 Because of this, all electrical ballasts can fire MH bulbs, but only a Switchable or HPS ballast can fire an HPS bulb without a conversion bulb.Usually a metal halide conversion bulb will be used in an HPS ballast since the MH conversion bulbs are more common.A switchable ballast is an HID ballast can be used with either a metal halide or an HPS bulb of equivalent wattage.