Typically, high-pressure sodium (HPS) ballasts support constant current rather than constant voltage. The ignition high voltage pulses varies from 1 kV to 5 kV, depending upon the type of lamp.
For example, if a 250-watt high pressure sodium ballast is operated on a 120-volt supply, the starting line current is only around 1.3 amps.
High pressure sodium lamps can have a large overwattage (three times rated wattage) and survive, at least in the short term (usually hours if the ballast provides enough voltage). Exceeding rated wattage by more than 15 percent, however, is not recommended.
The normal evidence of end of life for a high pressure sodium lamp is on-cycling. This results when an aging lamp requires more voltage to stabilize and operate than the ballast is able to provide. When the lamp's rising voltage exceeds the ballast output voltage the lamp extinguishes. Then, after a cooling down period of about a minute, the arc will restrike and the cycle begins again.
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