AND
All final circuits meet requirements of Regulation 411.3.1.2 for main protective bonding
Both conditions must be met. Not just RCDs. Not just assuming main bonding is correct. You verify:
RCD protection: Check consumer unit. All circuits feeding the bathroom—lighting, heating, shower, sockets if present—must be RCD-protected to 30mA. If the lighting circuit is on an old unprotected MCB, supplementary bonding cannot be omitted for that reason alone.
Main bonding adequacy: Visually inspect main bonding connections at gas and water entry points. Verify conductors meet sizing requirements (typically 10mm² minimum for TN-C-S, but depends on supply earthing conductor and system type). Check connections are tight and corrosion-free.
If either condition fails, supplementary bonding is required per 701.415.2.
When supplementary bonding IS definitely required:
Older installations without RCD protection: If circuits aren’t RCD-protected, supplementary bonding is mandatory regardless of main bonding quality.
Modified installations where RCD addition wasn’t possible: Sometimes main bonding is correct and you’d like to use the omission, but the consumer unit is old and RCD retrofitting isn’t economically viable for the client. Supplementary bonding provides the additional safety layer.
Special circumstances where resistance conditions aren’t met: Regulation 701.415.2 includes resistance criteria (R ≤ 50V/Ia) that can prevent omission even with RCDs. This is rare but possible in installations with poor earth fault loop impedance.
Common mistakes around supplementary bonding:
Assuming all modern bathrooms don’t need it: RCD protection is common now but not universal. Always verify rather than assume.
Not bonding because “everything’s plastic”: Supplementary bonding connects EXPOSED parts (shower casings, towel rail elements) to EXTRANEOUS parts (copper pipes, radiators). Even if waste pipes are plastic, you’re still creating equipotential between electrical equipment and heating system.
Bonding plastic-backed towel rails: If the towel rail has no metal connection to the heating system (plastic pipe connections, plastic backing plate), it’s not extraneous. Check continuity before bonding.
Incorrect conductor sizing: Supplementary bonding conductors are sized differently from main bonding. Minimum 4mm² if mechanically protected, 6mm² if not (Table 54.8). Learners sometimes use 2.5mm² thinking it’s adequate.
Assessment traps:
AM2 candidates must demonstrate understanding of when supplementary bonding can be omitted. Assessors ask directly: “Can you omit supplementary bonding in this scenario?” Correct answer requires checking RCD presence AND verifying main bonding quality—not just saying “yes, there’s an RCD.” The regulation requires both conditions.
Plastic Pipes and the "No Bonding Needed" Myth
The shift toward plastic plumbing created a widespread myth: “Plastic pipes mean no bonding required.” This is partially true but dangerously oversimplified.
The principle:
Extraneous-conductive-parts are metalwork capable of introducing earth potential. If incoming water service is entirely plastic from the meter to all taps, with no metal sections and no earth path, then technically it’s not extraneous and bonding isn’t required.
Why “plastic pipes = no bonding” is still wrong:
Metal still exists somewhere: Even with plastic incoming supply, there’s often a metal stopcock at the boundary, metal sections under the floor, or metal fittings where appliances connect. These create earth paths that make the system extraneous.
Boilers and heating systems: Central heating systems almost always contain metal components—boilers with earthed electrical parts, copper pipes within the boiler, metal radiators throughout the house. The heating system is extraneous even if cold water supply is plastic.
Gas services remain: Plastic water doesn’t affect gas bonding requirements. If there’s a metal gas pipe entering the building, it requires main protective bonding regardless of what the water pipes are made from.
Oil supplies: Oil-fired heating requires bonding of metal oil supply pipes entering the building. Again, unaffected by water pipe material.
Structural steel: If the building contains structural steelwork with an earth path (common in modern construction), it requires bonding regardless of plumbing materials.
The test remains essential:
Don’t assume plastic means no bonding. Test resistance to earth. If metal sections exist and resistance is below 23 kΩ, the system is extraneous and requires bonding. If it genuinely exceeds 23 kΩ across all metalwork—unlikely but possible—bonding isn’t required.
EICR implications:
Coders who mark “C2 – Absence of main bonding to water supply” on installations with entirely plastic water services make incorrect observations. The water supply isn’t extraneous if there’s no metal introducing earth potential. This damages professional credibility when reviewed.
Conversely, coders who see plastic incoming supply and assume NO bonding is required anywhere might miss extraneous heating systems, gas services, or structural steel that absolutely require bonding. Both over-coding and under-coding stem from not testing what’s actually extraneous.
The practical approach:
Visual inspection: Trace services from entry point through the building. Look for metal sections, metal valves, copper inserts in plastic fittings.
Resistance testing: Where uncertainty exists, test resistance between questionable metalwork and the MET. Above 23 kΩ, not extraneous. Below 23 kΩ, likely extraneous and requires bonding.
Apply regulation requirements: If it’s genuinely not extraneous (plastic throughout, no earth path, resistance test confirms), bonding isn’t required per Regulation 411.3.1.2. If any metal provides earth path, bonding is required.
Client communication:
When clients question bonding on mixed systems, explain the principle: “We’re bonding the metal sections that can introduce earth potential, not the plastic sections that can’t. Even though your cold water is plastic, your heating system contains metal components with earth paths, so bonding is required there.”
This is more credible than “regulations say we have to” without explaining why.
Common Assessment Failures and Site Errors
Understanding earthing versus bonding conceptually matters more than memorizing procedures, because assessments test understanding and site conditions require judgment.
NVQ Level 3 portfolio failures:
Terminology errors: Writing “bonding provides earth for the gas pipe” or “main bonding carries fault current.” These statements reveal you don’t understand the distinction. Bonding doesn’t provide earth—it connects extraneous parts to the earthing system. Bonding doesn’t carry fault current as its design purpose—earthing conductors do.
Incorrect part identification: Describing a metal appliance casing as extraneous (it’s exposed). Claiming a plastic-backed radiator requires bonding (it’s not extraneous if isolated). Failing to identify structural steel as extraneous (it absolutely is if it has earth connection).
Missing regulation justification: Claiming supplementary bonding isn’t required because “there’s an RCD” without referencing Regulation 701.415.2 or verifying main bonding compliance. Assessors need to see you understand the regulation, not just the outcome.
AM2/AM2E practical assessment failures:
Wrong conductor sizing: Using circuit protective conductor sizing methods (adiabatic equation, Table 54.7) for bonding conductors. Main bonding in TN-C-S is sized from the neutral conductor per Regulation 544.1.1, not from the circuit line conductor. Supplementary bonding uses Table 54.8.
Inadequate testing: Failing to verify bonding effectiveness through resistance testing. Just connecting a bonding conductor doesn’t prove equipotential—you must measure that the resistance between bonded parts is acceptably low.
Incorrect supplementary bonding omission: Omitting supplementary bonding in a bathroom with RCD protection but not verifying that main bonding meets 411.3.1.2 requirements. Both conditions must be checked.
Over-bonding demonstration: Bonding every radiator individually when they’re all connected through a system already bonded at the boiler. This shows you don’t understand what makes something extraneous (earth path introduction, not just being metal).
"On site, you'll encounter installations where bonding's been done incorrectly—pipes bonded that aren't extraneous, or extraneous parts missed entirely. Competent electricians identify these issues during initial surveys, price correctly for remedial work, and explain findings to clients. Those lacking earthing and bonding confidence lose jobs to contractors who demonstrate expertise."
EICR coding errors:
C2 coding plastic services: Marking absence of bonding to plastic water supply as “Potentially Dangerous” when the supply isn’t actually extraneous. This creates unnecessary remedial work and questions your competence.
Missing genuinely extraneous parts: Failing to identify and code missing bonding to structural steel, oil pipes, or metal gas services because you assumed plastic water meant no bonding anywhere.
Incorrect supplementary bonding observations: Coding bathrooms for missing supplementary bonding without checking whether omission is permitted under 701.415.2 (RCD protection + adequate main bonding).
Over-reliance on labels: Assuming bonding is adequate because “Safety Electrical Connection” labels exist, without testing bonding continuity or verifying connections are tight and effective. Understanding these distinctions separates electricians who command confidence from those who struggle with inspection work, and regional rates vary with capability—inspectors who consistently demonstrate earthing and bonding competence typically earn 15-20% more than those who over-code or under-code due to conceptual confusion.
Site installation errors:
Creating parallel earth paths: Over-bonding creates multiple parallel paths that can cause problems in fault conditions, particularly in TT systems where current might divide unpredictably.
Exporting earth potentials: Bonding conductive parts that extend outside the equipotential zone (metal fencing connected to structural steel, for example) can export dangerous potentials beyond the protected area.
Inadequate connections: Using connectors not suitable for the conductor size, making connections on painted surfaces without removing paint for metal-to-metal contact, or inadequate mechanical strength allowing connections to work loose.
How to avoid these failures:
Learn the principles, not just the procedures: Understand WHY exposed parts get earthed (fault current path) and WHY extraneous parts get bonded (potential equalisation). The “how” follows naturally from the “why.”
Test, don’t assume: If uncertain whether something is extraneous, test resistance to earth. If unsure bonding is effective, measure resistance between bonded parts.
Reference regulations correctly: When justifying decisions (in portfolios, in client discussions, in assessment interviews), cite specific regulations. “Regulation 411.3.1.2 requires main bonding” is more credible than “the regs say.”
Distinguish terminology precisely: Earth. Bond. Exposed. Extraneous. These aren’t interchangeable words. Using correct terms demonstrates understanding.
How This Applies to Modern Installations
Earthing and bonding principles remain constant, but modern installations introduce additional considerations that require applying fundamental understanding rather than following outdated procedures.
EV charger installations:
Electric vehicle chargers require careful earthing—the charger casing is an exposed-conductive-part that must be connected via the circuit protective conductor. In outdoor installations, the mounting post or wall bracket might be extraneous if it has an earth path through structural elements, requiring bonding.
The expansion of EV infrastructure means electricians comfortable with earthing and bonding distinctions are better positioned for emerging work, though policy changes affect specialization demands and market uncertainties require electricians to maintain broad competence across installation types rather than over-specializing too early.
TN-C-S supply systems (PME) create specific considerations for EV chargers. The IET Code of Practice for EV charging addresses earthing arrangements and bonding requirements that differ slightly from standard final circuits. Understanding exposed versus extraneous parts becomes critical when determining what requires additional protection.
Solar PV installations:
Photovoltaic systems introduce exposed-conductive-parts (inverter casings, mounting frames if accessible, DC isolators) that require earthing. Mounting systems on metal roofs might introduce extraneous-conductive-parts requiring bonding if the roof structure has an earth path.
Amendment 2 to BS 7671:2018 modified requirements for protective bonding of solar PV systems, particularly around DC side arrangements. The fundamental principles remain: earth exposed parts for fault protection, bond extraneous parts for potential equalisation, but application requires understanding what’s exposed versus extraneous in PV-specific contexts.
Heat pump installations:
Air source and ground source heat pumps contain electrical components (compressors, control systems) with exposed-conductive-parts requiring earthing. External units often have metal casings that must be connected via the circuit protective conductor.
Ground source systems introduce additional considerations—ground loop pipework might be extraneous if it provides an earth path through the ground. The installation becomes an outdoor electrical installation under Section 714, which has specific earthing and bonding requirements.
Smart home and IoT integration:
Modern homes contain numerous low-voltage electronic systems—smart thermostats, lighting controllers, security systems. These still have exposed-conductive-parts (metal faceplates, control boxes) that require appropriate earthing even though operating voltages are low.
Confusion sometimes arises when installers assume low-voltage equipment doesn’t need earthing. The principle remains: if it’s an exposed-conductive-part of electrical equipment, it requires connection to the protective conductor regardless of normal operating voltage. Fault protection requirements don’t disappear just because the device usually operates at 12V or 24V.
Consumer unit evolution:
Metal consumer units became mandatory under Amendment 3 (originally Amendment 2 proposal) to BS 7671:2018. The metal enclosure is an exposed-conductive-part requiring earthing. Many electricians correctly earth the enclosure via the incoming earthing conductor but miss that the enclosure itself must be bonded to the internal earthing bar, not just mechanically attached.
The distinction: the enclosure is exposed (part of the electrical installation) and must be earthed for fault protection. Any structural metalwork the consumer unit is mounted to might be extraneous (not part of electrical installation but capable of introducing potentials) and would require separate bonding.
Testing and verification technology:
Modern multifunction testers measure earth fault loop impedance, RCD operation, insulation resistance—all of which depend on proper earthing and bonding. Understanding the principles helps interpret test results correctly.
For example, unexpectedly low earth fault loop impedance might indicate unintentional parallel earth paths (over-bonding). Unexpectedly high readings might indicate poor earthing or bonding connections. You can’t interpret results competently without understanding what earthing and bonding are meant to achieve.
The constant principle:
Technology changes. Installation types evolve. Regulations get amended. But the fundamental distinction remains: earthing provides fault current paths for automatic disconnection, bonding equalises potentials to prevent touch voltage hazards. Apply this understanding to whatever installation type you encounter, and the specific requirements become logical rather than arbitrary rules to memorize.
The Bottom Line: Two Protective Measures, Not One
Earthing and bonding aren’t variations of the same thing. They’re distinct protective measures that work together to create the safety system we call automatic disconnection of supply under BS 7671 Chapter 41.
Earthing (Chapter 54, Regulation 411.3.1.1):
Connects exposed-conductive-parts to the main earthing terminal
Provides low-impedance path for fault currents
Enables protective devices to operate within disconnection times
Sized using adiabatic equation or simplified tables for circuit protective conductors
Design purpose: facilitate automatic disconnection
Bonding (Regulation 411.3.1.2, Section 701):
Connects extraneous-conductive-parts to the main earthing terminal
Creates equipotential zones preventing touch voltage hazards
Reduces potential differences between simultaneously accessible metalwork
Sized based on supply earthing conductor for main bonding, Table 54.8 for supplementary
Design purpose: potential equalisation
Why the distinction matters practically:
When you’re on site deciding what to connect where, you need to ask: “Is this part of the electrical installation (exposed) or is it introducing potential from outside (extraneous)?” The answer determines whether you earth it (via CPC) or bond it (via bonding conductor).
When you’re doing an EICR, you need to assess whether earthing provides adequate fault protection AND whether bonding creates effective equipotential zones. Missing either protective measure is a deficiency, but they’re different deficiencies requiring different remedial approaches.
When you’re explaining to a client why their gas pipe has a green-and-yellow conductor attached, “it’s required” is weak. “This bonding conductor keeps your gas pipe at the same electrical potential as your electrical system, preventing dangerous voltage differences if a fault occurs elsewhere in the house” demonstrates understanding.
When you’re sitting an AM2 assessment, the assessor isn’t just checking you can physically install conductors—they’re verifying you understand the principles behind automatic disconnection of supply. That requires distinguishing earthing’s role (fault current path) from bonding’s role (potential equalisation).
Call us on 0330 822 5337 to discuss electrical training pathways that build genuine understanding of protective measures rather than just teaching procedures, understand NVQ Level 3 assessment requirements for demonstrating earthing and bonding competence, explore AM2 preparation that focuses on principles enabling confident decision-making on site, or get guidance on inspection and testing qualification routes where earthing and bonding understanding determines EICR quality.
What we’re not going to tell you:
- That earthing and bonding are basically the same thing (they’re fundamentally different)
- That you can skip bonding if everything’s plastic (test what’s actually extraneous)
- That all metal requires bonding (only extraneous-conductive-parts do)
- That RCD protection alone means no supplementary bonding needed (must also verify main bonding adequate per 411.3.1.2)
- That memorizing procedures passes assessments (understanding principles does)
What we will tell you:
- How to distinguish exposed-conductive-parts requiring earthing from extraneous-conductive-parts requiring bonding
- Why testing resistance to earth (23 kΩ threshold) determines bonding requirements more reliably than assumptions
- What BS 7671 actually requires for main protective bonding sizing in TN-C-S systems (half earthing conductor, 10mm² minimum)
- When Regulation 701.415.2 permits omitting supplementary bonding (RCD protection AND adequate main bonding)
- How earthing conductors are sized differently from bonding conductors (adiabatic/Table 54.7 versus neutral conductor/544.1.1)
- Why touch voltage during the disconnection time makes bonding critical even though earthing handles the fault current
- What assessors look for in NVQ portfolios and AM2 assessments (conceptual understanding, not just procedural following)
- How to interpret EICR findings correctly (is missing bonding a genuine C2 or is the part not actually extraneous?)
- Why client communication improves when you explain bonding’s purpose (potential equalisation) rather than vague “safety” claims
The difference between electricians who consistently pass assessments, deliver compliant installations, and command client confidence versus those who struggle comes down to understanding these fundamental distinctions. Earthing and bonding aren’t complicated when you grasp what each one does and why both are necessary.
References
Primary Official Sources
- BS 7671:2018+A3:2024 (18th Edition Wiring Regulations): Part 2 (Definitions), Chapter 41 (Protection against electric shock), Chapter 54 (Earthing arrangements and protective conductors), Regulation 411.3.1.1 (Earthing of exposed-conductive-parts), Regulation 411.3.1.2 (Main protective bonding conductors), Regulation 544.1.1 (Sizing of main protective bonding conductors), Regulation 701.415.2 (Supplementary bonding in bathrooms and shower rooms): https://electrical.theiet.org/bs-7671/
- IET Guidance Note 8: Earthing & Bonding (6th Edition): https://shop.theiet.org/guidance-note-8-earthing-bonding
- Electricity at Work Regulations 1989, Regulation 8 (Earthing or other suitable precautions): https://www.legislation.gov.uk/uksi/1989/635/regulation/8/made
- HSE Guidance on Electrical Safety: https://www.hse.gov.uk/electricity/
Industry Standards and Technical Guidance
- NICEIC Technical Manual: Earthing and Bonding Explained: https://www.niceic.com/
- ECA Technical Bulletins on Foundation Earthing and Bonding Requirements: https://www.eca.co.uk/
- IET Wiring Matters: Technical Articles on Earthing and Bonding Misconceptions: https://electrical.theiet.org/wiring-matters/
- NET (National Electrotechnical Training) AM2/AM2E Assessment Guidance: https://www.netservices.org.uk/
Testing and Verification Standards
- BS 7671 Chapter 61: Initial Verification
- BS 7671 Chapter 62: Periodic Inspection and Testing
- IET Guidance Note 3: Inspection & Testing (8th Edition)
- IET Code of Practice for Electric Vehicle Charging Equipment Installation (4th Edition)
Additional Technical Resources
- BS EN 61008: Residual current operated circuit-breakers (RCCBs)
- BS EN 61009: Residual current operated circuit-breakers with integral overcurrent protection (RCBOs)
- IET Electrical Safety First: Bonding and Earthing in Domestic Installations
Note on Accuracy and Updates
Last reviewed: 02 February 2026. This page is maintained; we correct errors and refresh sources as BS 7671 amendments, HSE guidance, and assessment criteria change. Current content reflects BS 7671:2018 incorporating Amendment 3 (2024), which modified metal consumer unit requirements and updated protective bonding guidance. Key technical details—exposed-conductive-parts definition (Part 2), extraneous-conductive-parts definition (Part 2), earthing requirements (Chapter 54, Regulation 411.3.1.1), main bonding requirements (Regulation 411.3.1.2), supplementary bonding omission criteria (Regulation 701.415.2), bonding conductor sizing (Regulation 544.1.1, Table 54.8), resistance threshold for extraneous parts (23 kΩ derived from 230V ÷ 10mA), disconnection times (Table 41.1)—all align with current IET publications and NET assessment standards. Practical guidance reflects common NVQ Level 3, AM2/AM2E assessment requirements and EICR coding principles documented by NICEIC and other assessment bodies. Next review scheduled following publication of Amendment 4 to BS 7671:2018 or significant changes to earthing/bonding assessment criteria.
FAQs
What is the simplest definition of earthing, and what safety problem is it designed to solve?
Earthing is the connection of exposed-conductive-parts of an electrical installation to the main earthing terminal, providing a low-impedance path for fault current.
If a live conductor contacts a metal enclosure, the fault current flows safely to earth, causing protective devices (fuses or circuit breakers) to disconnect the supply quickly.
Safety problem solved:
It prevents electric shock from metal parts becoming live due to insulation failure. Without earthing, exposed metalwork could remain energised and dangerous to touch.
In UK installations, earthing follows BS 7671 principles to limit both the magnitude and duration of dangerous touch voltages.
What is the simplest definition of bonding, and what safety problem is it designed to solve?
Bonding is the electrical connection that keeps exposed-conductive-parts and extraneous-conductive-parts at substantially the same potential.
It connects metal items such as pipes or structural steel to the main earthing terminal to prevent voltage differences.
Safety problem solved:
It reduces the risk of electric shock caused by simultaneously accessible metal parts being at different voltages during a fault.
Bonding works alongside earthing under BS 7671 to limit shock risk where non-electrical metalwork can introduce earth potential.
In a fault where a live conductor touches a metal casing, what does the CPC actually do, step by step?
- A live conductor contacts the metal casing, creating a fault.
- The CPC provides a low-resistance path for fault current.
- Fault current flows back to the source via the earthing system.
- High fault current causes the protective device to operate within the required disconnection time.
- The supply disconnects, removing voltage from the casing.
This ensures automatic disconnection of supply (ADS) and prevents sustained touch voltage.
Why can a metal gas or water pipe be dangerous during a fault even though it is not part of the electrical installation?
Metal pipes can introduce earth potential into an installation and become energised during a fault.
Without bonding:
- A voltage difference can exist between the pipe and other metalwork.
- Touching both simultaneously can cause electric shock.
Under BS 7671, gas and water pipes are treated as extraneous-conductive-parts and must be bonded to prevent dangerous touch voltages and, in the case of gas, explosion risk.
What is the difference between an exposed-conductive-part and an extraneous-conductive-part?
- Exposed-conductive-part:
A conductive part of electrical equipment (e.g. metal casing) that can become live under fault conditions.
→ Requires earthing.
- Extraneous-conductive-part:
A conductive part not part of the electrical installation (e.g. pipes, steelwork) that can introduce earth potential.
→ Requires bonding.
This distinction determines whether fault current needs a return path (earthing) or potential differences need equalising (bonding).
How do earthing and bonding work together to achieve ADS under Chapter 41?
- Earthing provides the fault current path needed to operate protective devices within disconnection times (e.g. 0.4 s for TN systems).
- Bonding limits voltage differences between accessible metal parts during the fault.
Together, they create a protective equipotential zone, satisfying BS 7671 Chapter 41 requirements for fault protection. Verification is carried out through earth fault loop impedance testing.
What are the most common learner terminology mistakes, and why do they cause NVQ or AM2 failures?
Common errors include:
- Saying “earth bonding” instead of protective bonding
- Calling the CPC an “earth wire” without explaining its function
- Mixing up earthing and bonding roles
These mistakes show poor understanding of BS 7671 definitions. In NVQ and AM2 assessments, accuracy of language reflects safety knowledge, so incorrect terminology often results in lost marks or failure.
When can supplementary bonding in a bathroom be omitted under Regulation 701.415.2?
Supplementary bonding can be omitted if:
- Main protective bonding complies with 411.3.1.2
- All circuits are protected by 30 mA RCDs (411.3.3)
- Extraneous-conductive-parts are effectively connected to earth
- Resistance values comply with 415.2.2 (e.g. ≤ 1667 Ω for 30 mA RCDs)
Verification steps:
- Confirm main bonding to services
- Confirm RCD protection on all circuits
- Test continuity between extraneous parts and protective conductors
How do you test whether a metal part is genuinely extraneous, and what does the 23 kΩ test show?
- Measure resistance between the metal part and the main earthing terminal.
- Disconnect parallel paths where necessary.
Results:
- Above 23 kΩ: Not extraneous – bonding not required
- Below 23 kΩ: Extraneous – bonding required
The 23 kΩ threshold relates to limiting body current to safe values and prevents unnecessary bonding while maintaining safety.
On an EICR, what earthing and bonding issues are most commonly mis-coded, and how do you avoid them?
Common errors include:
- Bonding non-extraneous parts unnecessarily
- Coding omitted supplementary bonding as C2 when permitted by 701.415.2
- Missing ineffective main bonding
- Over-coding minor continuity issues
How to avoid mis-coding:
- Confirm whether parts are genuinely extraneous (test resistance)
- Remember BS 7671 is not retrospective
- Use calibrated instruments
- Reference Guidance Note 3
- Record test evidence clearly
Correct coding reflects actual risk, not just deviation from current standards.
