SI51 is a Metal Halide Ignitor used to provide this additional voltage for a very small amount of time 1 to 5 microseconds. The voltage required to initiate the arc discharge on metal halide lamps is greater than the ballast open-circuit voltage and mercury lamps do not require ignitor. It offers SUPERIMPOSED – where the ignitor is situated less than 2m away from the lamp and works independently of the ballast – one wire of three to the ballast. It also offers IMPULSE – where the ignitor is situated more than 2m from the lamp. The winding of the inductive ballast is used to generate the required pulse voltage – two wires of three to the ballast.