-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 appearance, 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 star