Why is a Grounded Fixture Needed for Reliable Starting?
Many fluorescent fixtures will not start reliably unless they are connected to a solid earth (safety) ground. This is most likely the case with rapid or trigger start magnetic ballasts. These will usually state on the label: “Mount tube within 1/2 inch of grounded metal reflector”. If this is not done or if the entire fixture is not grounded, starting will be erratic – possibly taking a long or random amount of time to start or waiting until you brush your hand along the tube. The reason is straightforward: The metal reflector or your hand provides a capacitive path to ground through the wall of the fluorescent tube. This helps to ionize the gases inside the tube and initiate conduction in the tube. However, once current is flowing from end-to-end, the impedance in the ballast circuit is much much lower than this capacitive path. Thus, the added capacitance is irrelevant once the tube has started. The reason that this is required is probably partly one of cost: it is cheaper to manufac
