Fluorescent tube failure mode

[Part timer]

Had a rather interesting (compared to the lecture) occurrence at uni today. An 8 foot fluorescent tube started to flicker. Everyone noticed it, I initially thought voltage sag, but it carried on. It went off, did its glowing purple/orange at one end trick, re-lit, probably repeated that once a minute for 10 minutes. Then it buzzed loudly and got brighter at the end that was dying. The brighter part spread about 2 feet down from the end that had been glowing purple, then it just died. No more flickering tube! I'd never seen it happen that quick before. The other end had no apparent symptoms or blackening to the phosphor. Mind you, at home, when a tube in a pair begins to die I will notice it, switch it off and remove the starter before replacing the tube asap. Related (lecture rooms): how much neglect can a VGA projector take in the air filter department? They all seem to flash up messages when powered on saying 50/100hr clean due, or check airflow. 50 hours isn't a lot when left mounted on a ceiling!

[misterroy]

you need to get out more.

[Part timer]

Oh trust me I do. Just not today! I spent too long in the same room and compared with what normally happens the light did the decent thing and died. Just wondered if anyone else on here had experience of the same.

[Dave Osborne]

No it's not, but the lamps run really hot and if the filters get clogged (as they will because they're quite fine) air flow will be reduced to the point that there's a risk of the heat from the lamp melting the innards and/or starting a fire. Consequently, the firmware has temperature monitoring and will shut the lamp down in the event of overtemp. On some projectors, they won't fire up until you do a service reset of the overtemp condition; others will fire up from cold, but turn off after an increasingly short period. Either way, blocked filters ultimately means an unserviceable projector.

[terry]

Interesting that UK fluorescent fixtures still use 'starters'? Here (eastern Canada) with 115 volt lighting and wall outlet circuits, and mainly 34 and 40 watt four footers and various other sizes there are either a conventional ballast or an electronic one, in all/most fixtures.

Only starters have seen are in very old (50 year?) style fixtures! Got a couple of them around to rebuild.

But is it because of the 230 volts (peak v. being around 320 v) that a ballast is unnecessary to 'strike' the arc?

BTW we obtained a couple of 347v fixtures from a being renovated s.market, they each contained a conventional looking 347v ballast (most likely 3 phase!) no starters, which we removed and in place installed 115v ones. They plus another couple of fixtures found dumped in a quarry have been fixed up cosmetically and electrically and are now part of a relatives home garage.

Just curious!

[Andrew Gabriel]

Well, starters work fine on 240V even for 8' tubes. They are used in all 240V countries. There is increasing use of electronic ballasts, but they aren't as reliable or cheap as switchstart ballasts, which pretty much never fail.

I can't tell from what the OP said if the fitting in question was switchstart - that's less likely in a lecture theatre (whatever the age of the lighting installation). My best guess from the symptoms would be SRS (semi- resonant start).

340V will strike an 8' tube with preheated cathodes, so no voltage spike is required on 240V mains. On 120V mains, a switchstart circuit relies on the back-EMF generated when the starter opens to strike the tube, which makes switchstart much less effective on 120V (starter doesn't always open at a point in the mains cycle when there's current flowing, so it doesn't work as well for you as it does for us). Also, you can't drive tubes longer than around 3' because there just isn't the voltage headroom for the series ballast without using a step-up transformer (which used to be done on 120V). Nowadays, a 120V ballast is more likely to be a leakage reactance transformer, which generates around 600V open circuit, but also limits the current flow, and this is cheaper/simpler to make than a step-up transformer and a (separate) series ballast, but not as cheap, simple, and reliable as our 240V series switchstart ballast.

[Mike Tomlinson]

In article , Andrew Gabriel writes

I remember seeing what must have been 12ft tubes at Paddington station about 30 years ago. They were mounted way high up on the passageway that divides the station and were not very bright at all. Wonder how they got started, must have needed a helluva ballast.

[Andrew Gabriel]

Sound like cold cathode tubes.

[Man at B&Q]

You mean rather than switch to something more complex and less reliable?

I'd say we have it about right.

MBQ