Short Time Emergencies. Disconnect switches can be subjected to currents higher than those calculated using Equation (3), above, for periods not exceeding the thermal time constant of the switch, generally 30 minutes. The permissible Short-Time Emergency (STE) and Drastic Action Limit (DAL) currents can be calculated using the following Equation (4) and substituting 15 minutes or 5 minutes, respectively, for the time duration, d: Ie(st) = ⎡ 1 ⎛ θ max − θ max e ⎞⎤ Ir ⎢ ⎜ + θ max− θa ⎟ 0.5 Equation (4) ⎜ ⎥ ⎢θr 1 − e−d /τ ⎟⎥ Where: ⎣ ⎝ ⎠⎦ Ie(st) is the permissible short time current, and d is the appropriate time duration (15 minutes for STE and 5 minutes for DAL) τ is the thermal time constant of the switch in minutes Values of τ may be conservatively assumed to be 30 minutes for switches rated 1200 Amperes or greater. Contact the manufacturer for switches rated less than 1200 Amperes.
Short Time Emergencies. A circuit breaker can be subjected to currents higher than those calculated using Equation (2), above, for shorter periods, provided that no component exceeds the limits of total temperature specified in Table 1 of ANSI/IEEE Standard 37.04-1999 by more than 15 °C. The permissible time for carrying short-time current is given by Equation (3), which can be restated to determine the permissible short-time current in terms of time and temperature (Equation (4)).
Short Time Emergencies. A circuit breaker can be subjected to currents higher than those calculated using Equation (2), above, for shorter periods, provided that no component exceeds the limits of total temperature specified in Table 1 of ANSI/IEEE Standard 37.04-1999 by more than 15 C. The permissible time for carrying short-time current is given by Equation (3), which can be restated to determine the permissible short-time current in terms of time and temperature (Equation (4)). max(1) a t = 1n1 (3) s I 1.8 s 1 Ii t
