Per IEC 60364-4-43, the thermal stability of cable shall be verified by the following method:
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434.5.2 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 not exceeding that which brings the insulation of the conductors to the permitted limit temperature.
For operating times of protective devices <0,1 s where asymmetry of the current is of importance and for current-limiting devices k2S2 shall be greater than the value of the let-through energy (I2t) quoted by the manufacturer of the protective device.
For short-circuits of duration up to 5 s, the time t, in which a given short-circuit current will raise the insulation of the conductors from the highest permissible temperature in normal duty to the limit temperature can, as an approximation, be calculated from the formula:
t=(k*S/I)2
where
t is the duration, in second;
S is the cross-sectional area, in mm2;
I is the effective short-circuit current, in A, expressed as an r.m.s. value;
k is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures. For common conductor insulation, the values of k for line conductors are shown in Table 43A.
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My question is: is there a counterpart regulation in NEC? Thanks.
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434.5.2 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 not exceeding that which brings the insulation of the conductors to the permitted limit temperature.
For operating times of protective devices <0,1 s where asymmetry of the current is of importance and for current-limiting devices k2S2 shall be greater than the value of the let-through energy (I2t) quoted by the manufacturer of the protective device.
For short-circuits of duration up to 5 s, the time t, in which a given short-circuit current will raise the insulation of the conductors from the highest permissible temperature in normal duty to the limit temperature can, as an approximation, be calculated from the formula:
t=(k*S/I)2
where
t is the duration, in second;
S is the cross-sectional area, in mm2;
I is the effective short-circuit current, in A, expressed as an r.m.s. value;
k is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures. For common conductor insulation, the values of k for line conductors are shown in Table 43A.
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My question is: is there a counterpart regulation in NEC? Thanks.