{"id":13188,"date":"2024-10-11T10:19:25","date_gmt":"2024-10-11T08:19:25","guid":{"rendered":"https:\/\/calcul-electrique.com\/non-classifiee\/too-much-thermal-stress-on-the-phase-conductor\/"},"modified":"2024-12-31T11:42:05","modified_gmt":"2024-12-31T10:42:05","slug":"too-much-thermal-stress-on-the-phase-conductor","status":"publish","type":"post","link":"https:\/\/calcul-electrique.com\/en\/faq-elec-calc-en\/calculation\/too-much-thermal-stress-on-the-phase-conductor\/","title":{"rendered":"Too much thermal stress on the phase conductor"},"content":{"rendered":"\n
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Article 434.5.2 of standard NF C 15-100 :<\/h5><\/div>\n\n\n\n

The breaking time of any current resulting from a short-circuit at any point in the circuit must not exceed the time required to bring the conductor temperature up to the permissible limit. <\/p>\n\n\n\n

For short-circuits lasting up to 5 s, the time required for a short-circuit current to raise the conductor temperature from the maximum permissible temperature in normal operation to the limit value can be calculated as a first approximation using the following formula : <\/p>\n<\/div>\n\n\n\n

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\"\"<\/figure><\/div>\n<\/div>\n\n\n\n
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Where: <\/p>\n\n\n\n

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<\/path><\/svg><\/span>t<\/strong> est la dur\u00e9e en secondes<\/span><\/div>\n\n\n\n
<\/path><\/svg><\/span>S<\/strong> est la section en mm\u00b2<\/span><\/div>\n\n\n\n
<\/path><\/svg><\/span>I<\/strong> est le courant de court-circuit effectif en A, exprim\u00e9 en valeur efficace (appel\u00e9 Ik<\/sub> <\/strong>dans le guide UTE C 15-105) <\/span><\/div>\n\n\n\n
<\/path><\/svg><\/span>k<\/strong> = facteur tenant compte de la r\u00e9sistivit\u00e9, du coefficient de temp\u00e9rature et de la tenue d’\u00e9chauffements du mat\u00e9riau du conducteur, ainsi que des temp\u00e9ratures initiales finales. Pour les isolations les plus communes des conducteurs, les valeurs de k<\/strong> pour les conducteurs de phase sont indiqu\u00e9es dans le tableau 43A <\/strong>ci-dessous :<\/span><\/div>\n<\/div><\/div>\n<\/div>\n\n\n\n
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\"\"<\/figure><\/div>\n<\/div>\n\n\n\n
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In elec calc\u2122 :<\/h5><\/div>\n\n\n\n

The permissible thermal stress of a cable is its capacity to withstand a circuit current during its elimination time without the insulation being damaged.<\/p>\n\n\n\n

The value of the thermal stress is expressed in A\u00b2.s (I\u00b2t).<\/p>\n<\/div>\n\n\n\n

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\"\"<\/figure><\/div>\n<\/div>\n\n\n\n
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In the absence of a manufacturer’s reference upstream of the conductor, the thermal stress on the conductor is calculated as a function of the maximum short-circuit current seen by the cable and the circuit-breaker opening time: I\u00b2 x t (I in A and t in s).<\/p>\n\n\n\n

This value must be less than the thermal resistance of the cable: k\u00b2xS\u00b2.<\/p>\n\n\n\n

Where: <\/p>\n\n\n\n

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<\/path><\/svg><\/span>S<\/strong> est la section du conducteur en mm\u00b2<\/span><\/div>\n\n\n\n
<\/path><\/svg><\/span>k<\/strong> est un coefficient calcul\u00e9 suivant l’annexe B de la norme NF C 15-500 et donn\u00e9e \u00e9galement par les diff\u00e9rents tableaux de la m\u00eame annexe B.<\/span><\/div>\n<\/div><\/div>\n<\/div>\n\n\n\n
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What’s the solution? Integrate a manufacturer reference on the upstream circuit breaker: <\/h5><\/div>\n\n\n\n

To avoid oversizing the cable cross-section, manufacturers incorporate a limiting effect in most protections. <\/p>\n\n\n\n

Limitation is read directly from a manufacturer’s curve for upstream protection (circuit-breaker or fuse): the energy limitation curve<\/strong>. It defines the maximum thermal stress borne by the conductor, according to the maximum short-circuit current flowing through the pipe. <\/p>\n\n\n\n

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\"\"<\/figure><\/div>\n<\/div>\n<\/div>\n\n\n\n
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Interpretation of thermal stress values in elec calc\u2122 : <\/h5><\/div>\n\n\n\n
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\"\"<\/figure><\/div>\n<\/div>\n\n\n\n

If i\u00b2t > k\u00b2s\u00b2<\/strong>, a thermal stress anomaly appears on the cable. <\/p>\n\n\n\n

For more information, see this article on checking thermal stresses in a conductor<\/a>.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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You can also find the tutorial on thermal constraints below:<\/p>\n<\/div>\n\n\n\n

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