400 Watt S51 High Pressure Sodium 5-Tap Ballast KitHPS Ballast Cross Reference: Advance Ballast 71A8453 001D Universal Ballast S400ML5AC5M 500K Howard S0400 81C 211 Sola E 971 WDD SO400 High Intensity Discharge (HID) lamps require a ballast to apply starting voltage to the lamp and establish current flow an arc between the lamp electrodes. Without a ballast, the arc will extinguish or draw increasing current until some circuit element burns up. Ballasts provide system stability by limiting the current