{"id":23215,"date":"2026-10-30T06:08:33","date_gmt":"2026-10-30T06:08:33","guid":{"rendered":"https:\/\/tradefoxapp.com\/?p=23215"},"modified":"2026-11-06T16:44:40","modified_gmt":"2026-11-06T16:44:40","slug":"adiabatic-equation","status":"publish","type":"post","link":"https:\/\/tradefoxapp.com\/fr\/adiabatic-equation\/","title":{"rendered":"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples"},"content":{"rendered":"<div data-elementor-type=\"wp-post\" data-elementor-id=\"23215\" class=\"elementor elementor-23215\" data-elementor-post-type=\"post\">\n\t\t\t\t<div class=\"elementor-element elementor-element-bd69d62 e-flex e-con-boxed e-con e-parent\" data-id=\"bd69d62\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-cfb4151 elementor-widget elementor-widget-text-editor\" data-id=\"cfb4151\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">The adiabatic equation checks one thing: whether a protective conductor has enough cross-sectional area to survive the heat of a fault until the protective device cuts the supply. It is a thermal test, nothing more. A conductor that passes it still has to satisfy automatic disconnection, mechanical protection, and the other conditions that apply to the installation.<\/span><\/p><p><span style=\"font-weight: 400;\">For electricians, contractors, and apprentices, the hard part is rarely the sum. It is picking inputs you can trust. Run the adiabatic equation with the wrong fault current, the wrong operating time, or the wrong k factor, and a tidy calculation hands you an unsuitable conductor.<\/span><\/p><p><span style=\"font-weight: 400;\">This guide works through the maths with three examples. The fault currents, operating times, and let-through values shown here are illustrative design inputs. They are not data for any specific device or installation, so never lift them into a real design.\u00a0 <\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-f299ff1 e-flex e-con-boxed e-con e-parent\" data-id=\"f299ff1\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-7e47bbf elementor-widget elementor-widget-heading\" data-id=\"7e47bbf\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h2 class=\"elementor-heading-title elementor-size-default\">Where the Adiabatic Equation Fits in UK Electrical Design\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-a42c1b7 elementor-widget elementor-widget-text-editor\" data-id=\"a42c1b7\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">For current design work, use BS 7671:2018+A4:2026 and its supporting guidance. The IET and BSI set 15 October 2026 as the withdrawal date for the previous edition, BS 7671:2018+A2:2022+A3:2024. An existing installation does not become unsafe just because it was designed to an earlier edition.<\/span><\/p><p><span style=\"font-weight: 400;\">BS 7671 is a non-statutory standard. The HSE&#8217;s position is that following it is likely to achieve compliance with the relevant parts of the Electricity at Work Regulations 1989. So a calculation on its own does not prove you have met every legal or technical duty. It is one piece of evidence, not the whole case.<\/span><\/p><p><span style=\"font-weight: 400;\">Protective conductor sizing follows one of two routes:<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-3d12577 elementor-icon-list--layout-traditional elementor-list-item-link-full_width elementor-widget elementor-widget-icon-list\" data-id=\"3d12577\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"icon-list.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<ul class=\"elementor-icon-list-items\">\n\t\t\t\t\t\t\t<li class=\"elementor-icon-list-item\">\n\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-icon\">\n\t\t\t\t\t\t\t<svg aria-hidden=\"true\" class=\"e-font-icon-svg e-fas-long-arrow-alt-right\" viewbox=\"0 0 448 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path d=\"M313.941 216H12c-6.627 0-12 5.373-12 12v56c0 6.627 5.373 12 12 12h301.941v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.569 0-33.941l-86.059-86.059c-15.119-15.119-40.971-4.411-40.971 16.971V216z\"><\/path><\/svg>\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-text\"><b>The Calculation Route: <\/b>Regulation 543.1.3 covers the adiabatic criterion, where you work out the minimum size directly.<\/span>\n\t\t\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t\t\t<li class=\"elementor-icon-list-item\">\n\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-icon\">\n\t\t\t\t\t\t\t<svg aria-hidden=\"true\" class=\"e-font-icon-svg e-fas-long-arrow-alt-right\" viewbox=\"0 0 448 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path d=\"M313.941 216H12c-6.627 0-12 5.373-12 12v56c0 6.627 5.373 12 12 12h301.941v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.569 0-33.941l-86.059-86.059c-15.119-15.119-40.971-4.411-40.971 16.971V216z\"><\/path><\/svg>\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-text\"><b>The Tabulated Route:<\/b> Regulation 543.1.4 lets you select a size against Table 54.7 instead, by relation to the line conductor.<\/span>\n\t\t\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t<\/ul>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-2667831 elementor-widget elementor-widget-text-editor\" data-id=\"2667831\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Each route carries its own conditions. A reduced circuit protective conductor (CPC) in an existing cable has to be judged against the right requirements. You cannot accept or reject it just by holding it up against the line conductor. <\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-ef2ab7d e-flex e-con-boxed e-con e-parent\" data-id=\"ef2ab7d\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-1241344 elementor-widget elementor-widget-heading\" data-id=\"1241344\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h2 class=\"elementor-heading-title elementor-size-default\">What the Adiabatic Equation Actually Says\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-0f6317a elementor-widget elementor-widget-text-editor\" data-id=\"0f6317a\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">The thermal sizing relationship is:<\/span><\/p><p><b>S \u2265 \u221a(I\u00b2t) \/ k<\/b><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-34f567b elementor-widget elementor-widget-html\" data-id=\"34f567b\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"html.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<style>\n  .pipe-table {\n    font-family: 'Inter', Arial, sans-serif;\n    font-size: 16px;\n    border-collapse: collapse;\n    color: #000;\n    width: 100%;\n    max-width: 800px;\n  }\n  .pipe-table th,\n  .pipe-table td {\n    text-align: left;\n    padding: 12px 16px 12px 16;\n    vertical-align: top;\n  }\n  .pipe-table th {\n    font-weight: 600;\n    text-align: center;\n    background: #f7f7f7;\n  }\n<\/style>\n\n<table class=\"pipe-table\">\n  <thead>\n    <tr>\n      <th>Symbol<\/th>\n      <th>Meaning<\/th>\n      <th>Unit\u00e9<\/th>\n  <\/tr>\n  <\/thead>\n  <tbody>\n    <tr>\n      <td>S<\/td>\n      <td>Selected protective conductor cross-sectional area<\/td>\n      <td>mm\u00b2<\/td>\n    <\/tr>\n    <tr>\n      <td>I<\/td>\n      <td>Relevant fault current, using the correct RMS value for AC work<\/td>\n      <td>A<\/td>\n    <\/tr>\n    <tr>\n      <td>t<\/td>\n      <td>Device operating time for that fault current<\/td>\n      <td>s<\/td>\n    <\/tr>\n    <tr>\n      <td>k<\/td>\n      <td>Factor set by conductor material and the applicable start and end temperatures<\/td>\n      <td>A\u221as\/mm\u00b2<\/td>\n    <\/tr>\n<\/tbody>\n<\/table>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-839cf08 elementor-widget elementor-widget-text-editor\" data-id=\"839cf08\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">The expression \u221a(I\u00b2t)\/k gives the calculated minimum area. You pick an available size at least that large, then finish the rest of the design checks. The current and the operating time together describe the fault energy. The material and temperatures set k.<\/span><\/p><p><span style=\"font-weight: 400;\">One assumption sits underneath all of this. Adiabatic heating treats the fault heat as if it all stays trapped in the conductor with none escaping. That short-duration method applies to fault times no longer than five seconds. Anything longer needs a different assessment.<\/span><\/p><p><span style=\"font-weight: 400;\">Read the five-second figure for what it is. It is the ceiling of this method, not a green light for every circuit to take five seconds to disconnect. \u00a0 <\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-7c6bdb4 elementor-hidden-mobile elementor-invisible elementor-widget elementor-widget-image\" data-id=\"7c6bdb4\" data-element_type=\"widget\" data-e-type=\"widget\" data-settings=\"{&quot;_animation&quot;:&quot;fadeInUp&quot;}\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<img fetchpriority=\"high\" decoding=\"async\" width=\"825\" height=\"550\" src=\"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current.png\" class=\"attachment-large size-large wp-image-23229\" alt=\"Relevant fault current\" srcset=\"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current.png 825w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current-300x200.png 300w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current-768x512.png 768w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current-18x12.png 18w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current-600x400.png 600w\" sizes=\"(max-width: 825px) 100vw, 825px\" \/>\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-018830d e-con-full elementor-hidden-desktop elementor-hidden-tablet e-flex e-con e-parent\" data-id=\"018830d\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t<div class=\"elementor-element elementor-element-da8b5b8 elementor-invisible elementor-widget elementor-widget-image\" data-id=\"da8b5b8\" data-element_type=\"widget\" data-e-type=\"widget\" data-settings=\"{&quot;_animation&quot;:&quot;fadeInUp&quot;}\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<img fetchpriority=\"high\" decoding=\"async\" width=\"825\" height=\"550\" src=\"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current.png\" class=\"attachment-large size-large wp-image-23229\" alt=\"Relevant fault current\" srcset=\"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current.png 825w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current-300x200.png 300w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current-768x512.png 768w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current-18x12.png 18w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Relevant-fault-current-600x400.png 600w\" sizes=\"(max-width: 825px) 100vw, 825px\" \/>\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-6fd6cc5 e-flex e-con-boxed e-con e-parent\" data-id=\"6fd6cc5\" data-element_type=\"container\" data-e-type=\"container\" data-settings=\"{&quot;background_background&quot;:&quot;classic&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-c65f8e7 elementor-widget elementor-widget-heading\" data-id=\"c65f8e7\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h2 class=\"elementor-heading-title elementor-size-default\">Choose the k Factor for the Real Conductor Arrangement\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-3f39b80 elementor-widget elementor-widget-text-editor\" data-id=\"3f39b80\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">The k factor stands for more than the metal in the conductor. Insulation, starting temperature, and the highest temperature the conductor may reach all feed into it.<\/span><\/p><p><span style=\"font-weight: 400;\">Two reference cases come up again and again:<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-b323184 elementor-widget elementor-widget-html\" data-id=\"b323184\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"html.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<style>\n  .pipe-table {\n    font-family: 'Inter', Arial, sans-serif;\n    font-size: 16px;\n    border-collapse: collapse;\n    color: #000;\n    width: 100%;\n    max-width: 800px;\n  }\n  .pipe-table th,\n  .pipe-table td {\n    text-align: left;\n    padding: 12px 16px 12px 16;\n    vertical-align: top;\n  }\n  .pipe-table th {\n    font-weight: 600;\n    text-align: center;\n    background: #f7f7f7;\n  }\n<\/style>\n\n<table class=\"pipe-table\">\n  <thead>\n    <tr>\n      <th>Insulation and Material<\/th>\n      <th>Initial Temperature<\/th>\n      <th>Final Temperature<\/th>\n      <th>k<\/th>\n  <\/tr>\n  <\/thead>\n  <tbody>\n    <tr>\n      <td>70\u00b0C thermoplastic (PVC), copper<\/td>\n      <td>70\u00b0C<\/td>\n      <td>160\u00b0C<\/td>\n      <td>115<\/td>\n    <\/tr>\n    <tr>\n      <td>90\u00b0C thermosetting (XLPE\/EPR), copper<\/td>\n      <td>90\u00b0C<\/td>\n      <td>250\u00b0C<\/td>\n      <td>143<\/td>\n    <\/tr>\n    \n<\/tbody>\n<\/table>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-7f0c492 elementor-widget elementor-widget-text-editor\" data-id=\"7f0c492\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">These values match BS 7671 Table 54.3. Check the correct protective-conductor table and its notes in your copy before you use them on a job.<\/span><\/p><p><span style=\"font-weight: 400;\">Here is the catch that trips people up. Both k values above assume the conductor is already sitting at the cable&#8217;s operating temperature, so 70\u00b0C or 90\u00b0C. A separately run conductor that does not carry load current may start nearer 30\u00b0C, which changes k and changes your answer. So do not read k off an insulation label, and do not reach for a bigger value just because it shrinks the result.<\/span><\/p><p><span style=\"font-weight: 400;\">The same warning covers material. Copper values do not carry across to steel armour or aluminium. Where armour acts as the protective conductor, its material, effective area, <a href=\"https:\/\/tradefoxapp.com\/fr\/cable-termination\/\">terminations<\/a>, and thermal conditions all need their own look.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-9c6cd6f e-flex e-con-boxed e-con e-parent\" data-id=\"9c6cd6f\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-a4a2d23 elementor-widget elementor-widget-heading\" data-id=\"a4a2d23\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h2 class=\"elementor-heading-title elementor-size-default\">Get Matching Fault-Current and Disconnection-Time Data\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-83bfce8 elementor-widget elementor-widget-text-editor\" data-id=\"83bfce8\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Work out the fault conditions that actually apply to the conductor and its protective arrangement. Then find out how the specified device behaves under those conditions. These two numbers have to belong to the same scenario.<\/span><\/p><p><span style=\"font-weight: 400;\">A few common mistakes turn a correct formula into an undersized cable:<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-ba37357 elementor-icon-list--layout-traditional elementor-list-item-link-full_width elementor-widget elementor-widget-icon-list\" data-id=\"ba37357\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"icon-list.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<ul class=\"elementor-icon-list-items\">\n\t\t\t\t\t\t\t<li class=\"elementor-icon-list-item\">\n\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-icon\">\n\t\t\t\t\t\t\t<svg aria-hidden=\"true\" class=\"e-font-icon-svg e-fas-long-arrow-alt-right\" viewbox=\"0 0 448 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path d=\"M313.941 216H12c-6.627 0-12 5.373-12 12v56c0 6.627 5.373 12 12 12h301.941v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.569 0-33.941l-86.059-86.059c-15.119-15.119-40.971-4.411-40.971 16.971V216z\"><\/path><\/svg>\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-text\"><b>The Rated Current Trap: <\/b>A device's rated current is not the fault current. A 32 A breaker rating is not an instruction to enter 32 A as I.<\/span>\n\t\t\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t\t\t<li class=\"elementor-icon-list-item\">\n\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-icon\">\n\t\t\t\t\t\t\t<svg aria-hidden=\"true\" class=\"e-font-icon-svg e-fas-long-arrow-alt-right\" viewbox=\"0 0 448 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path d=\"M313.941 216H12c-6.627 0-12 5.373-12 12v56c0 6.627 5.373 12 12 12h301.941v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.569 0-33.941l-86.059-86.059c-15.119-15.119-40.971-4.411-40.971 16.971V216z\"><\/path><\/svg>\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-text\"><b>The Operating-Time Trap: <\/b>Use the device's real time-current characteristic with its tolerance, or proper manufacturer data. Do not drop in a required maximum disconnection time and treat it as the device's actual operating time.<\/span>\n\t\t\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t\t\t<li class=\"elementor-icon-list-item\">\n\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-icon\">\n\t\t\t\t\t\t\t<svg aria-hidden=\"true\" class=\"e-font-icon-svg e-fas-long-arrow-alt-right\" viewbox=\"0 0 448 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path d=\"M313.941 216H12c-6.627 0-12 5.373-12 12v56c0 6.627 5.373 12 12 12h301.941v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.569 0-33.941l-86.059-86.059c-15.119-15.119-40.971-4.411-40.971 16.971V216z\"><\/path><\/svg>\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-text\"><b>The Let-Through Trap: <\/b>For fast current-limiting operation, use published let-through data. Multiplying prospective current squared by a guessed operating time can badly misstate the thermal stress.<\/span>\n\t\t\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t<\/ul>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-eb67729 elementor-widget elementor-widget-text-editor\" data-id=\"eb67729\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">A useful alternative form compares the stress against the withstand:<\/span><\/p><p><b>I\u00b2t \u2264 k\u00b2S\u00b2<\/b><\/p><p><span style=\"font-weight: 400;\">Here I\u00b2t is a thermal-stress quantity in A\u00b2s, often called let-through energy. It is not an energy in joules, and treating it as joules is a classic slip. When you use manufacturer I\u00b2t data, confirm the figure covers the total clearing duty for the conductor, not just a fuse&#8217;s pre-arcing stage. <\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-9c801d9 e-flex e-con-boxed e-con e-parent\" data-id=\"9c801d9\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-1ce91e8 elementor-widget elementor-widget-heading\" data-id=\"1ce91e8\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h2 class=\"elementor-heading-title elementor-size-default\">Worked Example 1: Calculate the Minimum CPC Size\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-30cef01 elementor-widget elementor-widget-text-editor\" data-id=\"30cef01\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Say a design assessment has fixed these inputs for a particular fault:<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-4fb5dc4 elementor-icon-list--layout-traditional elementor-list-item-link-full_width elementor-widget elementor-widget-icon-list\" data-id=\"4fb5dc4\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"icon-list.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<ul class=\"elementor-icon-list-items\">\n\t\t\t\t\t\t\t<li class=\"elementor-icon-list-item\">\n\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-icon\">\n\t\t\t\t\t\t\t<svg aria-hidden=\"true\" class=\"e-font-icon-svg e-fas-long-arrow-alt-right\" viewbox=\"0 0 448 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path d=\"M313.941 216H12c-6.627 0-12 5.373-12 12v56c0 6.627 5.373 12 12 12h301.941v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.569 0-33.941l-86.059-86.059c-15.119-15.119-40.971-4.411-40.971 16.971V216z\"><\/path><\/svg>\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-text\"><b>Fault Current: <\/b>I = 500 A<\/span>\n\t\t\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t\t\t<li class=\"elementor-icon-list-item\">\n\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-icon\">\n\t\t\t\t\t\t\t<svg aria-hidden=\"true\" class=\"e-font-icon-svg e-fas-long-arrow-alt-right\" viewbox=\"0 0 448 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path d=\"M313.941 216H12c-6.627 0-12 5.373-12 12v56c0 6.627 5.373 12 12 12h301.941v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.569 0-33.941l-86.059-86.059c-15.119-15.119-40.971-4.411-40.971 16.971V216z\"><\/path><\/svg>\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-text\"><b>Operating Time: <\/b>t = 0.20 s<\/span>\n\t\t\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t\t\t<li class=\"elementor-icon-list-item\">\n\t\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-icon\">\n\t\t\t\t\t\t\t<svg aria-hidden=\"true\" class=\"e-font-icon-svg e-fas-long-arrow-alt-right\" viewbox=\"0 0 448 512\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\"><path d=\"M313.941 216H12c-6.627 0-12 5.373-12 12v56c0 6.627 5.373 12 12 12h301.941v46.059c0 21.382 25.851 32.09 40.971 16.971l86.059-86.059c9.373-9.373 9.373-24.569 0-33.941l-86.059-86.059c-15.119-15.119-40.971-4.411-40.971 16.971V216z\"><\/path><\/svg>\t\t\t\t\t\t<\/span>\n\t\t\t\t\t\t\t\t\t\t<span class=\"elementor-icon-list-text\"><b>k Factor: <\/b>115 (copper, 70\u00b0C PVC)<\/span>\n\t\t\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t<\/ul>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-2f72977 elementor-widget elementor-widget-text-editor\" data-id=\"2f72977\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><b>Step one, calculate I\u00b2t:<\/b><\/p><p><span style=\"font-weight: 400;\">I\u00b2t = 500\u00b2 \u00d7 0.20 = 50,000 A\u00b2s<\/span><\/p><p><b>Step two, take the root and divide by k:<\/b><\/p><p><span style=\"font-weight: 400;\">S minimum = \u221a50,000 \/ 115 = 1.944 mm\u00b2<\/span><\/p><p><span style=\"font-weight: 400;\">The next standard size above that result is 2.5 mm\u00b2. A 1.5 mm\u00b2 conductor does not clear this thermal bar.<\/span><\/p><p><b>Step three, check the proposed size.<\/b><span style=\"font-weight: 400;\"> For a 2.5 mm\u00b2 conductor:<\/span><\/p><p><span style=\"font-weight: 400;\">k\u00b2S\u00b2 = 115\u00b2 \u00d7 2.5\u00b2 = 82,656.25 A\u00b2s<\/span><\/p><p><span style=\"font-weight: 400;\">The fault duty of 50,000 A\u00b2s sits below that withstand, so 2.5 mm\u00b2 passes the thermal check for these inputs. That is not the same as saying 2.5 mm\u00b2 is the final answer. If a separate copper conductor runs into a 4 mm\u00b2 mechanical minimum, that larger figure still wins.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-7e7a426 e-flex e-con-boxed e-con e-parent\" data-id=\"7e7a426\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-34c7700 elementor-widget elementor-widget-heading\" data-id=\"34c7700\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h2 class=\"elementor-heading-title elementor-size-default\">Worked Example 2: Use Protective-Device Let-Through Data\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-92dd134 elementor-widget elementor-widget-text-editor\" data-id=\"92dd134\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Now assume device data give a total clearing I\u00b2t of 45,000 A\u00b2s for the relevant fault duty. The arrangement again needs k = 115.<\/span><\/p><p><span style=\"font-weight: 400;\">You can feed the published I\u00b2t straight into the equation:<\/span><\/p><p><span style=\"font-weight: 400;\">S minimum = \u221a45,000 \/ 115 = 1.845 mm\u00b2<\/span><\/p><p><span style=\"font-weight: 400;\">A 2.5 mm\u00b2 conductor clears the calculated thermal minimum. But check an existing 1.5 mm\u00b2 conductor before you trust it:<\/span><\/p><p><span style=\"font-weight: 400;\">k\u00b2S\u00b2 = 115\u00b2 \u00d7 1.5\u00b2 = 29,756.25 A\u00b2s<\/span><\/p><p><span style=\"font-weight: 400;\">The duty of 45,000 A\u00b2s is bigger than 29,756.25 A\u00b2s, so the 1.5 mm\u00b2 conductor fails this thermal check. The 2.5 mm\u00b2 conductor holds its 82,656.25 A\u00b2s withstand, so it passes.<\/span><\/p><p><span style=\"font-weight: 400;\">That 45,000 A\u00b2s figure is made up for the example. On a real job, keep the exact device reference and the manufacturer&#8217;s data you used, and confirm the data apply to the prospective fault level, voltage, and protective arrangement. Breaking capacity alone tells you nothing about let-through performance.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-15635d5 e-flex e-con-boxed e-con e-parent\" data-id=\"15635d5\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-e6cc434 elementor-widget elementor-widget-heading\" data-id=\"e6cc434\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h2 class=\"elementor-heading-title elementor-size-default\">Why a Thermal Pass Does Not Finish the Job\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-3fcbf8d elementor-widget elementor-widget-text-editor\" data-id=\"3fcbf8d\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">A conductor can sail through the adiabatic equation and still be wrong for the installation. Three checks sit alongside it.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-3ad8394 elementor-widget elementor-widget-heading\" data-id=\"3ad8394\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Check the Mechanical Minimum Sizes\n<\/h3>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-5f2f3e7 elementor-widget elementor-widget-text-editor\" data-id=\"5f2f3e7\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Where a copper protective conductor is neither part of a cable nor in a common enclosure with the line conductor, the minimum is generally 2.5 mm\u00b2 with mechanical protection and 4 mm\u00b2 without it. These figures come from Regulation 543.1.1, so confirm the exact conditions in BS 7671. Do not stamp them onto every CPC that sits inside a cable.<\/span><\/p><p><span style=\"font-weight: 400;\">This matters most when the maths spits out a tiny number. Good thermal withstand does nothing to stop a physically fragile conductor from being snapped during service.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-d6f4016 elementor-hidden-mobile elementor-invisible elementor-widget elementor-widget-image\" data-id=\"d6f4016\" data-element_type=\"widget\" data-e-type=\"widget\" data-settings=\"{&quot;_animation&quot;:&quot;fadeInUp&quot;}\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<img decoding=\"async\" width=\"825\" height=\"550\" src=\"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor.png\" class=\"attachment-large size-large wp-image-23239\" alt=\"Overhead power line insulators and fuse cut-outs mounted on utility poles\" srcset=\"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor.png 825w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor-300x200.png 300w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor-768x512.png 768w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor-18x12.png 18w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor-600x400.png 600w\" sizes=\"(max-width: 825px) 100vw, 825px\" \/>\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-44424b6 e-con-full elementor-hidden-desktop elementor-hidden-tablet e-flex e-con e-parent\" data-id=\"44424b6\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t<div class=\"elementor-element elementor-element-bc9b019 elementor-invisible elementor-widget elementor-widget-image\" data-id=\"bc9b019\" data-element_type=\"widget\" data-e-type=\"widget\" data-settings=\"{&quot;_animation&quot;:&quot;fadeInUp&quot;}\" data-widget_type=\"image.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<img decoding=\"async\" width=\"825\" height=\"550\" src=\"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor.png\" class=\"attachment-large size-large wp-image-23239\" alt=\"Overhead power line insulators and fuse cut-outs mounted on utility poles\" srcset=\"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor.png 825w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor-300x200.png 300w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor-768x512.png 768w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor-18x12.png 18w, https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/copper-protective-conductor-600x400.png 600w\" sizes=\"(max-width: 825px) 100vw, 825px\" \/>\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-66073fc e-flex e-con-boxed e-con e-parent\" data-id=\"66073fc\" data-element_type=\"container\" data-e-type=\"container\" data-settings=\"{&quot;background_background&quot;:&quot;classic&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-dde5b41 elementor-widget elementor-widget-heading\" data-id=\"dde5b41\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Verify Loop Impedance and Automatic Disconnection\n<\/h3>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-83b3475 elementor-widget elementor-widget-text-editor\" data-id=\"83b3475\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">The conductor you pick feeds into earth fault loop impedance. Change its size and you change the fault current, which changes the device&#8217;s operating time.<\/span><\/p><p><span style=\"font-weight: 400;\">So after you change a conductor, go back and re-run the inputs and the disconnection assessment. A fault current worked out for a different arrangement is not safe to reuse without checking it still holds.<\/span><\/p><p><span style=\"font-weight: 400;\">One more point here. An RCD&#8217;s rated residual operating current, say 30 mA, is its residual-current sensitivity. It is not the earth-fault current for a conductor thermal calculation. Where residual and overcurrent protection work together, assess the actual arrangement.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-981f2ea elementor-widget elementor-widget-heading\" data-id=\"981f2ea\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Assess the Full Range of Fault Conditions\n<\/h3>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-4a3a823 elementor-widget elementor-widget-text-editor\" data-id=\"4a3a823\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">A high fault current may clear fast, while a lower current can hang on longer. The governing thermal duty depends on the device characteristics and the circuit conditions, not on a single convenient number.<\/span><\/p><p><span style=\"font-weight: 400;\">So look at the credible range of fault currents and the relevant locations. Do not assume the slowest disconnection always gives the worst I\u00b2t, and do not let one fault-current value stand in for the whole circuit. <a href=\"https:\/\/tradefoxapp.com\/fr\/earth-bonding\/\">Main protective bonding<\/a>, PEN conductors, and buried earthing conductors carry extra requirements of their own, and these CPC examples do not cover them.<\/span><\/p><p><span style=\"font-weight: 400;\">Want to build your electrical skills? <\/span><strong><a href=\"https:\/\/learn.tradefoxapp.com\/\">Tradefox <\/a><\/strong><span style=\"font-weight: 400;\">offers guided electrician simulations, including safe isolation, that you can revisit at your own pace. Explore the training and keep practising the skills used in electrical work. \u00a0 <\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-368c48a elementor-widget elementor-widget-text-editor\" data-id=\"368c48a\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<div class=\"tf-sim\"><div class=\"tf-sim-t\"><p class=\"tf-sim-k\">Practise this in a simulation<\/p><p class=\"tf-sim-p\">Practise protective conductor sizing and fault calculations in a safe virtual environment.<\/p><\/div><a class=\"tf-sim-btn\" href=\"https:\/\/tradefoxapp.com\/fr\/electric-courses\/\">Try the Electrician course<\/a><\/div>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-3c600ea e-con-full e-flex e-con e-parent\" data-id=\"3c600ea\" data-element_type=\"container\" data-e-type=\"container\" data-settings=\"{&quot;background_background&quot;:&quot;classic&quot;}\">\n\t\t\t\t<div class=\"elementor-element elementor-element-a241741 elementor-widget elementor-widget-heading\" data-id=\"a241741\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<h2 class=\"elementor-heading-title elementor-size-default\">Record a Calculation Another Electrician Can Verify\n<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-b4aa94d elementor-widget elementor-widget-text-editor\" data-id=\"b4aa94d\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">A design record earns its keep when someone else can follow it. A good one names the conductor material, insulation, installation arrangement, selected area, and the source of the k factor. It also captures the fault-current basis and the exact device data.<\/span><\/p><p><span style=\"font-weight: 400;\">Keep the thermal result apart from the automatic disconnection result. Include the mechanical minimum check and any conditions tied to manufacturer coordination data.<\/span><\/p><p><span style=\"font-weight: 400;\">Before certification, confirm through inspection and testing that what got installed matches what you designed. A swapped protective device, a rerouted cable, or a different conductor arrangement can undo an earlier result without anyone noticing.<\/span><\/p><p><span style=\"font-weight: 400;\">The adiabatic equation earns its trust when every input traces back to the real design. The final conductor has to satisfy the thermal calculation and the other requirements together, with the evidence kept in the installation records.\u00a0 \u00a0 \u00a0 <\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>","protected":false},"excerpt":{"rendered":"<p>The adiabatic equation checks one thing: whether a protective conductor has enough cross-sectional area to survive the heat of a fault until the protective device cuts the supply. It is a thermal test, nothing more. A conductor that passes it still has to satisfy automatic disconnection, mechanical protection, and the other conditions that apply to [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":23219,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"no-sidebar","site-content-layout":"","ast-site-content-layout":"full-width-container","site-content-style":"unboxed","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"default","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"set","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"_glsr_average":0,"_glsr_ranking":0,"_glsr_reviews":0,"footnotes":""},"categories":[1],"tags":[578,2750,2742,2752,2743,2748,2751,1241,2747,2236,2745,2744,2234,270,2746,2749],"class_list":["post-23215","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-electrical","tag-automatic-disconnection","tag-conductor-material","tag-conductor-sizing","tag-copper-conductor","tag-cross-sectional-area","tag-current-limiting","tag-fault-conditions","tag-fault-current","tag-let-through-energy","tag-loop-impedance","tag-mechanical-protection","tag-operating-time","tag-protective-conductor","tag-protective-device","tag-tabulated-route","tag-thermal-withstand"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples<\/title>\n<meta name=\"description\" content=\"Learn how the adiabatic equation sizes a protective conductor, guides fault-data selection and checks thermal withstand with other requirements.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/tradefoxapp.com\/fr\/adiabatic-equation\/\" \/>\n<meta property=\"og:locale\" content=\"fr_FR\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples\" \/>\n<meta property=\"og:description\" content=\"Learn how the adiabatic equation sizes a protective conductor, guides fault-data selection and checks thermal withstand with other requirements.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/tradefoxapp.com\/fr\/adiabatic-equation\/\" \/>\n<meta property=\"og:site_name\" content=\"Tradefox | Construction Training Online\" \/>\n<meta property=\"article:publisher\" content=\"https:\/\/www.facebook.com\/profile.php?id=61561067514381\" \/>\n<meta property=\"article:published_time\" content=\"2026-10-30T06:08:33+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-11-06T16:44:40+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Protective-Conductor.png\" \/>\n\t<meta property=\"og:image:width\" content=\"825\" \/>\n\t<meta property=\"og:image:height\" content=\"550\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"author\" content=\"Tradefoxapp\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:creator\" content=\"@Tradefox_app\" \/>\n<meta name=\"twitter:site\" content=\"@Tradefox_app\" \/>\n<meta name=\"twitter:label1\" content=\"\u00c9crit par\" \/>\n\t<meta name=\"twitter:data1\" content=\"Tradefoxapp\" \/>\n\t<meta name=\"twitter:label2\" content=\"Dur\u00e9e de lecture estim\u00e9e\" \/>\n\t<meta name=\"twitter:data2\" content=\"9 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/\"},\"author\":{\"name\":\"Tradefoxapp\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#\\\/schema\\\/person\\\/cad3ebca9b0c18ad17cdf9222b9e4d06\"},\"headline\":\"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples\",\"datePublished\":\"2026-10-30T06:08:33+00:00\",\"dateModified\":\"2026-11-06T16:44:40+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/\"},\"wordCount\":1706,\"publisher\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#organization\"},\"image\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/tradefoxapp.com\\\/wp-content\\\/uploads\\\/2026\\\/10\\\/Protective-Conductor.png\",\"keywords\":[\"automatic disconnection\",\"conductor material\",\"conductor sizing\",\"copper conductor\",\"cross-sectional area\",\"current-limiting\",\"fault conditions\",\"fault current\",\"let-through energy\",\"loop impedance\",\"mechanical protection\",\"operating time\",\"protective conductor\",\"protective device\",\"tabulated route\",\"thermal withstand\"],\"articleSection\":[\"Electrical\"],\"inLanguage\":\"fr-FR\"},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/\",\"url\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/\",\"name\":\"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#website\"},\"primaryImageOfPage\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/#primaryimage\"},\"image\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/tradefoxapp.com\\\/wp-content\\\/uploads\\\/2026\\\/10\\\/Protective-Conductor.png\",\"datePublished\":\"2026-10-30T06:08:33+00:00\",\"dateModified\":\"2026-11-06T16:44:40+00:00\",\"description\":\"Learn how the adiabatic equation sizes a protective conductor, guides fault-data selection and checks thermal withstand with other requirements.\",\"breadcrumb\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/#breadcrumb\"},\"inLanguage\":\"fr-FR\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/\"]}]},{\"@type\":\"ImageObject\",\"inLanguage\":\"fr-FR\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/#primaryimage\",\"url\":\"https:\\\/\\\/tradefoxapp.com\\\/wp-content\\\/uploads\\\/2026\\\/10\\\/Protective-Conductor.png\",\"contentUrl\":\"https:\\\/\\\/tradefoxapp.com\\\/wp-content\\\/uploads\\\/2026\\\/10\\\/Protective-Conductor.png\",\"width\":825,\"height\":550,\"caption\":\"Protective Conductor\"},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/adiabatic-equation\\\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Home\",\"item\":\"https:\\\/\\\/tradefoxapp.com\\\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#website\",\"url\":\"https:\\\/\\\/tradefoxapp.com\\\/\",\"name\":\"Tradefox | Construction Training Online\",\"description\":\"\",\"publisher\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#organization\"},\"potentialAction\":[{\"@type\":\"SearchAction\",\"target\":{\"@type\":\"EntryPoint\",\"urlTemplate\":\"https:\\\/\\\/tradefoxapp.com\\\/?s={search_term_string}\"},\"query-input\":{\"@type\":\"PropertyValueSpecification\",\"valueRequired\":true,\"valueName\":\"search_term_string\"}}],\"inLanguage\":\"fr-FR\"},{\"@type\":\"Organization\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#organization\",\"name\":\"Tradefox | Construction Training Online\",\"url\":\"https:\\\/\\\/tradefoxapp.com\\\/\",\"logo\":{\"@type\":\"ImageObject\",\"inLanguage\":\"fr-FR\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#\\\/schema\\\/logo\\\/image\\\/\",\"url\":\"https:\\\/\\\/tradefoxapp.com\\\/wp-content\\\/uploads\\\/2026\\\/03\\\/logo-transperent.png\",\"contentUrl\":\"https:\\\/\\\/tradefoxapp.com\\\/wp-content\\\/uploads\\\/2026\\\/03\\\/logo-transperent.png\",\"width\":1024,\"height\":208,\"caption\":\"Tradefox | Construction Training Online\"},\"image\":{\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#\\\/schema\\\/logo\\\/image\\\/\"},\"sameAs\":[\"https:\\\/\\\/www.facebook.com\\\/profile.php?id=61561067514381\",\"https:\\\/\\\/x.com\\\/Tradefox_app\",\"https:\\\/\\\/www.instagram.com\\\/tradefoxapp\\\/\",\"https:\\\/\\\/www.linkedin.com\\\/company\\\/tradefox-ltd\\\/\"]},{\"@type\":\"Person\",\"@id\":\"https:\\\/\\\/tradefoxapp.com\\\/#\\\/schema\\\/person\\\/cad3ebca9b0c18ad17cdf9222b9e4d06\",\"name\":\"Tradefoxapp\",\"sameAs\":[\"https:\\\/\\\/tradefoxapp.com\"]}]}<\/script>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples","description":"Learn how the adiabatic equation sizes a protective conductor, guides fault-data selection and checks thermal withstand with other requirements.","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/tradefoxapp.com\/fr\/adiabatic-equation\/","og_locale":"fr_FR","og_type":"article","og_title":"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples","og_description":"Learn how the adiabatic equation sizes a protective conductor, guides fault-data selection and checks thermal withstand with other requirements.","og_url":"https:\/\/tradefoxapp.com\/fr\/adiabatic-equation\/","og_site_name":"Tradefox | Construction Training Online","article_publisher":"https:\/\/www.facebook.com\/profile.php?id=61561067514381","article_published_time":"2026-10-30T06:08:33+00:00","article_modified_time":"2026-11-06T16:44:40+00:00","og_image":[{"width":825,"height":550,"url":"http:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Protective-Conductor.png","type":"image\/png"}],"author":"Tradefoxapp","twitter_card":"summary_large_image","twitter_creator":"@Tradefox_app","twitter_site":"@Tradefox_app","twitter_misc":{"\u00c9crit par":"Tradefoxapp","Dur\u00e9e de lecture estim\u00e9e":"9 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/#article","isPartOf":{"@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/"},"author":{"name":"Tradefoxapp","@id":"https:\/\/tradefoxapp.com\/#\/schema\/person\/cad3ebca9b0c18ad17cdf9222b9e4d06"},"headline":"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples","datePublished":"2026-10-30T06:08:33+00:00","dateModified":"2026-11-06T16:44:40+00:00","mainEntityOfPage":{"@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/"},"wordCount":1706,"publisher":{"@id":"https:\/\/tradefoxapp.com\/#organization"},"image":{"@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/#primaryimage"},"thumbnailUrl":"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Protective-Conductor.png","keywords":["automatic disconnection","conductor material","conductor sizing","copper conductor","cross-sectional area","current-limiting","fault conditions","fault current","let-through energy","loop impedance","mechanical protection","operating time","protective conductor","protective device","tabulated route","thermal withstand"],"articleSection":["Electrical"],"inLanguage":"fr-FR"},{"@type":"WebPage","@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/","url":"https:\/\/tradefoxapp.com\/adiabatic-equation\/","name":"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples","isPartOf":{"@id":"https:\/\/tradefoxapp.com\/#website"},"primaryImageOfPage":{"@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/#primaryimage"},"image":{"@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/#primaryimage"},"thumbnailUrl":"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Protective-Conductor.png","datePublished":"2026-10-30T06:08:33+00:00","dateModified":"2026-11-06T16:44:40+00:00","description":"Learn how the adiabatic equation sizes a protective conductor, guides fault-data selection and checks thermal withstand with other requirements.","breadcrumb":{"@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/#breadcrumb"},"inLanguage":"fr-FR","potentialAction":[{"@type":"ReadAction","target":["https:\/\/tradefoxapp.com\/adiabatic-equation\/"]}]},{"@type":"ImageObject","inLanguage":"fr-FR","@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/#primaryimage","url":"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Protective-Conductor.png","contentUrl":"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/10\/Protective-Conductor.png","width":825,"height":550,"caption":"Protective Conductor"},{"@type":"BreadcrumbList","@id":"https:\/\/tradefoxapp.com\/adiabatic-equation\/#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/tradefoxapp.com\/"},{"@type":"ListItem","position":2,"name":"Adiabatic Equation: Understanding Protective Conductor Sizing Through Worked Examples"}]},{"@type":"WebSite","@id":"https:\/\/tradefoxapp.com\/#website","url":"https:\/\/tradefoxapp.com\/","name":"Tradefox | Formation en construction en ligne","description":"","publisher":{"@id":"https:\/\/tradefoxapp.com\/#organization"},"potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/tradefoxapp.com\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"fr-FR"},{"@type":"Organization","@id":"https:\/\/tradefoxapp.com\/#organization","name":"Tradefox | Formation en construction en ligne","url":"https:\/\/tradefoxapp.com\/","logo":{"@type":"ImageObject","inLanguage":"fr-FR","@id":"https:\/\/tradefoxapp.com\/#\/schema\/logo\/image\/","url":"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/03\/logo-transperent.png","contentUrl":"https:\/\/tradefoxapp.com\/wp-content\/uploads\/2026\/03\/logo-transperent.png","width":1024,"height":208,"caption":"Tradefox | Construction Training Online"},"image":{"@id":"https:\/\/tradefoxapp.com\/#\/schema\/logo\/image\/"},"sameAs":["https:\/\/www.facebook.com\/profile.php?id=61561067514381","https:\/\/x.com\/Tradefox_app","https:\/\/www.instagram.com\/tradefoxapp\/","https:\/\/www.linkedin.com\/company\/tradefox-ltd\/"]},{"@type":"Person","@id":"https:\/\/tradefoxapp.com\/#\/schema\/person\/cad3ebca9b0c18ad17cdf9222b9e4d06","name":"Tradefoxapp","sameAs":["https:\/\/tradefoxapp.com"]}]}},"_links":{"self":[{"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/posts\/23215","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/comments?post=23215"}],"version-history":[{"count":62,"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/posts\/23215\/revisions"}],"predecessor-version":[{"id":25117,"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/posts\/23215\/revisions\/25117"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/media\/23219"}],"wp:attachment":[{"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/media?parent=23215"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/categories?post=23215"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/tradefoxapp.com\/fr\/wp-json\/wp\/v2\/tags?post=23215"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}