Short circuit current thermal effects

The prospective short circuit currents have to be determined at all relevant locations in the installation by calculation or measurement and protective devices selected to protect all conductors against thermal and mechanical effects. For new installations the designer will first have to ascertain the loop impedances and characteristics of the excess current protection at the intake from the supply company (see Regulation 313-01-01), except where private... 

Protection Against Short-circuit Current. A short circuit is a special case of excess current. However, a short-circuit failure differs from overload in that damage to the circuit and cables occurs very rapidly. Protective devices must therefore be able to respond quickly to these failures. As far as any type of fuse is concerned a short-circuit failure will cause it to operate more rapidly because of the increased thermal effect. Circuit-breakers, on the other hand, usually have a separate part of their operating characteristic which is specifically designed for short-circuit failures. A miniature... 

Before creating a fault condition, to obtain the required /sc the impedance of the test circuit is adjusted so that the required fault current is obtained in all the phases on creating a short-circuit. To provide the required thermal effect (/sc /). the duration of test, /, is then adjusted accor-dingly. The relevant standards therefore stipulate that the test current may be higher or lower than required and can be compensated by adjusting its duration, i. 

When the film is short-circuited and heated to high temperatures at which the molecules attain a sufficiently high mobility, a current is observed in the external circuit. This phenomenon is called pyroelectric effect, thermally stimulated current, or, when the film has been polarized by a static field prior to measurement, depolarization current. The conventional definition of pyroelectricity is the temperature dependence of spontaneous polarization Ps, and the pyroelectric constant is defined as dPJdd (6 = temperature). In this review, however, the term will be used in a broader definition than usual. The pyroelectric current results from the motion of true charge and/or polarization charge in the film. Since the piezoelectricity of a polymer film is in some cases caused by these charges, the relation between piezoelectricity and pyroelectricity is an important clue to the origin of piezoelectricity. 

The thermal effects caused by short-circuits can be calculated as follows the overtemperature AT of a conductor of length l, resistance R, cross-sectional area A and volume V, made of material with density cr and mass m, specific heat c and specific resistance p, caused by the current /(t), and without any heat dissipation to the environment, reads ... 

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