Bit of a problem with 70w metal halide external floodlight

[Nick]

Installed new 3-4 years ago. Controlled by photocell dusk-dawn. Lamp originally supplied was double ended Osram HQI-TS 70W/NDL. Couple of weeks ago noticed that the light output was considerably down. Ordered replacement lamp from CPC and received Osram Powerstar HQI-TS Excellence 70w/ndl. New lamp slightly different in the tubular ends but otherwise very similar.

Installed new lamp and, at dusk, all hunky dory. Light output very much improved. A few days later the lamp turned on at dusk and turned off after about 5 minutes. Then flickered for about five minutes, ignited and stayed on all night. It now does this every evening. The photocell is remote from the luminaire and is not receiving light from the luminaire or elsewhere.

My immediate thought was a duff lamp but this is probably not so as it re-ignites and continues to burn constantly until dawn (this verified by observation and cctv). The same process makes me think that the control gear is working correctly. So I'm thinking that the problem lies in the differences between original and replacement lamps, which appear to be very minor. Unfortunately I have no way of testing any of the components other than by replacement. Worries me a bit that the flickering may damage components.

Any thoughts on what the problem may be and how to fix it? Thanks,

Nick.

[Andrew Gabriel]

This is really strange. One thought is poor lampholder contact, triggered as it heats/expands, causing the arc to extinguish until the lamp cools enough for the igniter to restrike it.

Another thought - have you got any equipment which might generate voltage sags, such as a large motor starting? That might extinguish the arc.

There is a note on Osram's technical information

operation of HQI-TS Excellence at PT-TS ballast - which are not produced or sold anymore for very long time - is not permitted. You might want to check what the type of ballast is.

Copying sci.engr.lighting

[Brian Gaff]

Have you thought that the previous lamp may well have not been the problem but the internal circuit instead?

Brian

[Nick]

Thanks Brian,

I have tried the previous lamp in an identical luminaire (on the bench) and it shows exactly the same characteristics. Strikes immediately with light output very considerably reduced (40% ish). Have tried this for several hours. The replacement lamp strikes immediately and extinguishes after a few minutes. It then has a few minutes stuttering and will strike. It continues to burn perfectly for twelve hours or so until the photocell switches to off. Your comments on the internal circuitry are perhaps quite relevant given Andrew Gabriel's response in this thread. My thoughts are: The original lamp has had its day. The replacement lamp may not be compatible with the gear in existing luminaire. Try a new lamp of the same kind as the replacement. Check the gear in the luminaire. If necessary replace some or all.

Thanks for your response, Nick.