fluorescent lamp issues


Senior Team Emeritus
Premium Member
Story problem as it were.

Got a call today that a client with less than one month old 40 watt 48" T-10 Chroma 950 lamps already had almost half of them bad. They sent back the bad ones and they had characteristic bad lamp brown/black ends on one side of the lamp. Not extreme dark and fairly wide in darkened end, but not un-normal as it were to see on a bad lamp. Main problem is that these lamps are less than a month old.

The customer also noted in also sending back a half case of bad original lamps, that their 40 watt 48" T-12 Cool White lamps they used to use in the fixtures didn’t seem to have these lamp life problems. The T-12 lamps on the other hand seemed to have the same above brown/black ends on them, but again did not in a noticeable way seem to suffer from an overt short lamp life.

In further refining the information about the place the lamps were installed, it was stated that the lamps are not switched on and off much and in fact, it’s thought that they are normally on. Also that the lighting fixtures that the lamps are in are surmised to be in decently old drop ceiling fixtures. These are not new lighting fixtures and will no doubt have normal magnetic ballasts designed for a 40w lamp.

Main question by the customer is why the T-12 lamps seem to last just fine but the T-10 lamps died out within a month?

I'm fairly certain of the answers to this question.. have a hypothesis at least and am waiting on more conformation from the lamp manufacturers and from an electrician. Question is more one of what I'm suspecting and why?
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CB Mods
Premium Member
Personally I'm thouroughly convinced that Flourescents work purely by magick. So if they are not working right I think you say an incantation, wave a copy of Teslas biography at them and hope they start working again.


Senior Team Emeritus
Premium Member
This sounds like a Ballast issue....

Off the top of my head--the difference in a T10 and T12 is the diameter-- (1 1/2" vs 1 1/4" if I recall right) and the overall lumen output from a T10 is more then a T12 (the T10 is a Very High Output lamp usually)... But also it varies greatly in the over power efficiency of the lamp itself for differences. Usually the bulb failure you describe with the blackening of the ends is often the result of small cracks or seal breaks in the burner (bulb) or regular vaporization--which happens normally over time and regular use. When the lamps are switched ON and then OFF and then ON again I know this sudden occurance can cause a rapid expansion on some bulbs--which eventually will create leaks and cracks and failure prematurely. The expansion and contraction of the Fluorescent takes its toll too on the old magnetic ballasts which are in a nutshell "on/Off".. If I recall the new electronic ballasts in the compact flourescents do a an actual pre-heat of the electrodes to prevent and limit a lot of this, which is really strongly needed on some of the newer lamps for the best lifespan possibility and to minimize such problems happening prematurely....

We had a similar problem here with our lamps as they all were getting switched over to T10s and we had older ballasts--and they both would die pretty quick. As it was explained to me for the cause is what I said above... Interesting tho is the T10s tend to use less wattage--a 40 watt T12 will consume 40w--but a 40w T10 will consume something like 35w thus saving energy but being brighter overall...and additionally to the above comments--the power from the ballasts for the T12's vs the T10's is another part cause of this issue--not every magnetic ballast that can take a T12 is designed to handle the output of the T10's--in fact some ballasts specifically say they are NOT designed to use T10 lamps. I actually had ballasts fail when we switched to T10's from T12's. I would guess that a lot more thermal heating around the contact points/electrodes from the higher lumens and lower wattages and lack of preheating when using an older ballast with a T10 vs a T12 would create a thermal issue of sorts which would lead to early bulb failure. So--my guess would be that if they replaced their ballasts with a ballast that is designed FOR a T12 AND T10, then the newer T10 lamps and the heat/voltage imbalances created would be resolved. Not every Ballast for a T12 can take T10's. Rapid Start ballasts on the older T12's are often not suggested for the Very High Outputs of the T10 lamps.

I'm not sure as I don't have my books around me but I think a 40w T12 and a 40w T10 should both have a lifespan of around 4k-4500 hours....which should be around 170 days minimum on a constant 24/7 ON schedule...so they should be getting a TON of life out of the bulbs they have..

Dunno if that helps you with your theory or not....


Senior Team Emeritus
Premium Member
Yep, that theory(s) helps much in some thoughts I didn't have or explaining why the T-10's don't but the T-12's will.

Both lamps are the same wattage - I highest outputted these along with highest CRI for the chosen color temperature. Totally maxed out.

That's why innitially I was having that logic problem of 40w vs. 40w no matter the lamp size as long as the same wattage. I will have come more to your conclusion if it were say a 32w lamp having this problem but couldn't put my finger on how the 40w lamp T-10 was having a problem for shure. My Ushio rep of course never responded to the question about their lamp.

I knew the T-10 was a more high output and was theorizing but couldn't find anything to anser this question amongst a few books looked into about it. Mine was that it has less of a tolarance for older ballists due to it's high output smaller size - kind of like a HPL 575/115v vs. a HPL 575/120v, but your explination works well in forming it up also.

End result was to recommend that the customer change ballasts, and given the sales person didn't tell me the application in perhaps recommending say 20 year old ballasts might not work so well - possibly, believe we wrote off the lamps in not being any good for return but given it's what we specified...

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