Superior Electric Powerstat & reg; with Ace Warning Glow variable output transformer. Autotransformers have one wire continuous from input to load. If this wire is incorrectly connected to the "hot" side of the service, a 110-120v potential exists between it and the ground. Reversing the terminals, connecting the continuous wire to ground will resolve the problem and extinguish the warning. The light also warns when output voltage reaches potentially dangerous levels (50-55v). Model 3PN116C. cULus UL508 rating.