The standard has been revised twice in the past few years. A major safety testing initiative (UL1449 2nd edition) became effective February 2007, and additional administrative and methodology changes - UL1449 3rd edition - became effective in September 2009. The 3rd edition also includes new independent performance testing requirements. All new equipment brought to the market must now comply with the 3rd edition. No UL standard currently exist for Metal Oxide Varistors. Most are qualified to UL1449 since it is the most relevant for surge suppression components. Most UL standards will have a reference for surge protection. Typically UL1449 is the standard referenced for AC circuits.