{"id":2907,"date":"2026-02-02T06:21:16","date_gmt":"2026-02-02T06:21:16","guid":{"rendered":"https:\/\/xbrele.com\/?p=2907"},"modified":"2026-10-14T12:57:42","modified_gmt":"2026-10-14T12:57:42","slug":"ct-accuracy-classes-metering-protection-guide","status":"publish","type":"post","link":"https:\/\/xbrele.com\/fr\/ct-accuracy-classes-metering-protection-guide\/","title":{"rendered":"Explication des classes de pr\u00e9cision des transformateurs de courant (TC) (0,2\/0,5, 5P\/10P) : Mesure et protection"},"content":{"rendered":"<p class=\"xbrele-article-updated\"><small>Derni\u00e8re mise \u00e0 jour : <time datetime=\"2026-10-14\">October 14, 2026<\/time>.<\/small><\/p>\n\n\n\n<p>Current transformers supply measuring instruments and protection relays with scaled current signals. Metering classes describe measurement accuracy over a specified load-current range; protection classes describe performance under the fault conditions covered by their designation. A metering-class label alone does not establish suitability for protection, and a protection-class label alone does not establish revenue-metering accuracy.<\/p>\n\n\n\n<p>Ce guide d\u00e9compose les d\u00e9signations des classes de pr\u00e9cision de la CEI 61869-2, explique les diff\u00e9rences techniques entre les classes de mesure (0,1, 0,2, 0,5) et les classes de protection (5P, 10P), et fournit des conseils pratiques de s\u00e9lection pour les applications de moyenne tension.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"what-are-ct-accuracy-classes\">Qu'est-ce qu'une classe de pr\u00e9cision en tomodensitom\u00e9trie ?<\/h2>\n\n\n\n<p>A CT accuracy class defines permissible errors under specified current, burden, frequency and test conditions. For a metering CT, the class number identifies the rated-current ratio-error limit; other test currents have their own limits. In a 5P or 10P designation, the number identifies the composite-error limit at the rated accuracy-limit primary current, not the ratio-error limit at normal load.<\/p>\n\n\n\n<p>Il existe deux syst\u00e8mes de classification distincts. Les classes de mesure (0.1, 0.2, 0.5, 1.0) optimisent la pr\u00e9cision pour les courants de fonctionnement normaux - typiquement 5% \u00e0 120% du courant nominal. Les classes de protection (5P, 10P) donnent la priorit\u00e9 aux performances en cas de d\u00e9faut, en maintenant la pr\u00e9cision \u00e0 des multiples de courant d\u00e9passant de loin le fonctionnement normal.<\/p>\n\n\n\n<p>Metering and protection impose different design requirements. A CT may contain separate metering and protection cores, and some designs carry more than one accuracy designation. Select each core using its declared ratings and the connected equipment requirements; neither core size nor material alone proves its accuracy class.<\/p>\n\n\n\n<figure class=\"wp-block-embed is-type-video is-provider-youtube wp-block-embed-youtube wp-embed-aspect-16-9 wp-has-aspect-ratio\"><div class=\"wp-block-embed__wrapper\">\n<iframe title=\"Explication des classes de pr\u00e9cision des TC : Mesure 0,2\/0,5 vs protection 5P\/10P\" width=\"1290\" height=\"726\" src=\"https:\/\/www.youtube.com\/embed\/PVXlrxYOFzA?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe>\n<\/div><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"how-current-transformers-introduce-measurement-errors\">Comment les transformateurs de courant introduisent des erreurs de mesure<\/h2>\n\n\n\n<p>Part of the primary ampere-turns supplies the core excitation rather than the secondary load. The resulting exciting current, together with the secondary circuit impedance, produces ratio error and phase displacement.<\/p>\n\n\n\n<p><strong>Erreur de rapport (\u03b5)<\/strong> is the difference between the primary current inferred from the rated ratio and the actual primary current, expressed as a percentage of the actual primary current. With a rated ratio of 1000\/5 A, a secondary reading of 4.98 A at 1000 A primary gives (200 \u00d7 4.98 \u2212 1000) \/ 1000 \u00d7 100 = \u22120.4%. The error varies with current and burden.<\/p>\n\n\n\n<p><strong>D\u00e9placement de phase (\u03b4)<\/strong>&nbsp;mesure la diff\u00e9rence angulaire entre les phases des courants primaire et secondaire, exprim\u00e9e en minutes d'arc. La transformation id\u00e9ale produit un d\u00e9phasage nul ; les TC r\u00e9els pr\u00e9sentent de petits d\u00e9placements dus aux exigences du courant de magn\u00e9tisation.<\/p>\n\n\n\n<p>Core material, geometry, turns and excitation characteristics all influence CT performance. Rated output is stated in VA at the rated secondary current. Check the manufacturer\u2019s declared burden range and burden power factor as well as the VA rating: a very small connected burden is not automatically within the metering accuracy guarantee.<\/p>\n\n\n\n<p>The limits apply at the current and burden conditions specified for the class. At rated primary current, Class 0.2 has ratio-error and phase-displacement limits of \u00b10.2% and \u00b110 minutes. For protection classes, the rated-current ratio-error limits are \u00b11% for 5P and \u00b13% for 10P; their respective composite-error limits at the rated accuracy-limit current are 5% and 10%.<\/p>\n\n\n\n\n\n<h2 class=\"wp-block-heading\" id=\"metering-ct-accuracy-classes-0.1-0.2-0.5-and-extended-s-classes\">Classes de pr\u00e9cision des TC de comptage : 0,1, 0,2, 0,5 et classes \u201cS\u201d \u00e9tendues<\/h2>\n\n\n\n<p>Metering classes specify ratio-error and phase-displacement limits at several test currents. The table lists the limits at 100% rated current and the covered current range. It does not mean that the rated-current error limit applies unchanged over that entire range; use the full class table and the CT\u2019s declared burden conditions for selection.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Classe<\/th><th>Erreur de rapport \u00e0 100% In<\/th><th>Phase displacement at 100% In<\/th><th>Specified current range<\/th><th>Application typique<\/th><\/tr><\/thead><tbody><tr><td>0.1<\/td><td>\u00b10,1%<\/td><td>\u00b15 min<\/td><td>5-120%<\/td><td>\u00c9talons de r\u00e9f\u00e9rence de laboratoire<\/td><\/tr><tr><td>0.2<\/td><td>\u00b10,2%<\/td><td>\u00b110 min<\/td><td>5-120%<\/td><td>Comptage des recettes de transport<\/td><\/tr><tr><td>0.2S<\/td><td>\u00b10,2%<\/td><td>\u00b110 min<\/td><td>1-120%<\/td><td>Comptage des revenus de la charge variable<\/td><\/tr><tr><td>0.5<\/td><td>\u00b10,5%<\/td><td>\u00b130 min<\/td><td>5-120%<\/td><td>Comptage dans les sous-stations de distribution<\/td><\/tr><tr><td>0.5S<\/td><td>\u00b10,5%<\/td><td>\u00b130 min<\/td><td>1-120%<\/td><td>Comptage des recettes industrielles<\/td><\/tr><tr><td>1.0<\/td><td>\u00b11,0%<\/td><td>\u00b160 min<\/td><td>5-120%<\/td><td>Instruments d'indicateurs<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>The \u201cS\u201d suffix identifies metering classes with additional low-current requirements. Classes 0.2S and 0.5S include a 1% rated-current test point, whereas the ordinary classes listed above start at 5%. Error limits differ by test point. Select a class and ratio that cover the expected minimum load and the applicable metering requirements.<\/p>\n\n\n\n<p>For variable-load facilities, compare the expected minimum primary current with the CT rating before choosing between 0.2 and 0.2S. Billing value alone does not define the required class. Include the meter, voltage measurement and wiring in the overall measurement-accuracy assessment.<\/p>\n\n\n\n<p><strong>Instrument security factor (FS)<\/strong> describes the overcurrent behavior specified for a metering core at rated burden. FS5 and FS10 are common declared ratings, but the accuracy class alone does not prescribe a universal saturation multiple. Reducing the connected burden can increase the current at which the core saturates. Verify FS and the meter\u2019s withstand capability rather than assuming every metering CT saturates at 2\u20135 times rated current.<\/p>\n\n\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p><strong>Metering CT selection checks<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Compare the expected minimum load with the class test points and the CT ratio.<\/li>\n\n\n\n<li>Verify the complete connected burden is within the CT\u2019s declared metering accuracy range.<\/li>\n\n\n\n<li>Obtain the selected meter\u2019s actual input burden; verify accuracy at the resulting total burden.<\/li>\n\n\n\n<li>Demander des certificats d'essai d'usine indiquant les erreurs r\u00e9elles mesur\u00e9es, et pas seulement la conformit\u00e9 \u00e0 la classe.<\/li>\n<\/ul>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"protection-ct-accuracy-classes-decoding-5p-and-10p-designations\">Classes de pr\u00e9cision des TC de protection : D\u00e9codage des d\u00e9signations 5P et 10P<\/h2>\n\n\n\n<p>Les classes de protection suivent un syst\u00e8me de notation diff\u00e9rent qui refl\u00e8te leur fonction principale : la mesure pr\u00e9cise du courant de d\u00e9faut. La d\u00e9signation \u201c5P20\u201d codifie trois param\u00e8tres essentiels pour la coordination des relais.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"nomenclature-breakdown\"><strong>Ventilation de la nomenclature :<\/strong><\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>5<\/strong>\u00a0= Limite d'erreur composite (5% maximum)<\/li>\n\n\n\n<li><strong>P<\/strong>\u00a0= D\u00e9signation de la classe de protection<\/li>\n\n\n\n<li><strong>20<\/strong> = rated accuracy limit factor at rated burden; the specified composite-error limit is checked at 20 times rated primary current<\/li>\n<\/ul>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Classe<\/th><th>Erreur composite \u00e0 l'ALF<\/th><th>Valeurs communes de l'ALF<\/th><th>Application typique<\/th><\/tr><\/thead><tbody><tr><td>5P<\/td><td>\u22645%<\/td><td>10, 15, 20, 30<\/td><td>Overcurrent protection; verify each relay application<\/td><\/tr><tr><td>10P<\/td><td>\u226410%<\/td><td>10, 15, 20<\/td><td>Overcurrent protection where the scheme accepts 10P<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>Le <strong>accuracy limit factor (ALF)<\/strong> is the rated accuracy-limit primary current divided by the rated primary current. A 1000\/5 A, 5P20 CT has a rated accuracy-limit current of 20,000 A at its rated burden. This is a defined steady-state error test condition, not a sharp physical saturation threshold or a guarantee of transient performance.<\/p>\n\n\n\n<p>For a 5P20 CT, the composite error must not exceed 5% at 20 times rated primary current under the specified test conditions and rated burden. Composite error includes waveform distortion and is not a signed \u00b15% ratio-error band. DC offset, remanent flux and the required operating time can impose additional CT requirements.<\/p>\n\n\n\n<p><strong>Initial steady-state screening:<\/strong> if the scheme requires class accuracy at the maximum symmetrical through-fault current, compare the effective ALF at the actual burden with that fault-current multiple.<\/p>\n\n\n\n<p>Required effective ALF \u2265 Maximum symmetrical through-fault current \u00f7 CT rated primary current<\/p>\n\n\n\n<p>For a 12.5 kA symmetrical fault and a 500\/5 A CT, the fault-current multiple is 12,500 \/ 500 = 25.<\/p>\n\n\n\n<p>At rated burden, 5P20 alone does not demonstrate the above 25-times screening requirement; a 5P30 rating would cover that steady-state multiple. Final selection still requires the relay manufacturer\u2019s criteria, actual secondary burden, fault duration and transient assessment. Some protection functions tolerate partial CT saturation, so this screening rule is not a universal relay-sizing formula.<\/p>\n\n\n\n<p>Protection CT selection also involves excitation characteristics, secondary winding resistance and short-time thermal and dynamic ratings. A 5P or 10P designation does not replace these checks. For differential or transient duties, the relay manufacturer may specify PX, PR or a transient class, or give a calculation for an acceptable P-class CT.<\/p>\n\n\n\n\n\n<h2 class=\"wp-block-heading\" id=\"metering-vs-protection-ct-side-by-side-comparison\">TC de mesure et TC de protection : comparaison c\u00f4te \u00e0 c\u00f4te<\/h2>\n\n\n\n<p>The following comparison summarizes the different performance specifications. Individual CTs can have additional ratings beyond the class names shown.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Param\u00e8tre<\/th><th>CT de comptage<\/th><th>Protection CT<\/th><\/tr><\/thead><tbody><tr><td><strong>Fonction principale<\/strong><\/td><td>Facturation des recettes, contr\u00f4le de l'\u00e9nergie<\/td><td>D\u00e9tection des d\u00e9fauts, fonctionnement des relais<\/td><\/tr><tr><td><strong>Plage de pr\u00e9cision<\/strong><\/td><td>1-120% de valeur nominale (classe S)<\/td><td>Rated error limit at ALF \u00d7 rated current and rated burden<\/td><\/tr><tr><td><strong>Sp\u00e9cification des erreurs<\/strong><\/td><td>Erreur de rapport + d\u00e9phasage<\/td><td>Rated-current ratio error and composite error at ALF<\/td><\/tr><tr><td><strong>Comportement de saturation<\/strong><\/td><td>Specified by FS and burden; not by metering class alone<\/td><td>Specified accuracy-limit and transient characteristics<\/td><\/tr><tr><td><strong>Conception de base<\/strong><\/td><td>Design depends on accuracy, burden and FS<\/td><td>Design depends on burden, fault and transient duty<\/td><\/tr><tr><td><strong>Classes types<\/strong><\/td><td>0,2, 0,2S, 0,5, 0,5S, 1,0<\/td><td>5P10, 5P20, 10P10, 10P15<\/td><\/tr><tr><td><strong>Dispositifs connect\u00e9s<\/strong><\/td><td>Compteurs de kWh, analyseurs de puissance<\/td><td>Relais de protection, enregistreurs de d\u00e9fauts<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>A saturated CT produces a distorted secondary waveform. Depending on the relay function and settings, this can delay operation or affect differential stability. The magnitude and timing of the effect must be assessed for the actual protection scheme; they cannot be inferred from the metering class alone.<\/p>\n\n\n\n<p>Separate cores allow the metering burden and protection burden to be specified and tested independently when both functions are needed.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"burden-effects-and-practical-selection-considerations\">Effets de la charge et consid\u00e9rations pratiques de s\u00e9lection<\/h2>\n\n\n\n<p>Connected burden affects the voltage the CT must develop and therefore its accuracy and saturation behavior. Include the relay or meter input and the complete secondary wiring circuit. Keep the metering burden within its declared accuracy range, and calculate protection performance at the actual burden.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"burden-components-include\"><strong>Les \u00e9l\u00e9ments de la charge sont les suivants :<\/strong><\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Instrument burden: use the selected meter, relay or transducer input data.<\/li>\n\n\n\n<li>Charge de plomb : r\u00e9sistance du c\u00e2ble secondaire (importante pour les longs trajets)<\/li>\n\n\n\n<li>Connection burden: include terminals and test switches using their actual resistance.<\/li>\n<\/ul>\n\n\n\n<p><strong>Lead burden calculation for a separate outgoing and return pair:<\/strong><br>Lead VA = I<sub>secondary<\/sub>\u00b2 \u00d7 R<sub>loop<\/sub> = I<sub>secondary<\/sub>\u00b2 \u00d7 2 \u00d7 R<sub>one-way<\/sub><\/p>\n\n\n\n<p>Example: a 50 m one-way run with a measured or calculated one-way resistance of 0.35 \u03a9 has a 0.70 \u03a9 loop. At 5 A, its burden is 25 \u00d7 0.70 = 17.5 VA; at 1 A, it is 0.70 VA. Use the cable resistance at the design temperature and the actual return-circuit arrangement.<\/p>\n\n\n\n<p>Cela explique pourquoi les secondaires de 1 A conviennent aux longs parcours de c\u00e2bles - la charge est r\u00e9duite de 25 fois par rapport aux secondaires de 5 A pour un c\u00e2ble identique.<\/p>\n\n\n\n<p><strong>Liste de contr\u00f4le de la s\u00e9lection :<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Obtain maximum and minimum fault currents and the relay operating-time requirements.<\/li>\n\n\n\n<li>\u00a0Somme de la charge totale, y compris la r\u00e9sistance du plomb<\/li>\n\n\n\n<li>Verify the declared burden conditions for metering and the effective ALF for protection.<\/li>\n\n\n\n<li>Include credible future fault levels and transient duty using the relay manufacturer\u2019s calculation method.<\/li>\n\n\n\n<li>Choose the metering class and ratio to cover the actual load range and project requirements.<\/li>\n<\/ul>\n\n\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p><strong>Protection CT specification checks<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>For auto-reclosing, assess remanence, fault duration and the required operating cycle.<\/li>\n\n\n\n<li>For differential protection, follow the selected relay\u2019s CT ratio and saturation requirements.<\/li>\n\n\n\n<li>Use the relay\u2019s actual input burden and the complete wiring burden when calculating effective ALF.<\/li>\n\n\n\n<li>Request excitation curves and secondary winding resistance when required by the protection calculation.<\/li>\n<\/ul>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"dual-core-cts-and-switchgear-integration\">TC \u00e0 double c\u0153ur et int\u00e9gration de l'appareillage de commutation<\/h2>\n\n\n\n<p>La plupart des installations moyenne tension n\u00e9cessitent \u00e0 la fois un comptage et une protection de chaque d\u00e9part. Les TC bicanaux (ou multiconducteurs) r\u00e9pondent \u00e0 ce besoin en incorporant des noyaux magn\u00e9tiques distincts enroul\u00e9s sur le m\u00eame conducteur primaire.<\/p>\n\n\n\n<p>Illustrative dual-core specification, subject to the actual meter, relay and wiring calculations:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Noyau 1 : classe 0,5S, charge 10 VA (comptage des recettes)<\/li>\n\n\n\n<li>Noyau 2 : 5P20, charge de 30 VA (relais de protection)<\/li>\n<\/ul>\n\n\n\n<p>Specify and verify the accuracy and burden of each core separately. Confirm metering FS, protection performance, insulation level and the assembly\u2019s thermal and dynamic withstand ratings. Separate cores do not remove the need to coordinate the complete secondary circuits.<\/p>\n\n\n\n<p>Int\u00e9gration avec <a href=\"https:\/\/xbrele.com\/fr\/vacuum-circuit-breaker\/\">Disjoncteurs \u00e0 vide moyenne tension<\/a> requires a common system fault study. ALF is a dimensionless ratio and cannot be compared directly with a breaker breaking-current rating in kA. Evaluate the CT against the maximum through-fault current, its primary rating, actual burden and relay requirements. Separately verify the CT short-time\/dynamic withstand ratings and the breaker interrupting duty. Ask the <a href=\"https:\/\/xbrele.com\/fr\/switchgear-parts\/\">fabricants de composants<\/a> for the data needed to complete those checks.<\/p>\n\n\n\n<p>Lorsque vous sp\u00e9cifiez des TC pour de nouvelles installations, fournissez les informations suivantes \u00e0 votre fournisseur :<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Niveau de d\u00e9faut maximal du syst\u00e8me (kA)<\/li>\n\n\n\n<li>Plage de courant de charge (du minimum au maximum)<\/li>\n\n\n\n<li>Distance du c\u00e2ble jusqu'aux panneaux de mesure\/relais<\/li>\n\n\n\n<li>Mod\u00e8les sp\u00e9cifiques de compteurs et de relais avec donn\u00e9es de charge<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"common-ct-specification-errors-and-how-to-avoid-them\">Erreurs courantes dans les sp\u00e9cifications des TC et comment les \u00e9viter<\/h2>\n\n\n\n<p>Review the following specification errors before purchasing or replacing a CT.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"error-1-using-metering-ct-for-protection\"><strong>Erreur 1 : Utilisation d'un TC de mesure pour la protection<\/strong><\/h3>\n\n\n\n<p>A metering class alone does not verify fault-current performance. <strong>Solution :<\/strong> select a core with the protection characteristics required by the relay scheme and verify its performance at the actual burden, through-fault current and required operating time.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"error-2-ignoring-lead-burden-in-total-burden-calculation\"><strong>Erreur 2 : Ignorer la charge de plomb dans le calcul de la charge totale<\/strong><\/h3>\n\n\n\n<p>Long secondary circuits can add substantial burden, particularly at 5 A. <strong>Solution :<\/strong> calculate the complete loop resistance at the design temperature. Compare 1 A and 5 A options using the actual cable and compatible relay inputs; cable length alone is not a universal selection threshold.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"error-3-mismatched-cts-in-differential-protection\"><strong>Erreur 3 : TC mal appari\u00e9s dans la protection diff\u00e9rentielle<\/strong><\/h3>\n\n\n\n<p>Different saturation behavior can produce unwanted differential current during an external fault. <strong>Solution :<\/strong> use the relay manufacturer\u2019s ratio-compensation, excitation, winding-resistance and stability requirements. Transformer differential schemes may use different primary CT ratios; identical ratios and knee-point voltages are not a universal requirement.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"error-4-overlooking-remanent-flux-in-auto-reclose-schemes\"><strong>Erreur 4 : Le flux r\u00e9manent n'est pas pris en compte dans les sch\u00e9mas d'auto-refermeture<\/strong><\/h3>\n\n\n\n<p>Remanent flux can change CT saturation during a subsequent fault or reclosing sequence. <strong>Solution :<\/strong> assess the specified operating cycle, system time constant and relay requirements. Select suitable remanence or transient performance where required; a blanket 50% ALF increase is not a verified substitute.<\/p>\n\n\n\n<p>Compr\u00e9hension <a href=\"https:\/\/xbrele.com\/fr\/vacuum-circuit-breaker-ratings\/\">Caract\u00e9ristiques nominales des disjoncteurs \u00e0 vide<\/a> helps establish the switching duty. CT accuracy selection is a separate calculation based on the actual through-fault duty and protection scheme, while CT thermal\/dynamic withstand must also cover the clearing duty.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"frequently-asked-questions\">Foire aux questions<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"what-does-5p20-mean-on-a-current-transformer-nameplate\"><strong>Que signifie 5P20 sur la plaque signal\u00e9tique d'un transformateur de courant ?<\/strong><\/h3>\n\n\n\n<p>5P20 specifies a protection CT with a composite-error limit of 5% at a rated accuracy-limit current of 20 times rated primary current, at rated burden and the specified test conditions. For a 400\/5 A CT this test current is 8,000 A. It does not identify an exact saturation threshold or guarantee performance for every transient waveform.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"can-a-0-5-class-metering-ct-feed-protection-relays\"><strong>Un TC de comptage de classe 0,5 peut-il alimenter des relais de protection ?<\/strong><\/h3>\n\n\n\n<p>Do not assume that a Class 0.5 metering core is suitable for protection. Its class establishes metering accuracy under specified conditions, not the fault-current performance required by a relay. Use a core whose declared protection characteristics have been checked against the particular relay scheme.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"when-should-i-specify-0-2s-instead-of-standard-0-2-class\"><strong>Quand dois-je sp\u00e9cifier 0,2S au lieu de la classe standard 0,2 ?<\/strong><\/h3>\n\n\n\n<p>Consider 0.2S when accurate measurement is required at low load, especially between 1% and 5% of rated CT current. The required class also depends on the utility or project metering specification. Check the limits at each current test point and the declared burden range; 0.2S does not mean \u00b10.2% at every current.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"how-does-burden-affect-protection-ct-performance\"><strong>Comment la charge affecte-t-elle les performances des TC de protection ?<\/strong><\/h3>\n\n\n\n<p>A lower external burden generally increases the effective ALF of a P-class CT, but winding resistance must be included. For mainly resistive burdens, ALF<sub>actual<\/sub> \u2248 ALF<sub>\u00e9valu\u00e9e<\/sub> \u00d7 (R<sub>ct<\/sub> + R<sub>rated burden<\/sub>) \/ (R<sub>ct<\/sub> + R<sub>actual burden<\/sub>). Therefore, halving external burden does not generally double ALF. Use manufacturer data for the actual circuit and transient duty.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"what-is-knee-point-voltage-and-why-does-it-matter\"><strong>Qu'est-ce que la tension du point d'inflexion et pourquoi est-elle importante ?<\/strong><\/h3>\n\n\n\n<p>Knee-point voltage characterizes the bend in the secondary excitation curve. The IEC convention uses the point at which a 10% voltage increase produces a 50% increase in exciting current. It is particularly relevant when specifying PX-type CTs. Apply the relay manufacturer\u2019s knee-point, excitation-current and winding-resistance requirements; a simple voltage comparison does not establish suitability for every protection scheme.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"how-do-iec-and-ieee-ct-accuracy-standards-differ\"><strong>En quoi les normes de pr\u00e9cision des TC de la CEI et de l'IEEE diff\u00e8rent-elles ?<\/strong><\/h2>\n\n\n\n<p>IEC 61869-2 includes metering classes such as 0.2 and 0.5 and protection classes such as 5P, 10P and PX. IEEE C57.13 uses a different set of designations, including voltage-based relaying classes such as C100 and C200. The test conditions and burden definitions differ. Specify the standard required by the project and compare the full ratings rather than treating the class names as direct equivalents.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"what-accuracy-class-suits-capacitor-bank-switching-applications\"><strong>Quelle classe de pr\u00e9cision convient aux applications de commutation de batteries de condensateurs ?<\/strong><\/h3>\n\n\n\n<p>A capacitor bank can need phase overcurrent, earth-fault and sensitive unbalance protection, each with different measurement duties. Select CT ratio, accuracy, burden and transient performance from the bank configuration, expected unbalance level and relay instructions. A universal 5P10 or 0.5 recommendation cannot establish suitability for all capacitor banks.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p><em>Pour un support de sp\u00e9cification CT adapt\u00e9 \u00e0 vos besoins en mati\u00e8re d'appareillage, contactez l'\u00e9quipe d'ing\u00e9nieurs de XBRELE via <a href=\"https:\/\/xbrele.com\/fr\/contact\/\">la page de contact technique<\/a>. Nous proposons des solutions int\u00e9gr\u00e9es combinant&nbsp;<a href=\"https:\/\/xbrele.com\/fr\/what-is-a-vacuum-interrupter\/\">technologie de l'interrupteur \u00e0 vide<\/a>&nbsp;avec des transformateurs d'instrumentation correctement coordonn\u00e9s pour des performances fiables en mati\u00e8re de comptage et de protection, y compris <a href=\"https:\/\/xbrele.com\/fr\/transformer-protection-vcb-inrush-coordination-mistakes\/\">contr\u00f4le de la coordination des relais<\/a> pour les alimentations de transformateurs.<\/em><\/p>\n\n\n\n<p><strong>Technical references:<\/strong> <a href=\"https:\/\/webstore.iec.ch\/en\/publication\/6050\" target=\"_blank\" rel=\"noopener\">IEC 61869-2:2012 \u2014 inductive current transformers<\/a>; <a href=\"https:\/\/library.e.abb.com\/public\/194811e319ce2bb7c2256f9e00324a2f\/applicationCT_accuracylimitfactorENa.pdf\" target=\"_blank\" rel=\"noopener\">ABB \u2014 calculation of actual accuracy limit factor<\/a>; <a href=\"https:\/\/library.e.abb.com\/public\/eb71e99b34df40fac2256f230034b639\/applicationCTreqENa.pdf\" target=\"_blank\" rel=\"noopener\">ABB \u2014 CT requirements for non-directional overcurrent protection<\/a>; <a href=\"https:\/\/techdoc.relays.protection-control.abb\/r\/615-series-Technical-Manual\/5.0-FP1-IEC\/en-US\/Current-transformer-requirements-for-transformer-differential-protection?contentId=IOsDImuRMieCseqaGT6Irw\" target=\"_blank\" rel=\"noopener\">ABB \u2014 transformer differential CT requirements<\/a>. The ABB application notes explain calculation methods; final selection must use the applicable project standard and the selected relay\u2019s current instructions.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>","protected":false},"excerpt":{"rendered":"<p>Understand CT accuracy classes 0.2S, 0.5S, 5P and 10P, including rated burden, effective ALF and checks for metering and protection applications.<\/p>","protected":false},"author":3,"featured_media":4294,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_gspb_post_css":"\/* XBRELE: keep the image-free article hero readable. *\/\nbody.postid-2907 #main > .hero-section[data-type=\"type-2\"] {\n  background-color: var(--theme-palette-color-4, #080c0e);\n  --theme-text-color: #e6eef1;\n}\nbody.postid-2907 #main > .hero-section[data-type=\"type-2\"] .page-description {\n  color: var(--theme-text-color);\n}\n","footnotes":""},"categories":[27],"tags":[],"class_list":["post-2907","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-switchgear-parts-knowledge"],"blocksy_meta":[],"_links":{"self":[{"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/posts\/2907","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/comments?post=2907"}],"version-history":[{"count":5,"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/posts\/2907\/revisions"}],"predecessor-version":[{"id":4291,"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/posts\/2907\/revisions\/4291"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/media\/4294"}],"wp:attachment":[{"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/media?parent=2907"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/categories?post=2907"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/xbrele.com\/fr\/wp-json\/wp\/v2\/tags?post=2907"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}