{"id":29655,"date":"2026-06-30T15:11:52","date_gmt":"2026-06-30T07:11:52","guid":{"rendered":"https:\/\/www.cnspd.com\/?p=29655"},"modified":"2026-07-01T11:08:16","modified_gmt":"2026-07-01T03:08:16","slug":"spd-uc-calculation-for-tn-tt-it-and-split-phase-systems","status":"publish","type":"post","link":"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/","title":{"rendered":"Calcolo SPD Uc per sistemi TN, TT, IT e in fase spartita"},"content":{"rendered":"\t\t<div data-elementor-type=\"wp-post\" data-elementor-id=\"29655\" class=\"elementor elementor-29655\" data-elementor-post-type=\"post\">\n\t\t\t\t<div class=\"elementor-element elementor-element-7fc8633 e-flex e-con-boxed e-con e-parent\" data-id=\"7fc8633\" 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-5567304 elementor-widget elementor-widget-html\" data-id=\"5567304\" data-element_type=\"widget\" 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}\r\n\r\n    .cn-step-content strong { font-size: 16px; }\r\n\r\n    .cn-checklist li {\r\n      font-size: 15.5px;\r\n      margin: 9px 0;\r\n      padding-left: 26px;\r\n    }\r\n\r\n    .cn-btn {\r\n      width: 100% !important;\r\n      min-height: 46px !important;\r\n      padding: 12px 16px !important;\r\n    }\r\n\r\n    .cn-faq summary {\r\n      font-size: 16px;\r\n      line-height: 1.45;\r\n      padding-right: 24px;\r\n    }\r\n\r\n    .cn-related ul { padding-left: 18px; }\r\n\r\n    .cn-related span { font-size: 14.5px; }\r\n  }\r\n<\/style>\r\n\r\n<article class=\"cn-article\">\r\n\r\n  <div class=\"cn-article-label\">Calculation Guide<\/div>\r\n\r\n  <p class=\"cn-article-intro\">\r\n    SPD Uc calculation should start from the power system and the protection mode, not from a catalogue voltage name alone.\r\n    This guide explains how TN, TT, IT, and split-phase systems affect AC SPD Uc or MCOV checks for engineers, panel builders, and OEM buyers.\r\n  <\/p>\r\n\r\n  <div class=\"cn-article-meta\">\r\n    <span>For TN \/ TT \/ IT systems<\/span>\r\n    <span>For split-phase systems<\/span>\r\n    <span>For OEM SPD selection<\/span>\r\n    <span>Reading time: 8\u201310 minutes<\/span>\r\n  <\/div>\r\n\r\n  <div class=\"cn-note\">\r\n    <p>\r\n      <strong>Quick answer:<\/strong> SPD Uc is not calculated from the distribution board name alone.\r\n      First identify the system earthing arrangement, then identify the SPD protection mode such as L-N, L-PE, N-PE, L-PEN, or L-L.\r\n      After that, choose a Uc or MCOV rating that is higher than the continuous voltage the SPD will see, and confirm the final value with the approved datasheet and project requirement.\r\n    <\/p>\r\n  <\/div>\r\n\r\n  <nav class=\"cn-toc\">\r\n    <div class=\"cn-toc-title\">In this guide<\/div>\r\n    <ol>\r\n      <li><a href=\"#why-calculation-matters\">Why Uc calculation matters<\/a><\/li>\r\n      <li><a href=\"#terms\">Key terms used in Uc calculation<\/a><\/li>\r\n      <li><a href=\"#step-by-step\">Step-by-step calculation logic<\/a><\/li>\r\n      <li><a href=\"#iec-anchor\">IEC-style calculation anchors<\/a><\/li>\r\n      <li><a href=\"#worked-examples\">Worked examples for TN, TT, IT and split-phase systems<\/a><\/li>\r\n      <li><a href=\"#mistakes\">Common calculation mistakes<\/a><\/li>\r\n      <li><a href=\"#oem-checklist\">What OEM buyers should send to the supplier<\/a><\/li>\r\n      <li><a href=\"#faq\">FAQ<\/a><\/li>\r\n    <\/ol>\r\n  <\/nav>\r\n\r\n  <section id=\"why-calculation-matters\">\r\n    <div id=\"ez-toc-container\" class=\"ez-toc-v2_0_85 counter-hierarchy ez-toc-counter ez-toc-custom ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#why-spd-uc-calculation-matters\" >Why SPD Uc Calculation Matters<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#key-terms-used-in-uc-calculation\" >Key Terms Used in Uc Calculation<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#step-by-step-spd-uc-calculation-logic\" >Step-by-Step SPD Uc Calculation Logic<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#iec-style-calculation-anchors-for-tn-tt-and-it-systems\" >IEC-Style Calculation Anchors for TN, TT and IT Systems<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#worked-examples-for-common-systems\" >Worked Examples for Common Systems<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#common-spd-uc-calculation-mistakes\" >Common SPD Uc Calculation Mistakes<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#what-oem-buyers-should-send-before-asking-for-uc-confirmation\" >What OEM Buyers Should Send Before Asking for Uc Confirmation<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#faq-spd-uc-calculation\" >FAQ: SPD Uc Calculation<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-9\" href=\"#\" data-href=\"https:\/\/www.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/#related-spd-uc-mcov-guides\" >Related SPD Uc \/ MCOV Guides<\/a><\/li><\/ul><\/nav><\/div>\n<h2><span class=\"ez-toc-section\" id=\"why-spd-uc-calculation-matters\"><\/span>Why SPD Uc Calculation Matters<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <p>\r\n      Uc means maximum continuous operating voltage.\r\n      It tells buyers the highest voltage that the SPD can withstand continuously during normal service.\r\n      In UL-style documents, buyers often see MCOV for a similar selection role.\r\n    <\/p>\r\n\r\n    <p>\r\n      The calculation matters because the same nominal system voltage can expose different SPD modules to different continuous voltages.\r\n      A module connected L-N does not always see the same continuous voltage as a module connected L-PE or N-PE.\r\n    <\/p>\r\n\r\n    <p>\r\n      This is why IEC-style installation selection references, including IEC 60364-5-53 Table 534.2, link the minimum Uc requirement with the supply system configuration and the SPD connection mode.\r\n      The exact product selection should still follow the approved SPD datasheet, local requirements, and project electrical design.\r\n    <\/p>\r\n\r\n    <div class=\"cn-warning\">\r\n      <p>\r\n        <strong>Important:<\/strong> this guide gives a practical selection framework for buyers.\r\n        It is not a replacement for the official standard, the project designer, or the product certification file.\r\n      <\/p>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section id=\"terms\">\r\n    <h2><span class=\"ez-toc-section\" id=\"key-terms-used-in-uc-calculation\"><\/span>Key Terms Used in Uc Calculation<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <p>\r\n      Before checking any formula, make sure the voltage terms are clear.\r\n      Many selection errors happen because buyers mix system voltage, phase-to-neutral voltage, and the voltage across one SPD protection mode.\r\n    <\/p>\r\n\r\n    <div class=\"cn-table-wrap\">\r\n      <table class=\"cn-table\">\r\n        <thead>\r\n          <tr>\r\n            <th>Term<\/th>\r\n            <th>Meaning<\/th>\r\n            <th>Why it matters for SPD selection<\/th>\r\n          <\/tr>\r\n        <\/thead>\r\n        <tbody>\r\n          <tr>\r\n            <td>Uo<\/td>\r\n            <td>Nominal voltage between line and earth, or phase and neutral in many low-voltage systems.<\/td>\r\n            <td>Often used as the base voltage for IEC-style Uc checks in TN, TT and IT systems.<\/td>\r\n          <\/tr>\r\n          <tr>\r\n            <td>U<\/td>\r\n            <td>Nominal voltage between line conductors.<\/td>\r\n            <td>Important for three-phase systems and IT systems without distributed neutral.<\/td>\r\n          <\/tr>\r\n          <tr>\r\n            <td>Uc<\/td>\r\n            <td>Maximum continuous operating voltage in IEC-style SPD datasheets.<\/td>\r\n            <td>The SPD must continuously withstand this voltage without premature operation or ageing.<\/td>\r\n          <\/tr>\r\n          <tr>\r\n            <td>MCOV<\/td>\r\n            <td>Maximum continuous operating voltage wording commonly used in UL-style documents.<\/td>\r\n            <td>Used for similar voltage compatibility checks in UL-style product documents.<\/td>\r\n          <\/tr>\r\n          <tr>\r\n            <td>Protection mode<\/td>\r\n            <td>The SPD connection path, such as L-N, L-PE, N-PE, L-PEN, or L-L.<\/td>\r\n            <td>The voltage across each protection mode can be different, so Uc must be checked by connection path.<\/td>\r\n          <\/tr>\r\n        <\/tbody>\r\n      <\/table>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section id=\"step-by-step\">\r\n    <h2><span class=\"ez-toc-section\" id=\"step-by-step-spd-uc-calculation-logic\"><\/span>Step-by-Step SPD Uc Calculation Logic<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <p>\r\n      A good Uc or MCOV check is not only a mathematical step.\r\n      It is a system review.\r\n      Use the process below before confirming a catalogue value such as 275V, 320V, 385V, 440V or 480V.\r\n    <\/p>\r\n\r\n    <div class=\"cn-steps\">\r\n      <div class=\"cn-step\">\r\n        <div class=\"cn-step-num\"><\/div>\r\n        <div class=\"cn-step-content\">\r\n          <strong>Identify the supply system<\/strong>\r\n          <p>\r\n            Confirm whether the installation is TN-S, TN-C-S, TN-C, TT, IT with neutral, IT without neutral, or split-phase.\r\n            Do not select Uc only from the name \u201c230\/400V\u201d or \u201c120\/240V\u201d.\r\n          <\/p>\r\n        <\/div>\r\n      <\/div>\r\n\r\n      <div class=\"cn-step\">\r\n        <div class=\"cn-step-num\"><\/div>\r\n        <div class=\"cn-step-content\">\r\n          <strong>Identify the SPD protection mode<\/strong>\r\n          <p>\r\n            Check whether each SPD element is connected L-N, L-PE, N-PE, L-PEN, or L-L.\r\n            The voltage across this mode is the voltage that matters for Uc or MCOV.\r\n          <\/p>\r\n        <\/div>\r\n      <\/div>\r\n\r\n      <div class=\"cn-step\">\r\n        <div class=\"cn-step-num\"><\/div>\r\n        <div class=\"cn-step-content\">\r\n          <strong>Check the minimum continuous voltage requirement<\/strong>\r\n          <p>\r\n            In IEC-style selection, this is commonly checked against the system configuration and the earthing arrangement.\r\n            For many TN and TT low-voltage checks, buyers often see a margin above Uo, such as 1.1 \u00d7 Uo.\r\n          <\/p>\r\n        <\/div>\r\n      <\/div>\r\n\r\n      <div class=\"cn-step\">\r\n        <div class=\"cn-step-num\"><\/div>\r\n        <div class=\"cn-step-content\">\r\n          <strong>Choose the next suitable catalogue rating<\/strong>\r\n          <p>\r\n            After the minimum value is known, select the next suitable SPD catalogue rating above that value.\r\n            Do not choose a rating below the calculated minimum.\r\n          <\/p>\r\n        <\/div>\r\n      <\/div>\r\n\r\n      <div class=\"cn-step\">\r\n        <div class=\"cn-step-num\"><\/div>\r\n        <div class=\"cn-step-content\">\r\n          <strong>Confirm Up, TOV, SPD type and documentation<\/strong>\r\n          <p>\r\n            Uc or MCOV alone is not enough.\r\n            Also check voltage protection level Up, temporary overvoltage behaviour, Type 1 or Type 2 rating, In, Imax, Isccr, backup protection, and the product approval file.\r\n          <\/p>\r\n        <\/div>\r\n      <\/div>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section id=\"iec-anchor\">\r\n    <h2><span class=\"ez-toc-section\" id=\"iec-style-calculation-anchors-for-tn-tt-and-it-systems\"><\/span>IEC-Style Calculation Anchors for TN, TT and IT Systems<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <p>\r\n      IEC-style SPD selection does not treat every earthing system the same way.\r\n      The table below is a practical buyer-facing summary of common checking directions.\r\n      It is not a substitute for the official standard or project-specific design.\r\n    <\/p>\r\n\r\n    <div class=\"cn-formula-grid\">\r\n      <div class=\"cn-formula-card\">\r\n        <strong>1.1 \u00d7 Uo<\/strong>\r\n        <span>Common margin check<\/span>\r\n        <p>\r\n          Often used as a practical minimum reference for many TN and TT line-to-neutral or line-to-earth SPD checks.\r\n        <\/p>\r\n      <\/div>\r\n\r\n      <div class=\"cn-formula-card\">\r\n        <strong>3 \u00d7 Uo<\/strong>\r\n        <span>High-margin IT check<\/span>\r\n        <p>\r\n          May appear in IEC-style references for certain IT system line-to-earth checks where the fault condition changes the voltage stress.\r\n        <\/p>\r\n      <\/div>\r\n\r\n      <div class=\"cn-formula-card\">\r\n        <strong>U<\/strong>\r\n        <span>Line-to-line reference<\/span>\r\n        <p>\r\n          Important where the SPD element may continuously see the line-to-line voltage, especially in IT systems without distributed neutral or L-L protection.\r\n        <\/p>\r\n      <\/div>\r\n    <\/div>\r\n\r\n    <div class=\"cn-table-wrap\">\r\n      <table class=\"cn-table\">\r\n        <thead>\r\n          <tr>\r\n            <th>System or connection<\/th>\r\n            <th>Voltage check direction<\/th>\r\n            <th>Buyer note<\/th>\r\n          <\/tr>\r\n        <\/thead>\r\n        <tbody>\r\n          <tr>\r\n            <td>TN-S \/ TN-C-S<\/td>\r\n            <td>Check L-N, L-PE or L-PEN according to the actual SPD connection.<\/td>\r\n            <td>\r\n              For many 230\/400V projects, a 1.1 \u00d7 Uo check leads buyers to a catalogue value above 253V, often 275V as a starting point.\r\n            <\/td>\r\n          <\/tr>\r\n          <tr>\r\n            <td>TT system<\/td>\r\n            <td>Check line-to-neutral or line-to-earth modes, and review the N-PE protection path separately.<\/td>\r\n            <td>\r\n              In 3+1 arrangements, the N-PE element is not selected in the same way as a normal L-N MOV module.\r\n            <\/td>\r\n          <\/tr>\r\n          <tr>\r\n            <td>IT with distributed neutral<\/td>\r\n            <td>Line-to-earth voltage stress can be much higher under fault conditions.<\/td>\r\n            <td>\r\n              Ordinary 275V L-PE selection may be wrong. Review the IT system and the SPD datasheet carefully.\r\n            <\/td>\r\n          <\/tr>\r\n          <tr>\r\n            <td>IT without distributed neutral<\/td>\r\n            <td>The line-to-line voltage U may become the practical reference for some SPD checks.<\/td>\r\n            <td>\r\n              Do not select by phase-to-neutral voltage when neutral is not distributed.\r\n            <\/td>\r\n          <\/tr>\r\n          <tr>\r\n            <td>Split-phase systems<\/td>\r\n            <td>Check L-N and L-L protection modes separately.<\/td>\r\n            <td>\r\n              In UL-style documents, confirm MCOV for each service voltage and protection mode instead of directly copying IEC Uc values.\r\n            <\/td>\r\n          <\/tr>\r\n        <\/tbody>\r\n      <\/table>\r\n    <\/div>\r\n\r\n    <div class=\"cn-good\">\r\n      <p>\r\n        <strong>Simple professional rule:<\/strong> calculate or check the minimum continuous voltage for the actual protection mode, then select a catalogue Uc or MCOV value above that requirement.\r\n        After that, verify Up, TOV, short-circuit capability, backup protection and certification documents.\r\n      <\/p>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section id=\"worked-examples\">\r\n    <h2><span class=\"ez-toc-section\" id=\"worked-examples-for-common-systems\"><\/span>Worked Examples for Common Systems<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <p>\r\n      The examples below show how buyers can think about Uc or MCOV selection.\r\n      They are simplified examples for understanding and should not replace the final project design.\r\n    <\/p>\r\n\r\n    <h3>Example 1: 230\/400V TN-S Distribution Board<\/h3>\r\n    <p>\r\n      In a common 230\/400V TN-S system, Uo is usually 230V.\r\n      A practical IEC-style check using 1.1 \u00d7 Uo gives 1.1 \u00d7 230V = 253V.\r\n      The next common catalogue value above this minimum is often 275V.\r\n    <\/p>\r\n\r\n    <p>\r\n      This does not mean every 230\/400V board must always use 275V.\r\n      If the grid is unstable, if temporary overvoltage is expected, or if the buyer needs more voltage margin, 320V or 385V may also be reviewed with the datasheet.\r\n    <\/p>\r\n\r\n    <h3>Example 2: 230\/400V TT System with 3+1 SPD Arrangement<\/h3>\r\n    <p>\r\n      In a TT system using a 3+1 arrangement, the line-to-neutral SPD elements and the neutral-to-earth element should not be treated as the same selection problem.\r\n      The L-N modules may be checked from the phase-to-neutral voltage, while the N-PE path must follow the SPD design and system requirement.\r\n    <\/p>\r\n\r\n    <p>\r\n      This is why TT systems should be checked with a wiring diagram.\r\n      A buyer should not simply order a 4P SPD because the panel is three-phase with neutral.\r\n    <\/p>\r\n\r\n    <h3>Example 3: IT System with Distributed Neutral<\/h3>\r\n    <p>\r\n      IT systems need special attention because a first fault can change the voltage stress seen by SPD elements connected to earth.\r\n      In some IEC-style checks, the required line-to-earth Uc can be much higher than the value used in ordinary TN or TT systems.\r\n    <\/p>\r\n\r\n    <p>\r\n      For example, if Uo is 230V, a high-margin check such as 3 \u00d7 Uo points to 690V.\r\n      This does not mean a standard 275V or 385V SPD is suitable for that protection mode.\r\n      The IT system arrangement and product datasheet must be reviewed carefully.\r\n    <\/p>\r\n\r\n    <h3>Example 4: 120\/240V Split-Phase System<\/h3>\r\n    <p>\r\n      In a split-phase system, L-N and L-L are different checks.\r\n      A protection mode connected from line to neutral should be reviewed against the 120V service voltage, while a line-to-line protection mode should be reviewed against 240V service voltage.\r\n    <\/p>\r\n\r\n    <p>\r\n      In UL-style documents, buyers should confirm MCOV for each protection mode.\r\n      Do not directly translate IEC Uc values into MCOV labels without checking the target market, listing status, and product marking requirements.\r\n    <\/p>\r\n\r\n    <div class=\"cn-warning\">\r\n      <p>\r\n        <strong>Important:<\/strong> writing MCOV on a label does not mean the SPD is UL listed.\r\n        Certification status, listing file, product marking and datasheet wording must be confirmed separately.\r\n      <\/p>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section id=\"mistakes\">\r\n    <h2><span class=\"ez-toc-section\" id=\"common-spd-uc-calculation-mistakes\"><\/span>Common SPD Uc Calculation Mistakes<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <h3>Mistake 1: Selecting by system name only<\/h3>\r\n    <p>\r\n      \u201c230\/400V system\u201d is not enough information.\r\n      The buyer still needs to know the earthing arrangement and the SPD connection mode.\r\n    <\/p>\r\n\r\n    <h3>Mistake 2: Using line-to-line voltage for every SPD module<\/h3>\r\n    <p>\r\n      Some SPD modules are connected L-N, some L-PE, some N-PE, and some L-L.\r\n      The continuous voltage across the actual protection mode should be checked.\r\n    <\/p>\r\n\r\n    <h3>Mistake 3: Treating IT systems like TN systems<\/h3>\r\n    <p>\r\n      IT systems can expose SPD elements to different voltage stress, especially during fault conditions.\r\n      A normal TN-style Uc choice may not be suitable.\r\n    <\/p>\r\n\r\n    <h3>Mistake 4: Thinking higher Uc always means better protection<\/h3>\r\n    <p>\r\n      A higher Uc gives more continuous voltage margin, but it can also be linked with a different voltage protection level Up.\r\n      The protected equipment and coordination requirement still matter.\r\n    <\/p>\r\n\r\n    <h3>Mistake 5: Using the formula but ignoring TOV and datasheet limits<\/h3>\r\n    <p>\r\n      A calculated minimum value is only the starting point.\r\n      Temporary overvoltage behaviour, thermal disconnection, short-circuit withstand, backup protection and certification must still be checked.\r\n    <\/p>\r\n  <\/section>\r\n\r\n  <section id=\"oem-checklist\">\r\n    <h2><span class=\"ez-toc-section\" id=\"what-oem-buyers-should-send-before-asking-for-uc-confirmation\"><\/span>What OEM Buyers Should Send Before Asking for Uc Confirmation<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <p>\r\n      If you want a supplier to confirm the right AC SPD Uc or MCOV, do not only send the rated voltage.\r\n      Send the information below so the supplier can check the protection mode correctly.\r\n    <\/p>\r\n\r\n    <div class=\"cn-checklist\">\r\n      <ul>\r\n        <li>Target market: IEC-style, UL-style, GB\/T-style, or local project requirement.<\/li>\r\n        <li>System voltage: 230\/400V, 400\/415V, 480V, 120\/240V, 208Y\/120V, 277\/480V, or other.<\/li>\r\n        <li>Earthing system: TN-S, TN-C-S, TN-C, TT, IT with neutral, IT without neutral, or split-phase.<\/li>\r\n        <li>SPD connection mode: L-N, L-PE, N-PE, L-PEN, L-L, 3+0, 4+0, 3+1, or another arrangement.<\/li>\r\n        <li>Expected grid condition: stable utility, unstable grid, generator-fed system, or known TOV risk.<\/li>\r\n        <li>Required SPD type: Type 1, Type 2, or Type 1+2.<\/li>\r\n        <li>Required values: Uc or MCOV, Up, In, Imax, Isccr and backup protection arrangement.<\/li>\r\n        <li>Datasheet, label, packaging and certification requirements for OEM or private label orders.<\/li>\r\n      <\/ul>\r\n    <\/div>\r\n\r\n    <div class=\"cn-cta\">\r\n      <div class=\"cn-cta-title\">Need help checking SPD Uc or MCOV?<\/div>\r\n      <p>\r\n        Send us your system voltage, earthing system, protection mode and target market.\r\n        We can help review whether your AC SPD order should use 275V, 320V, 385V, 440V, 480V or another Uc \/ MCOV rating.\r\n      <\/p>\r\n      <a class=\"cn-btn leeyee-site-popup-btn\" href=\"javascript:void(0);\">\r\n        <span>Request SPD Voltage Check<\/span>\r\n      <\/a>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section id=\"faq\">\r\n    <h2><span class=\"ez-toc-section\" id=\"faq-spd-uc-calculation\"><\/span>FAQ: SPD Uc Calculation<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <div class=\"cn-faq\">\r\n      <details open>\r\n        <summary>Is SPD Uc calculated from line-to-line voltage or line-to-neutral voltage?<\/summary>\r\n        <div class=\"cn-faq-body\">\r\n          <p>\r\n            It depends on the SPD protection mode and the earthing system.\r\n            A module connected L-N is checked differently from a module connected L-PE, N-PE or L-L.\r\n            Do not use line-to-line voltage for every SPD module without checking the wiring.\r\n          <\/p>\r\n        <\/div>\r\n      <\/details>\r\n\r\n      <details>\r\n        <summary>Why does 1.1 \u00d7 230V often lead to a 275V SPD?<\/summary>\r\n        <div class=\"cn-faq-body\">\r\n          <p>\r\n            In many IEC-style checks, 1.1 \u00d7 Uo is used as a practical minimum reference.\r\n            If Uo is 230V, 1.1 \u00d7 230V equals 253V.\r\n            The next common catalogue value above 253V is often 275V.\r\n          <\/p>\r\n        <\/div>\r\n      <\/details>\r\n\r\n      <details>\r\n        <summary>Does a 230\/400V system always use 275V SPD?<\/summary>\r\n        <div class=\"cn-faq-body\">\r\n          <p>\r\n            No.\r\n            275V is common in stable 230\/400V systems, but the final selection depends on earthing system, connection mode, voltage fluctuation, TOV condition, Up and the SPD datasheet.\r\n          <\/p>\r\n        <\/div>\r\n      <\/details>\r\n\r\n      <details>\r\n        <summary>Why are IT systems different for SPD Uc selection?<\/summary>\r\n        <div class=\"cn-faq-body\">\r\n          <p>\r\n            In IT systems, a fault can change the voltage stress between live conductors and earth.\r\n            This can require a higher Uc for some protection modes than would be used in ordinary TN or TT systems.\r\n          <\/p>\r\n        <\/div>\r\n      <\/details>\r\n\r\n      <details>\r\n        <summary>Is MCOV calculated the same way as Uc?<\/summary>\r\n        <div class=\"cn-faq-body\">\r\n          <p>\r\n            MCOV and Uc serve a similar voltage compatibility role, but the documentation style and approval route can be different.\r\n            In UL-style markets, confirm MCOV according to the product file, service voltage and marking requirement.\r\n          <\/p>\r\n        <\/div>\r\n      <\/details>\r\n\r\n      <details>\r\n        <summary>Can I use this calculation for DC or solar PV SPD?<\/summary>\r\n        <div class=\"cn-faq-body\">\r\n          <p>\r\n            No.\r\n            This guide is for AC SPD Uc or MCOV checks.\r\n            DC and solar PV systems use Ucpv selection logic and should be checked separately.\r\n          <\/p>\r\n        <\/div>\r\n      <\/details>\r\n\r\n      <details>\r\n        <summary>What should I check after calculating the minimum Uc?<\/summary>\r\n        <div class=\"cn-faq-body\">\r\n          <p>\r\n            After calculating or checking the minimum Uc, confirm the next suitable catalogue rating, voltage protection level Up, TOV behaviour, In, Imax, Isccr, backup protection, SPD type and certification documents.\r\n          <\/p>\r\n        <\/div>\r\n      <\/details>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section class=\"cn-related cn-uc-cluster\">\r\n    <h2><span class=\"ez-toc-section\" id=\"related-spd-uc-mcov-guides\"><\/span>Related SPD Uc \/ MCOV Guides<span class=\"ez-toc-section-end\"><\/span><\/h2>\r\n\r\n    <p>\r\n      This article is part of our SPD Uc and MCOV guide series.\r\n      Use the guides below to compare cross-market terminology, voltage ratings, OEM documentation and country-related system differences.\r\n    <\/p>\r\n\r\n    <ul>\r\n      <li>\r\n        <a href=\"https:\/\/www.cnspd.com\/spd-uc-mcov-selection-across-markets\/\" target=\"_blank\" rel=\"noopener\">\r\n          SPD Uc and MCOV Selection Across IEC, UL and Export Markets\r\n        <\/a>\r\n        <br>\r\n        <span>\r\n          A full overview of why Uc and MCOV ratings vary across IEC-style, UL-style, GB\/T-style and export markets.\r\n        <\/span>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.cnspd.com\/spd-uc-selection-guide\/\" target=\"_blank\" rel=\"noopener\">\r\n          SPD Uc Selection Guide for Distribution Boards and OEM Orders\r\n        <\/a>\r\n        <br>\r\n        <span>\r\n          A practical guide for choosing 275V, 320V, 385V or 440V AC SPD options for panels and OEM orders.\r\n        <\/span>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.cnspd.com\/spd-uc-vs-mcov-oem-labels\/\" target=\"_blank\" rel=\"noopener\">\r\n          SPD Uc vs MCOV: Difference for OEM Labels and Datasheets\r\n        <\/a>\r\n        <br>\r\n        <span>\r\n          A guide for OEM buyers who need to align Uc, MCOV, product labels, datasheets, packaging and test files.\r\n        <\/span>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.cnspd.com\/common-spd-uc-mcov-ratings\/\" target=\"_blank\" rel=\"noopener\">\r\n          Common SPD Uc and MCOV Ratings: 150V, 275V, 320V, 385V, 440V and 480V\r\n        <\/a>\r\n        <br>\r\n        <span>\r\n          A catalogue-style explanation of common Uc and MCOV rating values and where buyers often see them.\r\n        <\/span>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.cnspd.com\/spd-uc-mcov-ratings-by-country\/\" target=\"_blank\" rel=\"noopener\">\r\n          Why SPD Uc and MCOV Ratings Vary by Country and Power System\r\n        <\/a>\r\n        <br>\r\n        <span>\r\n          A market-focused guide explaining why common SPD voltage ratings differ between regions, grid systems and project requirements.\r\n        <\/span>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/section>\r\n\r\n<\/article>\r\n\r\n<script>\r\n  document.addEventListener('DOMContentLoaded', function () {\r\n    document.querySelectorAll('.leeyee-site-popup-btn').forEach(function (btn) {\r\n      btn.addEventListener('click', function (e) {\r\n        e.preventDefault();\r\n        var trigger = document.querySelector('.w-popup.ush_popup_1 .w-popup-trigger');\r\n        if (trigger) {\r\n          trigger.click();\r\n        } else {\r\n          window.open('https:\/\/www.cnspd.com\/contact\/', '_blank');\r\n        }\r\n      });\r\n    });\r\n  });\r\n<\/script>\r\n\r\n<script type=\"application\/ld+json\">\r\n{\r\n  \"@context\": \"https:\/\/schema.org\",\r\n  \"@graph\": [\r\n    {\r\n      \"@type\": \"Article\",\r\n      \"headline\": \"SPD Uc Calculation for TN, TT, IT and Split-Phase Systems\",\r\n      \"description\": \"Learn how to check SPD Uc or MCOV for TN, TT, IT and split-phase systems. 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A module connected L-N is checked differently from a module connected L-PE, N-PE or L-L. Do not use line-to-line voltage for every SPD module without checking the wiring.\"\r\n          }\r\n        },\r\n        {\r\n          \"@type\": \"Question\",\r\n          \"name\": \"Why does 1.1 \u00d7 230V often lead to a 275V SPD?\",\r\n          \"acceptedAnswer\": {\r\n            \"@type\": \"Answer\",\r\n            \"text\": \"In many IEC-style checks, 1.1 \u00d7 Uo is used as a practical minimum reference. If Uo is 230V, 1.1 \u00d7 230V equals 253V. The next common catalogue value above 253V is often 275V.\"\r\n          }\r\n        },\r\n        {\r\n          \"@type\": \"Question\",\r\n          \"name\": \"Does a 230\/400V system always use 275V SPD?\",\r\n          \"acceptedAnswer\": {\r\n            \"@type\": \"Answer\",\r\n            \"text\": \"No. 275V is common in stable 230\/400V systems, but the final selection depends on earthing system, connection mode, voltage fluctuation, TOV condition, Up and the SPD datasheet.\"\r\n          }\r\n        },\r\n        {\r\n          \"@type\": \"Question\",\r\n          \"name\": \"Why are IT systems different for SPD Uc selection?\",\r\n          \"acceptedAnswer\": {\r\n            \"@type\": \"Answer\",\r\n            \"text\": \"In IT systems, a fault can change the voltage stress between live conductors and earth. This can require a higher Uc for some protection modes than would be used in ordinary TN or TT systems.\"\r\n          }\r\n        },\r\n        {\r\n          \"@type\": \"Question\",\r\n          \"name\": \"Is MCOV calculated the same way as Uc?\",\r\n          \"acceptedAnswer\": {\r\n            \"@type\": \"Answer\",\r\n            \"text\": \"MCOV and Uc serve a similar voltage compatibility role, but the documentation style and approval route can be different. In UL-style markets, confirm MCOV according to the product file, service voltage and marking requirement.\"\r\n          }\r\n        },\r\n        {\r\n          \"@type\": \"Question\",\r\n          \"name\": \"Can I use this calculation for DC or solar PV SPD?\",\r\n          \"acceptedAnswer\": {\r\n            \"@type\": \"Answer\",\r\n            \"text\": \"No. This guide is for AC SPD Uc or MCOV checks. DC and solar PV systems use Ucpv selection logic and should be checked separately.\"\r\n          }\r\n        },\r\n        {\r\n          \"@type\": \"Question\",\r\n          \"name\": \"What should I check after calculating the minimum Uc?\",\r\n          \"acceptedAnswer\": {\r\n            \"@type\": \"Answer\",\r\n            \"text\": \"After calculating or checking the minimum Uc, confirm the next suitable catalogue rating, voltage protection level Up, TOV behaviour, In, Imax, Isccr, backup protection, SPD type and certification documents.\"\r\n          }\r\n        }\r\n      ]\r\n    }\r\n  ]\r\n}\r\n<\/script>\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\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"<p>Calculation GuideSPD Uc calculation should start from the power system and the protection mode, not from a catalogue voltage name alone. This guide explains how TN, TT, IT, and split-phase systems affect AC SPD Uc or MCOV checks for engineers, panel builders, and OEM buyers.For TN \/ TT \/ IT systems For split-phase systems For&#8230;<\/p>","protected":false},"author":18,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1,488,422],"tags":[],"class_list":["post-29655","post","type-post","status-publish","format-standard","hentry","category-news","category-surge-protection","category-industry-news"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.9 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>SPD Uc Calculation for TN, TT, IT and Split-Phase Systems | LEEYEE Electrics<\/title>\n<meta name=\"description\" content=\"Discover SPD Uc Calculation for TN, TT, IT and Split-Phase Systems and further information about electrical protection. Find professional customized low-voltage SPD solutions at LEEYEE Electrics.\" \/>\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.cnspd.com\/it\/spd-uc-calculation-tn-tt-it\/\" \/>\n<meta property=\"og:locale\" content=\"it_IT\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"SPD Uc Calculation for TN, TT, IT and Split-Phase Systems | LEEYEE Electrics\" \/>\n<meta property=\"og:description\" content=\"Discover SPD Uc Calculation for TN, TT, IT and Split-Phase Systems and further information about electrical protection. 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