Earthing & Bonding Selection

BS 7671:2018+A4:2026 Tables 54.1, 54.7 and 54.8 for conductor sizing. The earthing conductor is sized from the line conductor under Table 54.7; on a PME supply the Table 54.8 value applies as well, and where it is buried the Table 54.1 minimum for the burial method applies as well (Reg 542.3.1): the greater value is returned. Table 54.8 is keyed on the PEN conductor of the supply, the DNO's PEN at the cut-out, service head or substation, in copper equivalent; where an installation is fed from a landlord or BNO sub-main, that sub-main has separate neutral and earth conductors and no PEN, and the DNO's PEN at the building intake is entered instead, since PME conditions apply at each installation where the intake has a PME earth terminal (ENA G87 Issue 2, 5.2 and 5.4). Main protective bonding on TN-S and TT is half the earthing conductor under Reg 544.1.1, not less than 6 mm² copper and need not exceed 25 mm² copper; on PME it follows Table 54.8 from the copper-equivalent PEN. Supplementary bonding is sized from the CPC under Regs 544.2.1 to 544.2.3, with the 4 mm² minimum without mechanical protection applied as IET Guidance Note 1 Table 3.9 does. The decision paths cite Regs 415.2.2, 542.2 to 542.4.2, 543.1.1 to 543.5.1, 544.1.1 to 544.2.5, 545.1.1 and 545.1.2, 514.4.2, 514.13.1, 708.415.1, 709.531.2, 710.415.2.101, 722.411.4.1 and 722.531.3.101, and Table 51 for conductor identification. Guidance: BS 7671 Section 545, IET Guidance Notes 1 and 8, BS 6701, BS EN 50310 and BS EN 62305-3. 17 guided questions lead to 22 outcomes, with 9 warnings raised where the answer depends on something the tool cannot see.

11/10/2026 (1.14.0): of 134,006 results compared with 1.13.4, 70 are larger, none smaller: 68 earthing conductors not buried on TN-S or TT with line conductors below 4 mm² (now 4 mm² Cu) and 2 with an 82 mm² aluminium PEN (now 25 mm² Cu). The wording changed on more results, stating more cases not checked. The supply question help now states how a TN-C-S supply with protective neutral bonding (PNB) is entered: TN-C-S (PME) if public, TN-S only if private with its source earth and neutral-earth link in the installation; the files on the site were compared with the release after upload. 26/09/2026 (1.13.4): the EV, caravan park or marina, and petrol filling station warnings that apply on every supply system were shown only on TN-S and TT; they are now also shown on TN-C-S (PME). The EV warnings now state the condition of Reg. 722.411.4.1 in the PME title, the electrical separation exception of Reg. 722.531.3.101 and the external-influence requirements of Reg. 722.512.2, and they no longer mention isolation. The caravan park or marina warnings now state the PME prohibition as ESQCR 2002 Regulation 9(4), Reg. 708.411.4 and Reg. 709.411.4 do, and they no longer name mobile units. Two installation types are now labelled as on their buttons: "Building" and "Caravan park or marina". A new automated test of these changes, with 25 checks, passes on 1.13.4 and fails on the 1.13.3 build. The full set of automated tests was re-run on both tiers, with no findings. No size, table value, calculation or verdict changed; verified live after upload. 26/09/2026 (1.13.3): on the printed summary sheet (Engineering Toolkit), every result line, worked example step, reference, designer note and warning now prints at 7 pt or more; a long result continues on a second A4 page. In a new automated test, 10 results are printed to PDF. On 1.13.3, all 318 lines shown in the tool for them are printed, the smallest at 7.4 pt, and no page is blank. On the 1.13.2 build, no line was missing, but 202 were below 7 pt, the smallest at 5.4 pt. The full set of automated tests was re-run with 300 enumerated outcomes on each tier, 1,308 numeric test runs on the Toolkit tier and 1,260 on the free tier, no findings. Nothing else changed on the printed sheet, in the tool, in the report emailed to free users or in the exports, and no conductor size, table value, calculation or verdict changed; verified live after upload. 26/09/2026 (1.13.2): five sentences and two warning titles, in eleven places, were corrected to use the verb of their source, "shall" or "shall not", in place of "may" or "must"; each was checked against BS 7671:2018+A4:2026 or ESQCR 2002 Regulation 9(4). On the five supplementary bonding results, the note on Reg. 544.2.4 said "may be provided by"; it now says "shall be provided by" and includes the regulation's opening exception, "except where Reg. 544.2.5 applies". In the four warnings for an EV charging point and for a caravan park or marina, the titles of the two PME warnings and two sentences in each of the other two said "must" or "must not"; they now say "shall" or "shall not", as in Reg. 722.411.4.1, Reg. 722.531.3.101, Section 722, Reg. 708.415.1, Reg. 709.531.2 and ESQCR 2002 Regulation 9(4). The corrected note and warnings are shown in the tool, in the report emailed to free users, and on the printed summary sheet and in the exports (Engineering Toolkit). The tool name in the dark bar at the top of the tool is now the main heading of the page, because the heading above the tool was removed; the tool name looks as it did in 1.13.1. The full set of automated tests was re-run with 300 enumerated outcomes on each tier, 1,308 numeric test runs on the Toolkit tier and 1,260 on the free tier, no findings; the automated check of the wording was re-run on both tiers, no findings. A new automated test of the corrected verbs, with 266 checks in all, passes on 1.13.2 and fails on the 1.13.1 build. The automated test of how the tool looks has 200 checks at desktop and phone width on both tiers, with the site's button, field and heading styles loaded; among them are the darker amber of a warning's left edge and the tool name as the only main heading of the page. It passes on 1.13.2 and fails on the 1.13.1 build, and it shows that the dark bar is the same height as on 1.13.1, 61.9 px at desktop width and 87 px at phone width. In this tool, the amber is used for the left edge of a warning that states a requirement; there is no green or grey mark, and there is no white text on any of these colours. The green for a pass, the amber for a point to check or a warning, and the grey for a neutral mark are now darker, so that any white text on them meets the 4.5:1 contrast that the Web Content Accessibility Guidelines 2.2 require at level AA for text of 12 px; this change applies only to the colours, not to any calculation, result, wording or layout. Apart from these changes, every question, option, help text, result line, designer note and warning is worded as before, and no size, table value, calculation or verdict changed; verified live after upload. 26/09/2026 (1.13.1): four corrections were made to the wording, each checked against BS 7671:2018+A4:2026. The note on the eight earthing conductor results stated that the termination at the MET must be labelled "Safety Electrical Connection - Do Not Remove", without the condition that the MET is separate from the main switchgear. It did not name the connection of the earthing conductor to the earth electrode or other means of earthing. It now states that the label is required at or near that connection (Reg. 542.3.2) and at or near the MET where it is separate from the main switchgear (Reg. 514.13.1). On both main bonding results, the note on where to connect gave the 600 mm distance from the meter outlet union without "where practicable" and without the case of an external meter. It now includes both. It also states that the connection shall be made as near as practicable to the point of entry and, where there is a meter, isolation point or union, to the consumer's hard metal pipework before any branch pipework (Reg. 544.1.2). The help under the installation type question named a "Standard building" button; it now names "Building", as the button has read since 1.8.0. The titles of the two medical location warnings and of the Common Bonding Network result now use the word "and" in place of "&" or "+". The corrected notes and titles are shown in the tool, in the report emailed to free users, and on the printed summary sheet and in the exports (Engineering Toolkit); the corrected help is shown in the tool only. Every other help text, result line, designer note and warning is worded as before, and so is every question and option; the tool looks as it did in 1.13.0. The full set of automated tests was re-run with 300 enumerated outcomes on each tier, 1,308 numeric test runs on the Toolkit tier and 1,260 on the free tier, no findings; the automated check of the wording was re-run on both tiers, no findings. A new automated test of the four corrections, with 152 checks in all, passes on 1.13.1 and fails on the 1.13.0 build. In a new automated test, every button and option of the tool was pressed and every field was filled in, with the mouse and the keyboard, at desktop and phone width on both tiers, 428 presses in all; it passes on 1.13.0 and on 1.13.1. No size, table value, calculation or verdict changed; verified live after upload. 26/09/2026 (1.13.0): the tool now looks like the PD Power Distribution tool, with the same font and colours. The other PD tools are being given the same look. The tool name and version are now shown in a dark bar at the top of the tool, and the Back and Start again buttons, which were at its top right from 1.10.0, now sit in that bar and work as before. Questions and results are now shown on white cards on a light grey background. The answer options are now white buttons with a grey border, where before they were filled blue with white text. The one-line answer at the top of a result now has a blue left edge and stays in view as before. A warning that states a requirement now has an amber left edge, and a warning that states a prohibition has a red one; the warnings with a red edge are those raised for PME at an EV charging point, a caravan park or marina, or a petrol filling station. The printed summary sheet (Engineering Toolkit) is now in the same colours as the tool; its text, layout, font and font sizes are unchanged, and so are its green-and-yellow and pink identification colours. One display fault was put right. In the Engineering Toolkit, under the heading "Find the PEN size from the DNO cable" at the PEN question, the aluminium PEN size box and its Convert button were shown all the time; they are now shown only after "Aluminium PEN size given by the DNO (convert to copper equivalent)" is chosen in the DNO cable list, as intended since 1.9.0. The conversion to copper equivalent is unchanged. The panel for saving and exporting, below the result, keeps its present colours for now. Every question, option, help text, result line, designer note and warning is worded as before. The sizes given in the results, the worked example for the case entered, the report emailed to free users, the exports and the free and Engineering Toolkit tiers are unchanged, and so are the plus and minus buttons on number fields on a touch screen. The full set of automated tests was re-run with 300 enumerated outcomes on each tier, 1,308 numeric test runs on the Toolkit tier and 1,260 on the free tier, the same counts as on 1.12.1, no findings; the automated check of the wording read 12,539 tool pages on the Toolkit tier and 12,290 on the free tier, no findings. A new automated test of how the tool looks, with 188 checks in all, was run at desktop width and at a 375 px phone width on both tiers; it passes on 1.13.0 and fails on the 1.12.1 build. On the case used in that test, the printed summary sheet has the same 3,034 characters of text and the same height on 1.13.0 as on 1.12.1. No table value, clause or verdict changed; verified live after upload. 15/09/2026 (1.12.1): the wording in the tool, in the result notes, in the readme and in the notes written inside the program code was revised in plain UK English. The identification notes now say a conductor has no protective function (Table 51, FE) or performs one (Reg. 543.5.1), and the functional earth bar note says no general protective bonding shall be connected to the bar. A new automated check reads every program file and the 12,539 tool pages on the Toolkit tier and 12,290 on the free tier that the full set of automated tests produces, and fails on the 1.12.0 build; the tests were re-run with 300 enumerated outcomes and 1,308 numeric test runs on the Toolkit tier and 1,260 on the free tier, no findings. No table value, clause or verdict changed; verified live after upload. 10/09/2026 (1.12.0): the confirmation screen after the supply and location questions was found to be a Continue click for every user that repeated the two answers, while its purpose, to stop the reader early with a warning, applies only where the combination raises one. It is now shown only where the supply and location raise a warning (PME at an EV charging point, a caravan park or marina, a petrol filling station or a medical location, any TT supply, medical on TN-S); on a plain building on PME or TN-S the role question follows the location directly and Back returns to the location question. The automated tests gained a check on every test run that the confirmation screen is present exactly where a warning applies and absent elsewhere, and that Back after a skipped screen returns to the location with its answer cleared; 300 enumerated outcomes and 1,308 numeric test runs on the Toolkit tier, 1,260 on the free tier, no findings, and the same tests fail on the 1.11.0 build. No table value, clause or verdict changed; verified live after upload. 10/09/2026 (1.11.0): the question about how the installation is supplied, added in 1.6.1 and reworded twice, was found to serve the multi-occupancy case only while being asked of every PME user, and to confuse the case it did not serve. It is removed. The PEN question is asked once per role, defined as the PEN conductor of the DNO supply at the cut-out, service head or substation in copper equivalent, and names the exception in one sentence: a landlord or BNO sub-main has a separate neutral and earth and no PEN, so the DNO's PEN at the building intake is entered instead (ENA G87 Issue 2, 5.2 and 5.4). The result prints the PEN used and prints a check line whenever the PEN entered equals the line conductor. The automated tests gained checks that no such question exists on the PME path, that the path runs straight from the installation method to the line conductor and the PEN, that the PEN question opens with its definition and names the exception with its basis, that the PEN is echoed on the result, and that the check line prints when the PEN equals the line conductor and never otherwise; 300 enumerated outcomes and 1,308 numeric test runs on the Toolkit tier, 1,260 on the free tier, no findings, and the same tests fail on the 1.10.0 build. No table value, clause or verdict changed; verified live after upload. 10/09/2026 (1.10.0): on a PME supply the question before the PEN entry was found to offer only the small-building form of a DNO supply (the cut-out), so an office block supplied from a DNO transformer on site fitted neither option. It was reworded with a description under each option. The worked example for the case entered now sits under a button on the result and the fixed reference example is no longer shown in the tool; the Back and Start again buttons sit at the top right of the tool. The automated tests gained checks that the two navigation buttons sit in the header and Print summary alone in the footer, and that the worked example sits under a button on every kind of result, closed until opened, with no reference example in the tool; 315 enumerated outcomes and 1,307 numeric test runs on the Toolkit tier, 1,259 on the free tier, no findings. No value, clause or verdict changed; verified live after upload. 05/09/2026 (1.9.0): the Engineering Toolkit gains a PEN cable helper under the PEN questions, built on a database of DNO service and mains cables with a source on every row: concentric service cables to BS 7870-3.11 from 4 to 35 mm² (copper equivalent of the concentric copper conductor by equal conductance against IET Guidance Note 3 Table B1, from the maximum resistance in the manufacturer datasheets), waveform mains and intake cables to BS 7870-3.40 and 3.50 from 95 to 300 mm² (the copper wires of the combined neutral and earth conductor, 22 or 42 wires of 1.85 mm, UK Power Networks EDS 02-0033), and the split concentric and 4-core cables that have no PEN, where the reader is sent to the DNO for the figure; an aluminium figure is converted by conductance (× 0.61, GN3 Table B1). The result, the worked example, the report and the printed sheet name the cable the figure came from. The automated tests gained their own transcription of every source and check against it the database, the helper on both tiers, every cable's fill and result band, the aluminium conversion at every Table 54.8 band edge, and a figure typed over the helper's value; 315 enumerated outcomes and 1,304 numeric test runs on the Toolkit tier, 1,256 on the free tier, no findings, and the same tests fail on the 1.8.1 build. No table value, clause or verdict changed; verified live after upload, with the helper tried while logged in. 05/09/2026 (1.8.1): on a PME supply the question before the PEN entry was found to read as ENA G87 text and to change nothing the reader could see. It now asks where the installation is supplied from, direct from the DNO cut-out or from a landlord or BNO sub-main, in plain words; each branch asks its own PEN question, the sub-main branch opening with the reminder that the sub-main neutral is not the PEN and that the PEN size at the building intake comes from the BNO or the DNO; the result on that branch shows a designer note naming the PEN used, in the tool, in the report and on the printed sheet, and the check line on a PEN equal to the sub-main line conductor reads as an instruction. The automated tests gained checks that each branch reaches its own PEN question, that the reminder opens the sub-main question and is absent from the cut-out question, that the feed question has no citation, and that the sub-main note reaches the result; 315 enumerated outcomes and 1,255 numeric test runs in each tier, no findings, and the same tests fail on the 1.8.0 build. No value, clause or verdict changed; verified live after upload. 05/09/2026 (1.8.0): the option buttons have shorter labels on every question so they read at a glance and never clip on a phone, with the detail kept in the question help and in the result, and every result with a size now opens with a one-line answer, the conductor role and its size, which stays in view while the rest of the result is read, with the full sizing line beneath it; the report and the printed sheet show the same answer line. The automated tests gained a check that the answer line is present wherever the sizing line opens with a figure, shows the same figure and the right role, and reaches the report and the printed sheet; 315 enumerated outcomes and 1,255 numeric test runs in each tier, no findings, and the same tests fail on the 1.7.0 build. No value, clause or verdict changed; verified live after upload. 04/09/2026 (1.7.0): every result now shows a worked example built from the case entered rather than a fixed illustration: the supply arrangement and location, each answer given, the table or regulation applied to the figures entered with the value at each step, the size that results and its identification, in the tool, in the emailed and exported report and on the printed sheet. The fixed examples stay below as reference examples. The automated tests gained a check that the worked example is present on every result, restates every figure entered, states the size the result prints and cites the table or regulation it came from; 315 enumerated outcomes and 1,255 numeric test runs in each tier, no findings, and the same tests fail on the 1.6.1 build. No value, clause or verdict changed; verified live after upload. 04/09/2026 (1.6.1): a landlord board fed through a building network was found to be answered from the sub-main neutral entered as the PEN. The PEN question now asks for the distributor's PEN at the intake in the standard's own term, names the sub-main and building network trap (ENA G87 5.2 and 5.4), and the result prints the PEN it used and checks a PEN equal to the line conductor on a sub-main. The automated tests gained a check that every input the standard names by a term of art is defined in the tool, echoed in the result and challenged where a substitution can be seen; 1,255 numeric test runs and 315 enumerated outcomes in each tier, no findings, and the same tests fail on the 1.6.0 build. No table value changed. 03/09/2026 (1.6.0): supplementary bonding now asks the CPC size and returns the Reg 544.2 size as a number, checked cell by cell against IET Guidance Note 1 Table 3.9; 1,245 numeric test runs in each tier, no findings. 03/09/2026: a buried earthing conductor on a TT supply was found to return the Table 54.1 minimum as its size. It was fixed in 1.5.3: every buried path now asks for the line conductor (and the PEN on PME) and returns the greater of the Table 54.7, Table 54.8 and Table 54.1 values. The automated tests gained a check that every derived size is reached through its input; 1,147 numeric test runs in each tier, no findings. 02/09/2026: every cited clause, table and note was read back again against BS 7671:2018+A4:2026, IET Guidance Note 8 (6th Edition) and the On-Site Guide; every computed size was also checked against On-Site Guide Tables 4.4(i) and 4.4(ii) as a second source, 28 further checks in each tier. Four wording corrections were released in 1.5.2 with no value or clause number changed; the full set of automated tests was re-run in both tiers, 634 scenarios each, with no findings. 12/08/2026: the full set of automated tests was re-run in both tiers after the limit of three anonymous sessions and the subscriber-only print were introduced, 634 scenarios each, with no findings; the free tier checks now confirm that the print button stays hidden and the email note is shown. August 2026 audit: 634 scenarios in each tier, free and Toolkit, were checked against hand transcriptions of Tables 54.1, 54.7 and 54.8, and every cited clause was read back against the standard itself.