May 2013 Electrical Apparatus

May 2013 Electrical Apparatus

This is a summary of the Electrical Apparatus May 2013 featured technical article,  by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical Apparatus

All electrical conductors are subject to damage by overheating when carrying current above some stated limit. That limit is determined by the balance between rates of heat generation and dissipation. A high fault current can be safely sustained for only a limited time. Hence, a maximum value of current squared times time, or I squared T, exists for cables, bus bars, and all other circuit components.

Whatever the thermal limit of conductor insulation may be, this maximum temperature is not reached instantly when a fault occurs. The higher the current, the shorter the allowable time. Cable manufacturers have therefore determined the allowable current-carrying capacity of their products based on the I squared T relationship that is derived from both theory and experience.

Mechanical stress is also caused by electromagnetic forces during a circuit fault, which are also functions of current squared. Such forces can dangerously reduce electrical clearances, bend bus bars, crack insulators, or break supports. Cables may whip back and forth.

The short-circuit current rating (SCCR), formerly identified in some standards as a withstand rating, is not the same as an interrupting rating, which is assigned to any protective device intended to interrupt fault current. Circuit breakers, in addition to their interrupting rating, also have a much lower short-time current rating, associated with a specific time—typically between 15 and 30 cycles.

For transformers, the equivalent of an SCCR is a through-fault capability. This is the current (or kVA) overload the transformer can safely withstand during a downstream fault. Although overheating does occur, the major effect of such fault current is the electromagnetic force that violently distorts the winding, leading to destructive internal short-circuiting.

When a circuit element is stressed to a thermal or mechanical limit, uncertainty often exists as to how often that stress can be repeated. Repetition involves fatigue. Circuit breakers and some switching devices are therefore assigned stated limits on the number of operations or faults to which they may safely be subjected. Similar repetition limits have not generally been stated for cables and connections themselves.

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