earance, defective ballast with charred windings, internal arc tube that looks black, and bulged or bluish outer bulb of the lamp. Placing construction material around an HID fixture may cause excessive temperature, or create a condition that restricts air flow and traps heat.
Test measurements. When troubleshooting, check the line voltage at the fixture, not at the panel, to account for any voltage drop. In most cases, the supply voltage should be within 10% of the rating for best operation. Some reactor ballasts require the voltage to be 55% for proper operation. If the line voltage exceeds the recommended range, erratic lamp and ballast operation may result. Identify and correct the external cause of the low or high voltage by running a larger gauge wire (if you find voltage drop to be the problem), changing taps on a distribution transformer, or balancing/adjusting the loads on multiphase power systems. Next, check the open circuit voltage (OCV). OCV is the voltage potential across the lamp socket and is higher than normal lamp operating voltage. This is because you need a high peak-to-peak voltage to strike an arc for starting the lamp. Read the OCV of an energized HID ballast by removing the lamp and placing one alligator clip of a true rms voltmeter on the lamp socket's center contact, and the other alligator clip on the screw shell of the socket. Using a meter other than a true-RMS reading tester can lead you incorrectly identify a ballast problem. If the OCV reading for the specific ballast is not within range, check to see if the ballast is receiving the correct line (input) voltage, and for a good connection of the correct ballast tap. If both are correct, the problem is usually a defective/marginal ballast or capacitor or an incorrect ballast/lamp combination.
Make sure you take specific safety precautions when working on an HPS ballast or some MH units, since they use a high-voltage starting circuit (the ignitor) to initiate arc conduction. Disconnect this starter circuit, since its high-voltage starting pulse can damage a multimeter. The voltage on 50W through 400W lamps is at least 2500V, and for the 1000W lamps at least 3000V. Therefore, determine if an ignitor is part of the circuit, and disconnect it before taking an OCV reading. A short-circuit current measurement gives you the highest current the lamp could see. It can also indicate if the ballast/capacitor combination is correct (or if the ballast/capacitor is defective).
To do the SCCM test, deenergize the fixture. Then, use a jumper lead with alligator clips at each end to create a dead short on the secondary side of the ballast; directly at the socket (the ballast acts as a choke and limits the current). One end of the of the jumper clips to the center contact; the other end clips to the screw shell. For maximum safety, use an HLR fuse holder with a 10A GLR fuse in the test wire, because the ballast could have an internal short. Although the fuse could blow, it usually doesn't. But for the few times it might, this precaution could save you from serious injury. While making sure your ammeter is not close to the ballast's magnetic field, since it could distort readings, clip the ammeter around the jumper wire. Next, energize the circuit, and take the reading. If the reading doesn't fall within the specified range, check the ballast input voltage and ballast wiring. If both are correct, the problem is probably a defective/marginal ballast or capacitor or the incorrect ballast/lamp combination.