Too much thermal stress on the phase conductor
Article 434.5.2 of standard NF C 15-100 :

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.

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 :

Where:

t est la durée en secondes
S est la section en mm²
I est le courant de court-circuit effectif en A, exprimé en valeur efficace (appelé Ik dans le guide UTE C 15-105)
k = facteur tenant compte de la résistivité, du coefficient de température et de la tenue d’échauffements du matériau du conducteur, ainsi que des températures initiales finales. Pour les isolations les plus communes des conducteurs, les valeurs de k pour les conducteurs de phase sont indiquées dans le tableau 43A ci-dessous :
In elec calc™ :

The permissible thermal stress of a cable is its capacity to withstand a circuit current during its elimination time without the insulation being damaged.

The value of the thermal stress is expressed in A².s (I²t).

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: I² x t (I in A and t in s).

This value must be less than the thermal resistance of the cable: k²xS².

Where:

S est la section du conducteur en mm²
k est un coefficient calculé suivant l’annexe B de la norme NF C 15-500 et donnée également par les différents tableaux de la même annexe B.
What’s the solution? Integrate a manufacturer reference on the upstream circuit breaker:

To avoid oversizing the cable cross-section, manufacturers incorporate a limiting effect in most protections.

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

Interpretation of thermal stress values in elec calc™ :

You can also find the tutorial on thermal constraints below:

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