Up until recently, the only MV fixtures I messed with were vertical with open diffuser...you know, yardlights. But now I have an M250 Pwr-Door with glass drop lense and noticed something when I shut lamp off.

Normaly when I shut off a MV bulb, I can see the electrodes glow orange for a couple seconds. But in this cobrahead when I shut the lamp off and immediately open lense to observe lamp, the whole arctube is seriously glowing red. After electrodes stop glowing, the glass continues to glow for a few more seconds. This is a brand new bulb. So, is this normal for a fully
enclosed, horizontally mounted 175w lamp?
Sincerely, Don

hmm i have noticed that lamps that are higher watts when they turn off still glow like hot coals for a few sec's feel the top of the fixture if it feels hotter then u think is good change the bulb might be just the bulb also so just seem to do that if u ever question or feel that something not right go with your feelings. example one day when i was with my "teacher" that was a jerk we came up to a 1000 watt box light at a ford dealership in back and this thing was HUMMING!!!!! sounded like a large bee flying. anyway i went up to "change the bulb" that was not lit and i said the whole time i doubt its the problem but u know im the new guy so i went up there new 1000 watt hps bulb did not light i went back down in the bucket he went up now this time im telling him lets cut the power ok it's not safe he said no im like ooookkkk he goes up starts playing with the top cover of the box light i see spark city !!!! the netural wire vaporized in his hand's and knocked him on his ass!!!! it was 480 v also he was ok scraed the poop out of me and ill never never forget about it!!! so if u think something is not right better to be safe then sorry. thought id share a story with u

Are you an electrician or something?

Some of my streetlights do glow whole arc tube orange...for MV........so I think it's OK....I said I THINK but not know...but I think it's OK since mine does the same for some of the lights....

yeas i work for a service comapany here in mesa arizona east of phoenix alot like steve's bge in a sense

Yes they can overheat because I had one running for a while right wattage and it still popped from the heat look at this pic I uploaded to see http://www.lighting-gallery.net/gallery/displayimage.php?album=70&pos=10

crazy was it a old bulb that poped ? mr big

I have a theory: Lamps in fully enclosed lumes will trap more heat. Also, a horizontal lamp will cause the arctube to get hotter because convection causes heat from the length of the arc to heat up the whole top length of the arc tube. I guess this is why a MH is more likely to explode when mounted horizontally.

Also, I read that the early mercs that were horizontal had an electromagnetic coil above the bulb to deflect the arc from bowing upwards and melting the arctube. Interesting stuff. I'd like to hear more info about that. 

When mercury lamps are operated horizontally, the arc bows upwards, nearly touching the quartz wall. That may be why they glow orange when operated horizontally. But this is still far from the temperatures of MH arc tubes, much less the melting point of quartz. Melted quartz actually glow white. I have a GE Bonusline 400 watt merc that was operated horizontally inside a Form 400 for many years. As a result the arc tube developed superfical cracks where the arc stream nearly touchs the quartz wall. Amazingly, this lamp still works. I have a Wheeler open type mercury street light with a horizontal bulb, so I will check to see if the overheating is from the arc bowing upwards or from being totally enclosed.

I feel a little better now. Come to think of it, some of the mercs from the 60's and 70's were known to last for 20 or more years in horizontal fixtures.
I think the early mercs I read about were from the 1940's or so and couldn't tolerate the heat very well.  Also only lasted somewhere around 3000 hours.

When mercury lamps are operated horizontally, the arc bows upwards, nearly touching the quartz wall. That may be why they glow orange when operated horizontally. But this is still far from the temperatures of MH arc tubes, much less the melting point of quartz. Melted quartz actually glow white. I have a GE Bonusline 400 watt merc that was operated horizontally inside a Form 400 for many years. As a result the arc tube developed superfical cracks where the arc stream nearly touchs the quartz wall. Amazingly, this lamp still works. I have a Wheeler open type mercury street light with a horizontal bulb, so I will check to see if the overheating is from the arc bowing upwards or from being totally enclosed.

I think I saw that lamp in question.  Nothing compared to what happens to MH's when operated horizontally.

I feel a little better now. Come to think of it, some of the mercs from the 60's and 70's were known to last for 20 or more years in horizontal fixtures.
I think the early mercs I read about were from the 1940's or so and couldn't tolerate the heat very well.  Also only lasted somewhere around 3000 hours.

you know....I have some 1970s bulbs that has been in the streetlights in my area that was there as early as 1973 until 2005-2006 and still works.......my bulb that was from 1973....is very dim.....but there was one point it would not start at all...but it came back to life..........also I have a clear bander bulb that would start hard...it would have hard time starting like starting then go off the n start very fast until it successully fires up....that one was from the late 1970s

That's interesting because I thought when a merc dies, it's done for good.
The burned out ones that I fooled with wouldn't start unless I put a couple thousand volts to it and the arc in the tube would be orange colored. Some times I had to remove the arc tube first because a few thousand volts would just arc at the bulb stem. But I never could get a burned out merc revived enough to operate on a normal ballast.

sometimes they work better on one ballast than the others..........my friend Steve said that he replaced some lights that would go on sometimes and sometimes it wouldn't at all...then the next night it goes back on and some nights it won't Like this light....this one was reported that sometimes it would work and other times it wouldn't it has a 1996 bulb which was 10 years old!

Where I work we run a ceramic MH philips master color not sure on the wattage but running these horizontally for 24/7 365 days a year no break unless the electricity goes out the previous ones were made by GE they were operated at a 90 degree angle and the arc tube still popped very, very loudly and we have 2 running with the ceramic metal halides when I replaced it the arc tube was as black as coal for the full length they glowed green before I replaced them and they had suddenly switched from the daylight blue color to green right as I was walking by it and as everyone knows when metal halides all of a sudden switch their color temprature they are at a very high risk of exploding anyway I replaced the lamp I looked at the old one and it was bulged just as the GE ones did