Switchgear (as an assembly) will have a number of current ratings. For short circuits, you are concerned with: Rated peak withstand current (Ipk) The rated peak withstand current is the “peak value of the short-circuit current, declared by the assembly manufacturer, that can be withstood under specified conditions. The rated peak withstand current shall be equal to or higher than the values stated for the peak value of the prospective short-circuit current of the supply system(s) to which the circuit(s) is (are) designed to be connected.” (IEC61439-1)
and Rated short-time withstand current (Icw) (of a circuit of an assembly) The rated short-time withstand current is “the r.m.s value of short-time current, declared by the assembly manufacturer that can be carried without damage under specified conditions, defined in terms of a current and time. The rated short-time withstand current shall be equal to or higher than the prospective r.m.s. value of the short-circuit current at each point of connection to the supply. Different values of Icw for different durations (e.g. 0,2 s; 1 s; 3 s) may be assigned to an assembly. For a.c., the value of the current is the r.m.s. value of the a.c. component.”
and Rated conditional short-circuit current of an assembly (Icc) The rated conditional short-circuit current of an assembly is “the value of the prospective short-circuit current, declared by the assembly manufacturer, that can be withstood for the total operating time (clearing time) of the short-circuit protective device (SCPD) under specified conditions. The rated conditional short-circuit current shall be equal to or higher than the prospective r.m.s. value of short-circuit current (Icp) for a duration limited by the operation of the short-circuit protective device that protects the assembly.”
It should be apparent that conditional ratings are based on specific device cut of characteristics whereas rated short time withstand is only limited by the available current and the stated time duration. 1 second is a long time to have a 50kA fault on the board, 3 seconds is a lifetime - but essentially the board must safely contain the fault energy (ie, doors don't blow open, etc)
In the absence of a time rating, the minimum time would be 4 full cycles.
Switchgear (as an assembly) will have a number of current ratings. For short circuits, you are concerned with: Rated peak withstand current (Ipk) The rated peak withstand current is the “peak value of the short-circuit current, declared by the assembly manufacturer, that can be withstood under specified conditions. The rated peak withstand current shall be equal to or higher than the values stated for the peak value of the prospective short-circuit current of the supply system(s) to which the circuit(s) is (are) designed to be connected.” (IEC61439-1)
and Rated short-time withstand current (Icw) (of a circuit of an assembly) The rated short-time withstand current is “the r.m.s value of short-time current, declared by the assembly manufacturer that can be carried without damage under specified conditions, defined in terms of a current and time. The rated short-time withstand current shall be equal to or higher than the prospective r.m.s. value of the short-circuit current at each point of connection to the supply. Different values of Icw for different durations (e.g. 0,2 s; 1 s; 3 s) may be assigned to an assembly. For a.c., the value of the current is the r.m.s. value of the a.c. component.”
and Rated conditional short-circuit current of an assembly (Icc) The rated conditional short-circuit current of an assembly is “the value of the prospective short-circuit current, declared by the assembly manufacturer, that can be withstood for the total operating time (clearing time) of the short-circuit protective device (SCPD) under specified conditions. The rated conditional short-circuit current shall be equal to or higher than the prospective r.m.s. value of short-circuit current (Icp) for a duration limited by the operation of the short-circuit protective device that protects the assembly.”
It should be apparent that conditional ratings are based on specific device cut of characteristics whereas rated short time withstand is only limited by the available current and the stated time duration. 1 second is a long time to have a 50kA fault on the board, 3 seconds is a lifetime - but essentially the board must safely contain the fault energy (ie, doors don't blow open, etc)
In the absence of a time rating, the minimum time would be 4 full cycles.
