While browsing through my mega-size lighting catalog, I noticed that MH fixtures can only be ordered with SCWA ballasts. What is it?
Super Constant Wattage Transformer, they are able to handle voltage variations of up to 45% so they are nice at the end of a long string of lights or remote locations, while they are not as efficient, when you have power issues and need the lights to stay running they are a good option, and for reference a CWA will take about 10% input voltage variation, A HX-HPF will only take about 5% Before either overpowering the lamp or having the lamp extinguish. SCWA's are not really commonly used except in remote areas, they are heavy, more expensive, larger and might not fit in the fixture, and use more power but not many 120 volt ballasts will support a lamp down to like 55 Volts AC at near full brightness without breaking a sweat (and are more common in higher voltages like 277,347,480, or 600, and really they aren't common outside the Canadian market, yes we have some really remote street lights, hundreds of miles from a highly reliable power grid)