Melting ballasts

[Ike]

I've just been replacing a load of sodium flood lights with metal halide ones and have found that the ones that were left with dead or dying lamps in them have very melted and deformed ballasts, a couple even went short circuit.

 

I know the lamps should have been replaced sooner and thermally protected ballasts could have been used but as we were converting to metal halides anyway for a nicer CT I wasn't too bothered, I was just curious as to why it happened.

 

Any suggestions of good reading on discharge lighting would also be appreciated.

 

 

Thanks in advance,

 

Ike

[Mush]

Try asking over at usenet sci.engr.lighting about the ballst meltdown.

 

Good sites about lamps of all sorts :

 

http://members.misty.com/don/

 

http://www.lamptech.co.uk/

[Jivemaster]

Discharge lamps first conduct as rectifiers so for the first few loops the draw is only half cycles so the current is only limited by the DC resistance of the ballast. In a good lamp this state lasts only a few loops then the arc stabilises both ways becoming AC and ballast impedance limited. In a dead lamp this can last until thermal failure happens to break the cct.