The FS4 fluorescent starter is a "twist-and-lock" glow-switch designed for preheat fluorescent lighting fixtures, typically operating 30W to 40W lamps on 120V circuits. It utilizes a gas-filled glass bulb with bimetallic electrodes and an internal capacitor to heat the tube’s filaments and initiate a voltage spike for ignition. The FS-4 starter is cULus approved. A starter should be replaced when the lamp flickers constantly without lighting, the lamp glows at the ends but fails to light up fully, delayed startup, or repeated, erratic blinking.