The joule is a common misleading parameter for gauging surge protectors. Any ampere and voltage combination can occur in time, but surges commonly occur for microseconds to nanoseconds, and experimentally modeled surge energy has been far under 100 joules. Well designed surge protectors should not rely on MOVs to absorb surge energy but more to survive the process of redirecting it. The UL 1449 testing standard addresses the safety of a TVSS device under temporary and abnormal overvoltage conditions, but does not specifically mandate a design that will give a reliable, long length of service in the real world. Specifically, UL 1449 tests that the TVSS remains operational at 10% above nominal supply voltage, allowing SPD manufacturers to design products that permanently disconnect just above that. Most reputable manufacturer’s designs allow for up to a 25% overvoltage.