The History of Safety in the SPD Industry

If one reviews the history of Surge Protective Devices (SPDs), they were created without a full understanding of surges and safety issues. Most thought SPD operation would only occur when suppressing lighting stroke associated surges. In real world scenarios this assumption proved to be incorrect. In reality SPDs tried to control numerous electrical disturbances such as Transient Overvoltages (TOVs), fault conditions, and ferroresonace. It is these types of disturbances that typically cause catastrophic failures.

In the infancy of the industry the utilization of SPDs was low and resulted in low visibility of incidents. As the usage of SPDs increased and the industry grew the results of unsafe SPDs became known, i.e. fires and/or explosions. In the mid 1990’s the surge industry attracted a lot of attention from gear companies because of liability concerns and business possibilities. Placing the suppressor integral to the gear required that the gear companies had to greatly increase safety in order not to damage other electrical components within the gear. SPDs are intentionally put in harm’s way and their failure effects needed to be fully understood.

SPDs/MOVs operate by varying impedance based on voltage. It is not immediately obvious that MOV failures also involve varying impedance. There are nearly infinite failure impedances resulting in equally numerous failure scenarios. Coordinating overcurrent protection is much trickier than a motor that tends to fail hard short. There is also the paradox of SPD fusing needing to pass large surge currents, yet clear low fault currents. After copious amounts of time and money APT and its gear company partners determined that selecting conservative fusing and incorporating thermal disconnectors that operated correctly and allowed for mild fluctuations resulted in passively failed SPDs.

In order to take advantage of a perceived weakness in conservatively-fused SPDs, as a marketing tactic, some manufacturers promoted their products ability to pass humongous surge currents that cannot exist within building wiring. In order to pass these excessively large surge currents these manufacturers were skewing their fusing and in some cases deleting it, making their product unsafe. They were trading safety for marketing purposes. As fusing is skewed to pass larger surge currents, it creates a safety blind spot at lower fault currents. This is a safety hazard because high impedance, low current faults tend to create SPD fires. For example, an MOV failed at 10Ω, while connected to a 277V system will draw 28A. 277V x 28A = 7.8kW, which will generate substantial heat, yet not be cleared by a 30A breaker/fuse.

In 2002 a proposal was submitted to UL pointing out fusing problems in SPDs and included one brands installation manual citing published fusing deficiencies. This ultimately led to a two part revision of UL 1449. The initial portion of that is UL 1449^2nd Edition Revision (aka Rev. 2.5) and the second is UL 1449^3rd Edition.