{"id":48051,"date":"2025-07-01T10:10:44","date_gmt":"2025-07-01T09:10:44","guid":{"rendered":"https:\/\/www.gt-engineering.it\/?post_type=approfondimento&p=48051"},"modified":"2025-07-04T18:09:17","modified_gmt":"2025-07-04T17:09:17","slug":"the-let-through-energy-for-protective-devices","status":"publish","type":"approfondimento","link":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/","title":{"rendered":"The let through energy for protective devices"},"content":{"rendered":"\n\n \n \n \n
\n\n \n
\n \n
\n \n \n

\n The doubt <\/h2>\n <\/div>\n <\/div>\n \n \n \r\n\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n <\/picture>\r\n\r\n \r\n\r\n \r\n \r\n <\/div>\n \n
\n \n
\n

What’s the maximum current a cable can withstand before it deteriorates or gets on fire?<\/p>\n

What’s the parameter to consider? The current or the energy that goes throught the cable?<\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n
\n \n
\n \n \n

\n Consideration 1: The electric energy <\/h2>\n <\/div>\n <\/div>\n \n \n
\n \n
\n

Electrical components have a certain lifetime, which is affected by the current and, in general, the energy that flows through them.<\/p>\n

The electrical energy defined by the english physician James Prescott Joule can be calculated using the following formula:<\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n \n \n \r\n\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n <\/picture>\r\n\r\n \r\n\r\n \r\n \r\n <\/div>\n \n
\n \n
\n

Q= I 2*R<\/strong>, from which one obtains I 2*t=Q\/R,<\/strong> that is from the current passing through the resistance in the unit of time [A^2*s],<\/strong><\/em> analogue as [Joule\/Ohm].<\/strong><\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n \n \n
\n \n
\n

If we take for example a short-circuit current on a line with a value of 10kA, a Circuit Breaker will protect it: a modular circuit breaker (MCB) normally opens in 30ms. The cable, during that time, will be subject to a let through teorical energy of I^2*t=3*10^6[A^2*s].<\/strong><\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n
\n \n
\n \n \n

\n Consideration 2:The prospective short-circuit current <\/h2>\n <\/div>\n <\/div>\n \n \n \r\n\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n <\/picture>\r\n\r\n \r\n\r\n \r\n \r\n <\/div>\n \n
\n \n
\n

This value of the let through energy would be correct if we won’t consider the limitation properties of Circuit Breakers or Fuses, which, limit the energy during the short-circuit phenomena.<\/p>\n

Important to consider for the case presented is the concept of the prospective short-circuit current<\/strong>, which is the current that would flow in a circuit if all devices would be replaced by a conductor.<\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n \n \n
\n \n
\n

[IEC 60947-1:2008 ]: <\/i>2.5.5 prospective current <\/i><\/b>(of a circuit and with respect to a switching device or a fuse) <\/i><\/p>\n

Current\u00a0 that\u00a0 would\u00a0 flow\u00a0 in\u00a0 the\u00a0 circuit\u00a0 if\u00a0 each\u00a0 pole\u00a0 of\u00a0 the\u00a0 switching\u00a0 device\u00a0 or\u00a0 the\u00a0 fuse\u00a0 were\u00a0 replaced by a conductor of negligible impedance.<\/i><\/p><\/blockquote>\n

In reality, like we see in the chart, the real short-circuit current its smaller that the perspectuve one. The reason is because the opening of the protection doesn’t determine the immediate interruption of the current. The current continues to flow even when the CB opens; and it flows through the air inside the CB (electrical-arc).<\/p>\n

For that reason, the energy that flows through the CB during the Short circuit clearance is much less than the theoretical one.<\/p>\n

 <\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n
\n \n
\n \n \n

\n Consideration 3: The let through energy <\/h2>\n <\/div>\n <\/div>\n \n \n \n \n
<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n \n \n \r\n\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n <\/picture>\r\n\r\n \r\n\r\n \r\n \r\n <\/div>\n \n
\n \n
\n

We assume a 50Hz frequency, which implies a current with a period of 20ms.<\/p>\n

The thermal and dynamic stresses that occur during the short circuit are proportional to I^2*t, where I^2 is the square of the short circuit current and t is the time taken by the CB to extinguish it.<\/p>\n

During the clearance of the short circuit, energy is developed which is let through by the short circuit protection device, and it transformed into heat. It is defined as specific let-through energy, or the energy dissipated during a short circuit, which can be calculated based on the intensity of the short circuit current and the time for which it persists, it can also be called the Joule integral.<\/strong><\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n
\n \n
\n \n \n

\n Consideration 4: The Cable Ampacity or Current Carring Capacity <\/h2>\n <\/div>\n <\/div>\n \n \n
\n \n
\n

Regarding the verification of short circuit currents, supposing we use a protection devices, the energy that the cable can withstand is given by the product of the square of the cable section, expressed in mm2, and the square of K, i.e. a constant, which takes into account the parameters of resistivity, temperature coefficient and thermal tolerability of the material, which in the case of PVC is = 115. The energy passing through the cables must be greater than that passed through the protection, if the cable is to be properly protected.<\/p>\n

If we take for example a PVC cable with a section of 10mmq, its maximum let through energy will be 1 322 500\u00a0 A^2*s. Refering to the chart, considering a Circuit Breaker of 32 A and supposing a short-circuit of 10kA, the energy that the protection lets through is near the value of 60K A^2*s, which is smaller than the one that the cable can withstand.<\/p>\n

The formula used assume that the cable doesen’t have an exchange of heat with the external environment (adiabatic phenomena).<\/p>\n

IEC 60364-4-43] <\/i>431.5.4 Characteristics of short-circuit protective devices [\u2026]<\/i><\/b><\/p>\n

431.5.4.2 For cables and insulated conductors, all current caused by a short-circuit occurring at any point of the circuit shall be interrupted in a time <\/i>not exceeding that which brings the insulation of the conductors to the permitted limit temperature<\/i><\/b>.<\/i><\/p>\n

For operating times of protective devices <\/i>< 0,1 s <\/i><\/b>where asymmetry of the current is of importance and for current-limiting devices k^<\/i>2*<\/i>S^<\/i>2<\/i>\u00a0 shall be greater than the value of the let-through energy I^<\/i>2*<\/i>t quoted by the manufacturer of the protective device.<\/i><\/p>\n

I^2*t \u2264 K^2*S^2<\/strong><\/em><\/p><\/blockquote>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n\n\n\n\n \n \n \n
\n\n \n
\n \n
\n \n \n

\n Conclusions <\/h2>\n <\/div>\n <\/div>\n \n \n
\n \n
\n

The limitation of the current in a cable isn’t the only parameter to use to protect it against fires. A protection against overload protects the cable against that phenomena, but not against a short-circuit. In that case the current is so high and unexpected that a protection against overload cannot assure that the cable will be protected. The electrical standards address this problem by analysing the phenomena in terms of Let-Though Energy.<\/strong><\/p>\n

Important, because it plays a role in favor of safety, is the concept of Let-Through Energy of a Circuit Breaker or of a Fuse, considering that most electrical protections are actually energy limiters.<\/strong><\/p>\n<\/div> <\/div>\n \n \n

<\/div>\n \n\n <\/div>\n <\/section>\n\n \n","protected":false},"excerpt":{"rendered":"

Like we well know electrical components have a lifetime, which is calculated considering the energy that trasverse them. The electrical energy like expressed from the english physician James Prescott Joule its explained like it follows:<\/p>\n","protected":false},"author":2,"featured_media":47829,"parent":0,"menu_order":10,"template":"","meta":{"_acf_changed":false,"footnotes":""},"tipologia_approfondimento":[51],"class_list":["post-48051","approfondimento","type-approfondimento","status-publish","has-post-thumbnail","hentry","tipologia_approfondimento-electrical-hazard-and-arc-flash-mitigation"],"acf":{"sottotitolo":"","allegati":null},"yoast_head":"\nThe let through energy for protective devices - Gt-Engineering<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The let through energy for protective devices - Gt-Engineering\" \/>\n<meta property=\"og:description\" content=\"Like we well know electrical components have a lifetime, which is calculated considering the energy that trasverse them. The electrical energy like expressed from the english physician James Prescott Joule its explained like it follows:\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/\" \/>\n<meta property=\"og:site_name\" content=\"Gt-Engineering\" \/>\n<meta property=\"article:modified_time\" content=\"2025-07-04T17:09:17+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2025\/07\/Senza-titolo.png\" \/>\n\t<meta property=\"og:image:width\" content=\"275\" \/>\n\t<meta property=\"og:image:height\" content=\"183\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data1\" content=\"1 minute\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/\",\"url\":\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/\",\"name\":\"The let through energy for protective devices - Gt-Engineering\",\"isPartOf\":{\"@id\":\"https:\/\/www.gt-engineering.it\/en\/#website\"},\"primaryImageOfPage\":{\"@id\":\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#primaryimage\"},\"image\":{\"@id\":\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#primaryimage\"},\"thumbnailUrl\":\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2025\/07\/Senza-titolo.png\",\"datePublished\":\"2025-07-01T09:10:44+00:00\",\"dateModified\":\"2025-07-04T17:09:17+00:00\",\"breadcrumb\":{\"@id\":\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#breadcrumb\"},\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"ReadAction\",\"target\":[\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/\"]}]},{\"@type\":\"ImageObject\",\"inLanguage\":\"en-US\",\"@id\":\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#primaryimage\",\"url\":\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2025\/07\/Senza-titolo.png\",\"contentUrl\":\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2025\/07\/Senza-titolo.png\",\"width\":275,\"height\":183},{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#breadcrumb\",\"itemListElement\":[{\"@type\":\"ListItem\",\"position\":1,\"name\":\"Home\",\"item\":\"https:\/\/www.gt-engineering.it\/en\/\"},{\"@type\":\"ListItem\",\"position\":2,\"name\":\"Insights\",\"item\":\"https:\/\/www.gt-engineering.it\/en\/approfondimenti\/\"},{\"@type\":\"ListItem\",\"position\":3,\"name\":\"Electrical Safety and Arc Flash Mitigation\",\"item\":\"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/\"},{\"@type\":\"ListItem\",\"position\":4,\"name\":\"The let through energy for protective devices\"}]},{\"@type\":\"WebSite\",\"@id\":\"https:\/\/www.gt-engineering.it\/en\/#website\",\"url\":\"https:\/\/www.gt-engineering.it\/en\/\",\"name\":\"Gt-Engineering\",\"description\":\"bizonweb\",\"potentialAction\":[{\"@type\":\"SearchAction\",\"target\":{\"@type\":\"EntryPoint\",\"urlTemplate\":\"https:\/\/www.gt-engineering.it\/en\/?s={search_term_string}\"},\"query-input\":{\"@type\":\"PropertyValueSpecification\",\"valueRequired\":true,\"valueName\":\"search_term_string\"}}],\"inLanguage\":\"en-US\"}]}<\/script>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The let through energy for protective devices - Gt-Engineering","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:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/","og_locale":"en_US","og_type":"article","og_title":"The let through energy for protective devices - Gt-Engineering","og_description":"Like we well know electrical components have a lifetime, which is calculated considering the energy that trasverse them. The electrical energy like expressed from the english physician James Prescott Joule its explained like it follows:","og_url":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/","og_site_name":"Gt-Engineering","article_modified_time":"2025-07-04T17:09:17+00:00","og_image":[{"width":275,"height":183,"url":"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2025\/07\/Senza-titolo.png","type":"image\/png"}],"twitter_card":"summary_large_image","twitter_misc":{"Est. reading time":"1 minute"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"WebPage","@id":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/","url":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/","name":"The let through energy for protective devices - Gt-Engineering","isPartOf":{"@id":"https:\/\/www.gt-engineering.it\/en\/#website"},"primaryImageOfPage":{"@id":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#primaryimage"},"image":{"@id":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#primaryimage"},"thumbnailUrl":"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2025\/07\/Senza-titolo.png","datePublished":"2025-07-01T09:10:44+00:00","dateModified":"2025-07-04T17:09:17+00:00","breadcrumb":{"@id":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#breadcrumb"},"inLanguage":"en-US","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/"]}]},{"@type":"ImageObject","inLanguage":"en-US","@id":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#primaryimage","url":"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2025\/07\/Senza-titolo.png","contentUrl":"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2025\/07\/Senza-titolo.png","width":275,"height":183},{"@type":"BreadcrumbList","@id":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/the-let-through-energy-for-protective-devices\/#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.gt-engineering.it\/en\/"},{"@type":"ListItem","position":2,"name":"Insights","item":"https:\/\/www.gt-engineering.it\/en\/approfondimenti\/"},{"@type":"ListItem","position":3,"name":"Electrical Safety and Arc Flash Mitigation","item":"https:\/\/www.gt-engineering.it\/en\/insights\/electrical-hazard-and-arc-flash-mitigation\/"},{"@type":"ListItem","position":4,"name":"The let through energy for protective devices"}]},{"@type":"WebSite","@id":"https:\/\/www.gt-engineering.it\/en\/#website","url":"https:\/\/www.gt-engineering.it\/en\/","name":"Gt-Engineering","description":"bizonweb","potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/www.gt-engineering.it\/en\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-US"}]}},"_links":{"self":[{"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/approfondimento\/48051","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/approfondimento"}],"about":[{"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/types\/approfondimento"}],"author":[{"embeddable":true,"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/users\/2"}],"version-history":[{"count":11,"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/approfondimento\/48051\/revisions"}],"predecessor-version":[{"id":48175,"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/approfondimento\/48051\/revisions\/48175"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/media\/47829"}],"wp:attachment":[{"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/media?parent=48051"}],"wp:term":[{"taxonomy":"tipologia_approfondimento","embeddable":true,"href":"https:\/\/www.gt-engineering.it\/en\/wp-json\/wp\/v2\/tipologia_approfondimento?post=48051"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}