Signs may be designed in ways that make it difficult to inspect the secondary wires, or faults may exist inside the ballast. In these cases, you must test the system to determine the source of the problem before replacing the ballast. First, however, let's review how ballasts operate. Lamps start in a sequence corresponding to the order in which they are numbered on the ballast's wiring diagram. To test any ballast, begin by checking the primary input voltage with the lamps and ballasts connected to the circuit.
Low voltage caused by overloads or undersized feed wires is the most common problem.
But the voltage drop won't show up on your voltmeter unless the sign is connected when you test. If you notice that the insulated, primary wires are very warm or hot to the touch, the wiring is probably too small for the connected load. If any significant voltage is present, the cabinet is not properly grounded. In this case, it should not be used as a substitute for the neutral wire when testing. Because ungrounded signs are hazardous and troublesome, this problem should be corrected as part of the repair.