sence of some form of foreign coating or material on the inside of the lamp's outer jacket can also indicate trouble. This can appear as a light yellow (with an MH lamp) or a silver mirrored appearance (with an HPS lamp). Remember, an HPS lamp will have some normal silver appearance around the lamp base. But, if all, or most, of an HPS lamp's outer jacket has that coating, there's usually a defect. Check the metal parts making up the mount or support for the arc tube for separation or damage. Also observe if the arc tube is cracked or broken. Another procedure is to put an ohmmeter (at the highest setting) across the lamp's center base contact and the base's screw shell. There should be no continuity (it should be open). If the ohmmeter shows continuity, there's a short in the base or mount, and the lamp will not operate. If these procedures don't help you uncover the problem, you can confirm the lamp is defective by putting it in a fixture known to be operational.
Ballast. An HID ballast typically steps up the incoming line voltage to a higher voltage to start the lamp. Thus, a common problem with a multiple-tap ballast can be an incorrect tap connection for the line voltage available. A defective core and coil ballast will usually show signs of its condition, such as darkening of the case or cracked or crumbling insulation, which definitely indicates excessive current flow or excessive heat buildup. Because of its construction, an F-can or encapsulated ballast does not allow you to see the winding. However, these two types often give off a strong odor if they have burned or melted ballast windings. You can make voltage measurements on an HID ballast, similar to a fluorescent ballast, using a multimeter. When you replace a defective ballast, also consider replacing the capacitor, ignitor (if used), and lamp.
Capacitor. Generally, a capacitor fails by either shorting or opening. When a capacitor shorts, it can damage the ballast, lamp, and/or cause excessive current flow, which may cause a breaker to trip. This type of failure is more typical in the older oil-filled (metal oval can) capacitor type than with the dry metalized film (round plastic can) type or the non-PCB oil-filled capacitors, which typically have an integral interrupter. A metal can type capacitor may bulge when it fails, particularly if it doesn't have an integral interrupter.
Usually, you use a single capacitor in a ballast circuit. However, sometimes a fixture has a pair of capacitors in the ballast compartment, wired in parallel to increase the capacitance. Thus, a common error in the field is the incorrect wiring of dual capacitors, which should operate in parallel, as seen in Fig. 3, in the original article. Verify the rating marked on the capacitor matches that on the ballast. Then, verify the actual capacitance in microfarads (mf), by taking a measurement. If low light output is the problem, the third rating to check is the mf tolerance (5) percentage. Some capacitors have a tolerance of 510%. A 110% would lower the wattage at which the lamps are operating. In applications where assurance of light level is vital, capacitors with a tight tolerances, such as 51%, are used (e.g. sports lighting applications).
You can test a capacitor with an ohmmeter set at a high resistance scale, after you turn off the branch circuit power and disconnect the capacitor from the circuit. Of course, always safely discharge the capacitor first. If the ohmmeter measures a very high resistance, the capacitor is open, so replac e it. If the reading is zero or a low resistance, the capacitor has a short. You should also replace it. If the reading is zero, or if a very low resistance shows initially, and the resistance slowly increases, the capacitor is good.
Ignitor. Some HID lamps require a high-voltage pulse, provided by an external starting aid, to excite mercury ions in the arc tube initially. This ignitor circuit produces a sharp impulse typically with a rating of 2500V to 4000V. The ignitor emits this pulse for one millionth of a second, at a minimum of once per half