 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-base MH lamps that have additional protection inside the bulb, so you can use them in an open fixture. To ensure proper lamp usage, the open fixture has a special socket that accepts only an open-rated MH lamp. This socket is longer, and the opening is narrower than a standard socket.
Open-rated mogul base socket. In the last five years, certain MH lamps have required the use of a higher voltage rated socket we call open-rated. An open-rated, mogul-base MH lamp has an extended center contact on its base to fit into the cavity of a corresponding open-rated socket. For easy identification, this pulse-start socket, rated to withstand the up to 4000V pulse (open voltage) needed to start to start the lamp, is pink.
What is the purpose of an open-rated mogul base socket and an open-rated lamp? Recently, MH lamp manufacturers extended "pulse-start" technology, already used in metal-halide lamps 150W and smaller, to other portions of their product line. Superior to the older pinched arc tube design, a pulse-start lamp offers impressive improvements. It can operate at lower ambient temperatures; about 10DegrF lower than the pinched body lamps (140DegrF instead of 130DegrF), higher lamp efficacy (up to 110 lumens per watt), improved lumen maintenance (up to 80%), and consistent lamp-to-lamp color (within 100 degrees K). Smaller than a pinched arc tube, a pulse-start lamp's arc tube uses an ignitor to create a high-voltage starting pulse, as does an HPS lamp. While both types of lamps use a mogul base, a pulse-start lamp's socket (and ballast) is incompatible with a pinched tube MH lamp's socket (and ballast), and visa-versa. (See Fig.4, in the original article.)
Heat. Excessive temperatures can cause lamps, ballasts, capacitors, and ignitors to fail prematurely. This condition can also affect light output. In particular, high-wattage, compact fixtures using reduced jacket MH lamps can have elevated temperatures. Good indicators of excessive heat include: continual short life of components, cover lens with a discolored app