IEEE C37.24-2003 Historical

Switchgear will perform satisfactorily and have a reasonable life when operated within the temperature limits estab-lished in ANSI/IEEE C37.20.1, C37.20.2, C37.20.3, C37.21 and C37.23. These standards specify the temperature rise limits above a standard (maximum) ambient temperature of 40 ?C. This is satisfactory for indoor applications where the temperature rise is due entirely to heat release (internal losses). In outdoor applications, the limiting tem-peratures result from the net effect of internal losses and external influences, principally the sun, wind and local am-bient temperatures. All of these must be considered in determining the current-carrying capacity of outdoor metal-enclosed switchgear. The magnitude of these factors will vary geographically and from season to season. The time relationship of maxi-mum circuit loads with respect to maximum ambient temperature is important. It is not practical to design switch-gear on the basis that all adverse factors reach their maxima coincident with maximum loads. If this does not occur, full current ratings may be realized. Recommendations will be made to point out the cumulative effect of these various influences.