ype 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 cycle, until the lamp starts, and then shuts off when it senses a current draw. It will also pulse when a lamp fails or the socket is empty. You can partially base ignitor life on the number of times the unit pulses. For that reason, you should repair or deenergize a nonoperating fixture as soon as possible. An oscilloscope with a proper high-voltage probe can show the pulse. HPS and some newer MH lamps use an ignitor as part of the ballast. We'll discuss this factor, because it can cause a maintenance person to troubleshoot problems that don't exist.
Socket. Because of the higher voltages carried and heat generated in its vicinity, you should check the socket carefully for cracking of the ceramic housing or discoloring of the metal parts. One note of caution: Manufacturers configure many MH lamp sockets specifically for a particular type of lamp. Thus, they should not accept any other type of MH lamp. MH sockets can produce the following confusing situations:
Position-oriented mogul base (POMB) socket for MH lamps. This type of socket fits a horizontal-burning lamp only, which has a bowed arc tube. When operated with that orientation, it offers increased light output. The lamp's screw-shell base has a protruding pin, about 1/4 in. in length. This configuration allows you to install the lamp in a POMB socket that has a slot to accept the pin, thereby ensuring proper burning position. For easy identification, the POMB socket is yellow. Thus, a POMB lamp will not screw all the way into a standard mogul base socket. Do not attempt to cut, grind, or bend the pin to make the lamp fit. Altering the pin may allow the lamp to descend further into the socket.
Extended husk medium-base socket. You can use this type of socket for a special MH lamp in an open fixture; one without a cover lens. Generally, you must use an MH lamp in a fixture with a cover lens. This requirement is a safety measure in the rare event a lamp ruptures or explodes at the end of its life. Over the past 10 years, lamp manufacturers introduced medium