Black boxes beget blind spots—How to avoid pitfalls in SPD testing

The current standards for testing surge-protective devices (SPDs), leave some ambiguities for their implementation. Field failure data show that some SPDs that pass standard tests can still fail in unacceptable or unexpected modes, perhaps because of “blind spots” in the test regimen. This paper shows examples of such unresolved issues and progress towards a resolution. It also urges closer attention to the disconnector design, function, and coordination with overcurrent-protective devices in the installation.