Do you mean the AIC rating of a protective device?
How familiar are you with short circuit currents? Do you understand what it means to have a bolted fault and the amount of current that flows as a result of that fault?
Do you know the requirements of NEC 110.9 and 110.10?
AIC stands for Amps, Interrupting Capacity which is basically the amount of fault current the device can 'clear' without failing (i.e. exploding).
A Series Rating is the effective AIC which results from having two protective devices tested and listed in series. The downstream device is not rated to interrupt the fault current on its own.
