5.1 Thermal effect

When a short-circuit occurs, a much larger current than the nominal current flows (10 to 100 times its value). This results in cables overheating which may damage the insulating material. The current must therefore be switched by a circuit- breaker or a fuse in a short enough time t _s for the cable temperature not to reach a critical value.

The thermal losses per unit of length is proportional to the square of the current:

R _L : resistance per unit length of the cable

If the current is not sinusoidal, the energy stored by the cable is:

t _s : switching time of the switching device

If we take the approximate value of a short-circuit sinusoidal current I _sc, then:

I _sc : r.m.s. value of the short-circuit current

In practice, the energy able to be stored in the cable depends on the conductor cross-section, the material of the core and the maximum temperature admissible in the insulating material.

A coefficient k, a function of the core material and the type of insulating material, is defined in LV (see Table 5-1) and can be applied in MV as follows:

hence:

or where

where:

• I _sc : short-circuit current at A

• t _s : switching time of the switching device in seconds

• S:...