Class 1, Class 2, Class 3
Let-through voltage, or Suppressed Voltage Rating (SVR) performance Classes defined by the Federal government CID in their power line surge suppressor specification. Class 1 products have a SVR of 330 volts (the best rating). Class 2 products have a SVR of 400 volts, and Class 3 have a SVR of 500 volts. The Class of a given product may change depending on the Grade it is tested to, since the test conditions are different for different Grades, and this may influence the SVR, hence the Class.
Note – This standard is no longer used by the US Government but you may still see it listed as a property for surge protection.