Technical guide · Earthing & bonding
Being metal is not the deciding factor. The question is whether a part can introduce a potential — usually Earth potential — from outside the electrical installation.
For competent electricians: classification, practical examples, resistance interpretation and the precautions needed before any bonding is disturbed.
Never disconnect bonding just to get a reading. External current can remain with the main switch off. Suspected dangerous metalwork or a PEN fault needs a safe assessment and DNO assistance on 105.
“A conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.”
BS 7671, Part 2: extraneous-conductive-part. Current edition: BS 7671:2018+A4:2026 (previous A2:2022+A3:2024 version also current until 15 October 2026). Public IET guidance reproduces this definition; exact current provisions must be checked in the licensed standard.
Earth-connected service
Potential from outside
Accessible metal pipe
Extraneous if liable to introduce it
Simultaneous contact
Possible touch-voltage hazard
Likely extraneous
A metallic service connected to the general mass of Earth can introduce Earth potential. Establish the actual incoming material, insulating sections and other connections; do not judge only the pipe visible indoors.
Not automatically extraneous
Internal copper does not become extraneous merely because it is conductive. Check for other Earth connections, such as buried pipework, building steel or shared services. A plastic service alone is not proof of separation.
Assess its connections
Connection through a boiler’s CPC can produce a low reading to the MET without the radiator independently introducing Earth potential. Other connections may change the result; do not bond every radiator by default.
Potentially extraneous
Accessible steelwork connected to foundations or other Earth-connected structures may introduce a potential. Assess continuity, accessibility and the actual construction.
Metal alone is insufficient
A conductive object with no relevant connection outside the electrical installation is not automatically an extraneous-conductive-part. Conductive waste pipes, supports or structural connections can change the assessment.
Exposed-conductive-part
It forms part of electrical equipment and can become live if basic insulation fails. Its protective connection is normally through the circuit protective conductor, not a separate main bonding wire simply because it is metal.
Connects relevant extraneous-conductive-parts to the main earthing terminal (MET) for protective equipotential bonding. Refer to Regulation 411.3.1.2 and Chapter 54, including 544.1.1 for sizing and 544.1.2 for connection position. PME sizing needs the applicable Table 54.8 assessment, not a blanket “always 10 mm²” rule.
Provides additional local equipotential bonding between relevant simultaneously accessible parts where required. Refer to 415.2 and the applicable special location. For bathrooms, 701.415.2 permits omission only when all its conditions are satisfied; an RCD by itself is not sufficient justification.
Earthing provides a protective connection for the installation. Bonding limits potential differences. An RCD does not remove a main bonding requirement, and bonding does not replace a required CPC.
The commonly quoted 22 kΩ figure can be illustrated using a simplified series-resistance model with assumed body current and body resistance. That arithmetic is not, by itself, an authoritative classification method. The public primary sources checked for this guide do not establish a universal 22 kΩ rule or a prescribed test procedure; BS 7671’s definition is not simply “less than 22 kΩ”.
Illustrative Ohm’s-law model — not a BS 7671 acceptance equation
RCP > U0 / IB − RB
Illustration only: 230 V ÷ 0.010 A − 1,000 Ω = 22,000 Ω (22 kΩ). Using 0.005 A instead gives 45,000 Ω (45 kΩ). Neither is a universal safety boundary; 10 mA is an assumption in this example, not a declaration that such a shock is safe.
Wet conditions, contact area, damaged skin, changing pipework and parallel paths can invalidate a simple comparison. A high reading today does not prove that a part cannot introduce a potential later. A low reading to the MET may merely show its existing bonding or an appliance CPC.
This is a classification discussion — not the supplementary-bonding verification equation in Regulation 415.2.2, an electrode resistance limit or an insulation-resistance acceptance test.
Identify incoming service material, insulating sections, buried metal, structural connections and shared services. Assess whether the part independently introduces a potential, not simply whether a continuity buzzer sounds.
Confirm the supply arrangement as far as practicable. Check for evidence of dangerous touch voltage, overheating and current in bonding using suitable instruments. Current needs investigation; a clamp reading alone cannot distinguish a defective PEN from normal parallel neutral paths or leakage.
Identify and isolate all relevant supplies, including PV, batteries, generators and shared circuits; lock off and prove dead with an appropriate two-pole voltage detector proved before and after. The main switch does not isolate DNO equipment or remove externally supplied current in bonding.
A competent electrician may use suitable resistance testing as supporting evidence, after establishing that the test is safe. Record the test points, instrument and range, connections left in place, conditions and parallel paths. A measurement to the MET is not automatically the independent resistance to true Earth.
Existing bonding or CPC connections can mask the independent path, but disconnecting them can create a shock or arcing hazard. Where separation is necessary, it needs a specific safe method and control of external currents. If this cannot be established, stop and document the limitation rather than invent a classification.
Record the evidence, assumptions and limitations and apply the relevant main/supplementary bonding requirements. Any connection legitimately disturbed must be correctly restored and verified before returning the installation to service. Reassess after service or building alterations.
Test suspect housings, pipework and metal surfaces using a voltage contact indicator — a contact-type tester, pen-shaped and often single-pole, such as the Martindale VT7 — proved before and after use. A non-contact “volt stick” is not a substitute. A single-pole surface check does not replace the appropriate two-pole proving-dead procedure, and an absence of indication does not establish that bonding can safely be disconnected.
On TN-C-S/PME, interconnected metal services can form parallel neutral return paths. Current in bonding does not automatically prove a broken PEN, but a high-resistance or open PEN can make those paths hazardous and raise bonded metalwork above true Earth. Switching off the customer’s installation may leave current supplied by neighbouring properties flowing.
Do not disconnect bonding to “see whether the current stops” or remove required bonding to cure diverted neutral current. Stop work where safety cannot be established, keep people away from suspect metalwork and report a suspected supply fault to the DNO on 105. Call 999 for an immediate threat to life or fire.
An EICR observation should describe the actual defect and resulting risk. Do not assign a code solely because a metal object has no bonding wire, or solely from one resistance value. Missing required bonding must be assessed against the installation’s protective measures and current coding guidance.
References: BS 7671:2018+A4:2026 (previous A2:2022+A3:2024 version also current until 15 October 2026), Part 2; Regulations 411.3.1.2, 415.2, 544.1.1 and 544.1.2; Table 54.8; bathroom Regulation 701.415.2. Public explanatory sources can predate Amendment 4 and are not substitutes for the current licensed text. No printed page numbers are asserted.
Part 2 definition, qualifying parts and the distinction between main and supplementary bonding. Historical explanatory guidance, not verified Amendment 4 wording.
Plastic incoming services, fortuitous Earth connections and why metal alone does not require bonding.
The authoritative standard for the installation and relevant special location.
Selection and safe use of electrical test equipment.
Isolation, proving dead and precautions for electrical work.