{"id":27980,"date":"2026-11-09T17:37:37","date_gmt":"2026-11-09T17:37:37","guid":{"rendered":"https:\/\/tradefoxapp.com\/?page_id=27980"},"modified":"2026-11-09T23:58:49","modified_gmt":"2026-11-09T23:58:49","slug":"earthing-and-bonding","status":"publish","type":"page","link":"https:\/\/tradefoxapp.com\/fr\/bs-7671\/earthing-and-bonding\/","title":{"rendered":"Earthing and bonding under BS 7671:2018+A4:2026"},"content":{"rendered":"\n<style>\n.tfg-page{--fg:#1d1d1f;--fg2:#6e6e73;--line:#d2d2d7;--soft:#f5f5f7;--brand:#F15927;--brand-ink:#C2410C;font-family:-apple-system,BlinkMacSystemFont,\"SF Pro Text\",\"SF Pro Display\",\"Helvetica Neue\",Helvetica,Arial,sans-serif!important;color:var(--fg);-webkit-font-smoothing:antialiased;font-size:19px;line-height:1.58;letter-spacing:-.012em;text-align:left;background:#fff}\n.tfg-page *,.tfg-page *::before,.tfg-page *::after{box-sizing:border-box}\n.tfg-page h1,.tfg-page h2,.tfg-page h3,.tfg-page p,.tfg-page li,.tfg-page td,.tfg-page 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class=\"tfg-local\" aria-label=\"BS 7671 guide\"><div class=\"tfg-local-in\"><a class=\"tfg-local-title\" href=\"https:\/\/tradefoxapp.com\/bs-7671\/\">BS 7671 Guide<\/a><div class=\"tfg-local-links\"><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/amendment-4\/\">What&#x27;s new in Amendment 4<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/before-you-start\/\">Before you start<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/protection-against-electric-shock\/\">Electric shock<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/fire-arc-fault-and-surge\/\">Fire, arc and surge<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/cable-selection-and-sizing\/\">Cable sizing<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/earthing-and-bonding\/\" aria-current=\"page\">Earthing and bonding<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/special-locations\/\">Special locations<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/ev-solar-batteries-and-heat-pumps\/\">EV, solar and batteries<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/inspection-and-testing\/\">Inspection and testing<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/certificates-and-eicr-codes\/\">Certificates and codes<\/a><a href=\"https:\/\/tradefoxapp.com\/bs-7671\/quick-reference\/\">Quick reference<\/a><\/div><\/div><\/nav><article class=\"tfg\" data-title=\"Earthing and bonding under BS 7671:2018+A4:2026\" data-desc=\"Earthing vs bonding, main and supplementary bonding sizes, PME, TT electrodes, CPC sizing and the new A4 Section 545, explained for UK electricians.\" data-slug=\"earthing-and-bonding\">\n  <p class=\"tfg-eyebrow\">BS 7671 Guide \u00b7 Section 6<\/p>\n  <h1>Earthing and bonding<\/h1>\n  <p class=\"tfg-lede\">Earthing gives fault current a way back to the source so a device can trip. Bonding stops dangerous voltages between metal parts you can touch at the same time. You need both, and they do different jobs.<\/p>\n  <aside class=\"tfg-brief\"><h2>In brief<\/h2><ul>\n    <li>Earthing makes disconnection happen. Bonding limits the touch voltage while it happens.<\/li>\n    <li>Only bond metalwork that can bring in a potential from outside. A quick resistance test to the MET tells you whether it can.<\/li>\n    <li>On a PME supply, main bonding is sized from the supply neutral. A 25 mm\u00b2 neutral needs 10 mm\u00b2 copper bonds.<\/li>\n    <li>You can leave out supplementary bonding in a bathroom if three conditions are all met (Regulation 701.415.2).<\/li>\n    <li>On TT, R<sub>A<\/sub> \u00d7 I\u0394n must not exceed 50 V. Aim for an electrode resistance as low as you can get, because readings above 200 \u03a9 may not be stable.<\/li>\n    <li>A4 adds Section 545, which covers functional earthing for ICT. Functional earthing and bonding conductors are identified in pink.<\/li>\n  <\/ul><\/aside>\n\n  <h2>Earthing and bonding: two different jobs<\/h2>\n  <p><strong>Earthing<\/strong> connects exposed-conductive-parts (the metal cases of Class I equipment) to the means of earthing through circuit protective conductors (CPCs), the main earthing terminal (MET) and the earthing conductor. When a line conductor faults to a metal case, current flows round the earth fault loop. If the loop impedance is low enough, the protective device disconnects in time.<\/p>\n  <p><strong>Protective equipotential bonding<\/strong> doesn&#8217;t make anything trip. While a fault lasts, it keeps exposed and extraneous metalwork at close to the same potential, so a person touching two pieces of metal at once gets a much smaller voltage across their body.<\/p>\n  <p>Automatic disconnection of supply needs both. For disconnection times and loop impedance limits, see <a href=\"https:\/\/tradefoxapp.com\/bs-7671\/protection-against-electric-shock\/\">protection against electric shock<\/a>.<\/p>\n  <p class=\"tfg-reg\">Regulation references: 411.3.1.1, 411.3.1.2, Chapter 54<\/p>\n\n  <h2>The earthing conductor and the MET<\/h2>\n  <p>The MET is where everything comes together: the earthing conductor, the main bonding conductors, the CPCs and any functional earthing conductors. It is usually a terminal bar in the consumer unit or a separate earth block next to it.<\/p>\n  <p>Size the earthing conductor in the same way as any other protective conductor, by table or by calculation. Buried earthing conductors have their own minimum sizes, which depend on whether the conductor has protection against corrosion and mechanical damage. A buried conductor with no protection of either kind needs at least 25 mm\u00b2 copper. Check Table 54.1 for the other cases.<\/p>\n  <p>On TN-S and TN-C-S supplies, many distributors set their own minimum for the earthing conductor, often 16 mm\u00b2 copper. Check the distributor&#8217;s requirements before you fit a new tail set or consumer unit.<\/p>\n  <div class=\"tfg-note\"><strong>On site:<\/strong> make the MET easy to get at, and make sure you can disconnect the earthing conductor with a tool for testing. Record its size and type on the certificate.<\/div>\n  <p class=\"tfg-reg\">Regulation references: 542.3, 542.4, 543.1, Table 54.1<\/p>\n\n  <h2>Main protective bonding<\/h2>\n  <h3>What counts as extraneous<\/h3>\n  <p>An extraneous-conductive-part is metalwork that isn&#8217;t part of the electrical installation but can introduce a potential, usually earth potential. Typical examples are metal water, gas and oil service pipes coming in from outside, structural steel in contact with the ground, and exposed metal of other services such as central heating or air conditioning where it is connected to the outside world.<\/p>\n  <p>Metal is not automatically extraneous. A copper pipe fed by plastic and running only between internal fittings may introduce no potential at all, so it doesn&#8217;t need bonding.<\/p>\n\n  <h3>Deciding with a test<\/h3>\n  <p>If you can&#8217;t tell by looking, measure the resistance between the metalwork and the MET with an insulation resistance tester. A high reading means the part has no low-resistance path to earth, so it can&#8217;t bring a dangerous potential into the building.<\/p>\n  <ul>\n    <li>Test at a low voltage setting (250 V or 500 V). You are testing a path through the building fabric and the ground, not a cable.<\/li>\n    <li>IET guidance (Guidance Note 8) works out the threshold by limiting body current to a safe level. For normal dry locations, this commonly comes out at about 22 k\u03a9. If you measure above that, you don&#8217;t need to bond the part. Below it, bond.<\/li>\n    <li>Be stricter where people may be wet or in poor contact with earth, such as bathrooms, swimming pools and agricultural sites. Check the guidance note before you rely on a reading in those places.<\/li>\n    <li>If the result is borderline, bond.<\/li>\n  <\/ul>\n  <div class=\"tfg-warn\"><strong>Watch out:<\/strong> parallel paths through other bonds and CPCs can give a falsely low reading. Think about what you are actually measuring. Record your decision and the reading, so the next electrician knows why there is no bond.<\/div>\n\n  <h3>Where and how to connect<\/h3>\n  <p>Connect main bonding to an incoming service as close as practicable to its point of entry. For gas, water or oil, where practicable, connect within 600 mm of the meter outlet union on the consumer&#8217;s side, before any branch. If the meter is outside, connect at the point of entry. Clamp onto rigid pipework, never onto a flexible meter connection.<\/p>\n  <p>Use clamps to BS 951, protected against corrosion where needed. Fix a durable label at the connection that says &#8220;Safety Electrical Connection \u2013 Do Not Remove&#8221;. The label supplied with a BS 951 clamp is acceptable for this.<\/p>\n  <p class=\"tfg-reg\">Regulation references: 411.3.1.2, 514.13.1, 544.1.1, 544.1.2<\/p>\n\n  <h2>Sizing main bonding conductors<\/h2>\n  <p>The rule depends on the earthing arrangement.<\/p>\n  <ul>\n    <li><strong>Not PME (TN-S, TT):<\/strong> use at least half the size the earthing conductor is required to be, with a minimum of 6 mm\u00b2 copper. You never need more than 25 mm\u00b2 copper.<\/li>\n    <li><strong>PME (TN-C-S):<\/strong> size from the supply neutral conductor, using Table 54.8. This is larger because on PME, bonding conductors can carry diverted neutral current if the supply PEN conductor is lost or has a high resistance.<\/li>\n  <\/ul>\n  <table class=\"tfg-table\">\n    <thead><tr><th>Typical supply<\/th><th>Earthing arrangement<\/th><th>Minimum copper main bond<\/th><\/tr><\/thead>\n    <tbody>\n      <tr><td>Domestic, 25 mm\u00b2 tails\/neutral<\/td><td>PME (TN-C-S)<\/td><td>10 mm\u00b2<\/td><\/tr>\n      <tr><td>Domestic, 25 mm\u00b2 tails, 16 mm\u00b2 earthing conductor<\/td><td>TN-S<\/td><td>10 mm\u00b2<\/td><\/tr>\n      <tr><td>Small supply, 10 mm\u00b2 tails, 10 mm\u00b2 earthing conductor<\/td><td>TN-S<\/td><td>6 mm\u00b2 (minimum applies)<\/td><\/tr>\n      <tr><td>Larger supply, 50 mm\u00b2 neutral<\/td><td>PME (TN-C-S)<\/td><td>16 mm\u00b2<\/td><\/tr>\n    <\/tbody>\n  <\/table>\n  <p>These are examples only. For other sizes, check Table 54.8 and Regulation 544.1.1 in the current edition. Some distributors ask for more than the BS 7671 minimum on PME, so check their requirements too.<\/p>\n  <div class=\"tfg-note\"><strong>On site:<\/strong> on a domestic EICR, a 6 mm\u00b2 main bond on a PME supply is undersized by today&#8217;s rules. Whether you code it, and how, depends on the condition of the installation. Section 10 covers <a href=\"https:\/\/tradefoxapp.com\/bs-7671\/certificates-and-eicr-codes\/\">EICR coding<\/a>.<\/div>\n  <p class=\"tfg-reg\">Regulation references: 544.1.1, Table 54.8<\/p>\n\n  <h2>Supplementary bonding<\/h2>\n  <p>Supplementary bonding connects exposed and extraneous parts that can be touched at the same time, locally within one area.<\/p>\n  <p>You need it when:<\/p>\n  <ul>\n    <li>a circuit can&#8217;t meet its required disconnection time and you use supplementary bonding as the remedy (Regulation 415.2), or<\/li>\n    <li>a Part 7 special location requires it, for example some agricultural premises, swimming pools and medical locations. See <a href=\"https:\/\/tradefoxapp.com\/bs-7671\/special-locations\/\">special locations<\/a>.<\/li>\n  <\/ul>\n  <h3>Bathrooms: when you can leave it out<\/h3>\n  <p>In a room with a bath or shower, you can omit supplementary bonding if all three of these are true:<\/p>\n  <ol>\n    <li>Every final circuit in the location meets the automatic disconnection requirements of Regulation 411.3.2.<\/li>\n    <li>Every final circuit in the location has 30 mA RCD additional protection.<\/li>\n    <li>Every extraneous-conductive-part in the location is effectively connected to the main protective bonding.<\/li>\n  <\/ol>\n  <p>The third condition is where it usually goes wrong. Bonding the services at the point of entry is often enough, but only if the pipework in the bathroom is metal and continuous back to that point. If there are plastic sections, check with a continuity test.<\/p>\n  <h3>Conductor size<\/h3>\n  <p>Supplementary bonding conductors must be at least 2.5 mm\u00b2 copper if sheathed or otherwise mechanically protected, or 4 mm\u00b2 if not. If you are bonding an exposed-conductive-part, the conductor must also be related to the size of the circuit&#8217;s CPC. Check Regulation 544.2 for the exact relationships.<\/p>\n  <p class=\"tfg-reg\">Regulation references: 415.2, 544.2, 701.415.2<\/p>\n\n  <h2>Earth electrodes on TT systems<\/h2>\n  <p>On TT, the installation has its own electrode. Loop impedance is usually far too high for a fuse or MCB to clear a fault, so an RCD provides fault protection.<\/p>\n  <p>The basic check is that the electrode resistance R<sub>A<\/sub> (including the earthing conductor and protective conductors) multiplied by the RCD&#8217;s rated residual operating current I\u0394n must not exceed 50 V. For a 30 mA RCD, that allows up to about 1,667 \u03a9 in theory. Don&#8217;t design to that.<\/p>\n  <ul>\n    <li>Get the electrode resistance as low as practicable. BS 7671 notes that values above 200 \u03a9 may not be stable, so treat 200 \u03a9 as an upper limit, not a target. Many designers aim lower to allow for seasonal change.<\/li>\n    <li>Soil that dries out or freezes can raise resistance many times over. Drive rods deep enough to reach consistently moist ground.<\/li>\n    <li>Keep the electrode clear of buried metalwork connected to a PME earth. The distributor may specify a separation distance.<\/li>\n    <li>Never use a gas or other flammable-fluid pipe, or the water utility&#8217;s supply pipe, as the earth electrode (Regulation 542.2.6).<\/li>\n  <\/ul>\n  <div class=\"tfg-note\"><strong>On site:<\/strong> record the electrode type and the measured R<sub>A<\/sub> on the certificate. If the reading is marginal, add a second rod spaced well apart, rather than accepting a number you&#8217;ll struggle to repeat in August.<\/div>\n  <p class=\"tfg-reg\">Regulation references: 411.5.3, 542.2.2, 542.2.4, 542.2.6, Table 41.5<\/p>\n\n  <h2>Sizing CPCs: table or calculation<\/h2>\n  <p>You have two ways to size a protective conductor.<\/p>\n  <ul>\n    <li><strong>By table (Table 54.7):<\/strong> a simple rule based on the line conductor size, assuming the CPC is the same material. Up to 16 mm\u00b2, the CPC matches the line conductor. Above that, it can be smaller. It is quick but often oversizes.<\/li>\n    <li><strong>By calculation (the adiabatic equation, Regulation 543.1.3):<\/strong> work out the minimum size from the fault current, the device&#8217;s disconnection time and a factor <em>k<\/em> for the conductor material and insulation. Use this for twin and earth cable, where the CPC is smaller than the line conductor, and for armour used as a CPC.<\/li>\n  <\/ul>\n  <p>The calculation checks the CPC won&#8217;t overheat before the device disconnects. Loop impedance is still a separate check. Section 5 covers <a href=\"https:\/\/tradefoxapp.com\/bs-7671\/cable-selection-and-sizing\/\">cable selection and sizing<\/a> in more detail.<\/p>\n  <p class=\"tfg-reg\">Regulation references: 543.1.1, 543.1.3, 543.1.4, Table 54.7<\/p>\n\n  <h2>Functional earthing and Section 545 (new in A4)<\/h2>\n  <p>Functional earthing is not for safety. It exists so that equipment works properly: reducing interference, giving electronics a stable reference and controlling electromagnetic disturbance.<\/p>\n  <p>A4 adds Section 545, which covers functional earthing and functional bonding for information and communication technology (ICT). It covers minimum sizes and continuity for functional bonding conductors, conductors that are both protective and functional, the main functional earthing terminal (MFET) and bonding ring conductors. It supports the electromagnetic compatibility measures in Section 444.<\/p>\n  <p>Identify functional earthing and bonding conductors in <strong>pink<\/strong>. Pink replaced the older cream colour in an earlier revision of the 2018 edition, not in A4. You may still find cream on older installations. A combined protective and functional conductor is still a protective conductor and must keep its green-and-yellow identification.<\/p>\n  <div class=\"tfg-warn\"><strong>Watch out:<\/strong> never let a functional earth do the job of a protective one. If a conductor carries fault current, treat it as protective and size, identify and terminate it to protective conductor rules.<\/div>\n  <p class=\"tfg-reg\">Regulation references: Section 444, Section 545, Table 51<\/p>\n\n  <h2>Protective neutral bonding (PNB)<\/h2>\n  <p>A4 recognises PNB as a variant of TN-C-S. The supply PEN conductor is earthed at an electrode close to the consumer&#8217;s supply terminals, not at the transformer, which limits the voltage rise if the PEN conductor breaks. You will mostly see it on private or single-customer supplies where PME isn&#8217;t permitted or recommended (see ENA Engineering Recommendation G12). Where a PNB arrangement connects to a public network, expect PME-style conditions to apply. Ask the distributor or system designer before you size bonding.<\/p>\n  <p class=\"tfg-reg\">Regulation references: 312.2.1.1, Part 2 (definitions)<\/p>\n\n  <aside class=\"tfg-tools\"><h2>Tradefox tools<\/h2><ul>\n    <li><a href=\"https:\/\/tradefoxapp.com\/calculators\/electrical\/adiabatic-protective-conductor-calculation\/\">Adiabatic protective conductor calculation<\/a><\/li>\n    <li><a href=\"https:\/\/tradefoxapp.com\/calculators\/electrical\/calculated-earth-fault-loop-impedance\/\">Calculated earth fault loop impedance<\/a><\/li>\n    <li><a href=\"https:\/\/tradefoxapp.com\/calculators\/electrical\/radial-line-and-cpc-resistance\/\">Radial line and CPC resistance<\/a><\/li>\n    <li><a href=\"https:\/\/tradefoxapp.com\/calculators\/electrical\/prospective-fault-current-estimate\/\">Prospective fault current estimate<\/a><\/li>\n    <li><a href=\"https:\/\/tradefoxapp.com\/courses\/\">Tradefox courses: earthing, bonding and inspection and testing<\/a><\/li>\n  <\/ul><\/aside>\n<\/article><nav class=\"tfg-pager\" aria-label=\"Guide sections\"><a class=\"tfg-prev\" href=\"https:\/\/tradefoxapp.com\/bs-7671\/cable-selection-and-sizing\/\"><span>Previous<\/span>\u2039 Cable sizing<\/a><a class=\"tfg-all\" href=\"https:\/\/tradefoxapp.com\/bs-7671\/\">All sections<\/a><a class=\"tfg-next\" href=\"https:\/\/tradefoxapp.com\/bs-7671\/special-locations\/\"><span>Next<\/span>Special locations \u203a<\/a><\/nav><div class=\"tfg-legal\"><p><strong>Important.<\/strong> This guide is written by Tradefox to help qualified electricians understand BS 7671:2018+A4:2026. It is a plain-English summary for learning and reference only. It is not the standard, it does not reproduce it, and it is not professional advice. Always work to the current edition of BS 7671, the IET guidance notes, manufacturers\u2019 instructions, Building Regulations and your scheme provider\u2019s requirements. Electrical work must be carried out by a competent person.<\/p><p>To the extent permitted by law, Tradefox accepts no liability for loss or damage arising from use of this guide. BS 7671 is published by the IET and BSI; Tradefox is not affiliated with or endorsed by either. See our <a href=\"https:\/\/tradefoxapp.com\/terms-of-use\/\">Terms of Use<\/a>. Spotted something out of date? 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