How to dim a high-intensity discharge lamp

The National Electrical Manufacturers Association (NEMA) recently updated its guidelines for dimming high-intensity discharge lamps. Available in a document dubbed LSD 14, the guidelines cover dimming applications, combinations, and practices. The three technologies discussed are high-pressure sodium, metal halide, and mercury vapor, each of which responds differently to manipulation. Typical applications for such HID dimming systems include parking garages, warehouses, shipping docks, streetlights, supermarkets, ball fields, factories, and security lighting.

Lumen output vs. power output for high-intensity discharge lamps: (a) mercury vapor, (b) quartz metal halide, (c) high-pressure sodium.

Part of the difficulty associated with dimming such lighting stems from a change in output color temperature that happens as light input voltage changes. Also, ballasts used for dimming HID lamps must meet an ANSI specification for starting and operating a reference lamp at a rated lamp power. Magnetically and electronically ballasted dimming systems should not drop below these wattage specifications when using a reference lamp at the lowest recommended ANSI ballast input voltage.

Another point to note: NEMA says dimming systems also may not be capable of maintaining sufficient sustaining voltage in the dimmed mode. This may cause premature lamp dropout and short life. In particular, older lamps may be prone to dropout during dimming. Consequently, NEMA recommends a slower rate of dimming and recommends staying away from magnetic lag and regulated lag ballast dimming.

Download the report free here: http://www.nema.org/download.cfm?docId=17667&filename=/LSD%2014%20final.pdf