Grade A, Grade B, Grade C
The Federal CID defined three grades of surge suppressors based on endurance testing at several surge test levels. Grade A is for 1,000 surge endurance with 6,000 volts, 3,000 amperes applied. Grade B is for 1,000 surges with 4,000 volts, 2,000 amperes. Grade C is for 1,000 surges with 2,000 volts, 1,000 amperes applied.
This standard is no longer being used by the US Government but you may find it referenced in our older literature.