wrote in message
oups.com...
When the outside lights in this apartment house come on at night by
photocell control they cause a loud buzzing noise in the buildings'
intercom system. These lights are the type that come on dim and warm up
to full brightness over several minutes. There are two large lighting
units mounted on the front of the building and three smaller units
mounted in the back. One of the large units is inoperatve in the front
and one small unit is also not working in the back. Are these what are
called "high pressure sodium"?

It's impossible to say without a description of the bulbs and the light they
emit. They could be metal vapour lights like Sodium, Cadmium or Mercury,
but they could also be HID lamps.

Dave