Metal housings are frequently used for TVSS because of the risk of failure causing fires or even explosions. UL1449 2nd Edition dictates that TVSS units MUST have safety features that prevent fires or explosion in the event of failure. All ASC products are independently tested by UL to ensure that they fail safely. In addition, the Thermo plastic box is NEMA 4X rated with gasket doors. This means that it is an Indoor/Outdoor unit. The housing is corrosion proof and UV stabilized. The clear door allows the status of the modules to be read clearly through the door, removing the necessity of lights in the door and the associated circuitry