Is it safe to change fluorescent lamps when the unit is live

[Iggifer]

90 degrees

Thats what I said!

 

[333rocky333]

Must confess, when I was ill-educated, I have done that, but never got a shock.

Surely, there's no "connection" between the two pairs of pins until both pairs are connected to their respective lampholders?

I recall It was more prone in the 80's when starterless transtar units were used, i think somehow a voltage on the filament was induced via the gas.

As you say there aint no electrical connection so you might assume it would be ok to touch the end

 

[333rocky333]

Oddly enough, some philips fittings have two wires to one filament, and only one wire to the other filament, so who knows what goes on inside the glass tube

Wheres john he will know

 

[SimonH2]

The majority of tubular lamp holders don't make the circuit until you rotate the lamp ...

Fairly common these days, but older fittings didn't have namby pamby safety features like that. Though I suspect it's more a case of being easier/cheaper to make "insert and rotate" tube holders than to make spring out holders.

I too have done tubes live, and as described, if it's a long tube you pretty well have to "feel" the orientation of the pins as it's not possible/practical to holed the tube so you can see the pins while also lifting it up and putting the pins in the other end of the fitting. I was never really comfortable doing that with live sockets at the other end of the tube.

 

[JohnW2]

Oddly enough, some philips fittings have two wires to one filament, and only one wire to the other filament, so who knows what goes on inside the glass tube ... Wheres john he will know

You reckon? I have no idea, and have never seen such an animal. If there's literally only one connection to one end, that presumably must mean that there is no filament at that end (there ain't going to be a conductor going along the tube) - which would seem odd. Of course, 'instant start' tubes have/had no filaments at all (just used a high enough striking voltage to strike 'cold', like a 'cold cathode tube'), hence just one pin at each end, but it would seem pretty odd to have a filament at one end and not the other. Maybe, for some reason, it's a half-way house between 'ordinary' and 'instant start' - do they use 'special' ballasts?

Kind Regards, John

 

[SimonH2]

I read it as the tube still having 4 terminals, but the fitting only using 3 of them.

Thinking back <quite a few years> to physics lessons, I think you probably only need a heater at one end to strike up an arc - if the polarity of the applied voltage is correct.

But since we use AC, and with a magnetic ballast/switch start fitting it's indeterminate which polarity the fitting would try and strike up on. Also, there's probably little saving to be had by making non-symmtrical tubes - so there'll have been no reason in the past to make 3 terminal fittings.

Of course, now we can make electronic ballasts, instant start, etc - but to use anything but a 4 terminal tube would mean something non-standard and given the installed base I cannot see the basic pattern of flouro tubes changing any time soon.

 

[JohnW2]

I read it as the tube still having 4 terminals, but the fitting only using 3 of them.

On reflection, maybe - but that doesn't alter what I wrote, which was essentially about 3 connections to the tube, regardless of the number of pins.

Thinking back <quite a few years> to physics lessons, I think you probably only need a heater at one end to strike up an arc - if the polarity of the applied voltage is correct.

That's also my understanding - it's only the cathode (negative side, at that moment) which needs heating

But since we use AC, and with a magnetic ballast/switch start fitting it's indeterminate which polarity the fitting would try and strike up on. Also, there's probably little saving to be had by making non-symmtrical tubes - so there'll have been no reason in the past to make 3 terminal fittings.

Quite so.

Kind Regards, John

 

[333rocky333]

The electronic ballasts have 3 lamp output wires so are non standard really.
I have only seen them as 4 foot T8 fittings.
The fittings use standard 4 pin tubes which seem to work fitted either way round, though it may be coindidence the lamps are fitted right way round first time.

They also do emergency versions, but in emergency mode only the 2 wire end lights up for aprox 1/3 of the tube.

 

[JohnW2]

The electronic ballasts have 3 lamp output wires so are non standard really. I have only seen them as 4 foot T8 fittings. The fittings use standard 4 pin tubes which seem to work fitted either way round.

Interesting. It does sound a bit like the 'half way house' scenario I mentioned - with the 'ballast' (odd word, really, for an electronic one!) providing enough start up voltage for the tube to strike (and 'stay struck') with only one filament (at one end) working. It would be interesting to play and see if it would strike with no filament at all (i.e. just one connection at each end, but you'd have to experiment to find out which one at the 2-wire end!).

Kind Regards, John

 

[JohnW2]

It would be interesting to play and see if it would strike with no filament at all (i.e. just one connection at each end, but you'd have to experiment to find out which one at the 2-wire end!).

This quote from Wikipedia suggests that they may well strike with no filament at all:

Low cost ballasts mostly contain only a simple oscillator and series resonant LC circuit. ... This principle is called the current resonant inverter circuit. After a short time the voltage across the lamp reaches about 1 kV and the lamp ignites. The process is too fast to preheat the cathodes, so the lamp instant-starts in cold cathode mode. The cathode filaments are still used for protection of the ballast from overheating if the lamp does not ignite.

Kind Regards, John

 

[333rocky333]

I will try that next time I find one, and also get the ballast info John W2

I would not know off hand as they are twist holders and as you would know the connections not active till you twist it, usually with both ends in.

But i can easily just try various combos of twisting one end at a time also trying the lamp either way in each end

EDIT
Oh just realised i think you meant 1 wire at each end

 

[333rocky333]

...

 

[333rocky333]

The cathode filaments are still used for protection of the ballast from overheating if the lamp does not ignite.

Didnt quite get that bit

So as per the OP
Could that create a shock risk from the exposed pin or pins to earth via your body if the opposite tube end was in and connected

 

[JohnW2]

I will try that next time I find one, and also get the ballast info John W2 ... I would not know off hand as they are twist holders and as you would know the connections not active till you twist it, usually with both ends in. But i can easily just try various combos of twisting one end at a time also trying the lamp either way in each end EDIT Oh just realised i think you meant 1 wire at each end

Yes, I meant with only one of the two wires connected at the 2-wire end - so just one wire connected at each end. However, as I said, one probably has to get the right one of the two wires (shouldn't matter with of the two pins it connects to).

Kind Regards, John

 

[JohnW2]

The cathode filaments are still used for protection of the ballast from overheating if the lamp does not ignite.

Didnt quite get that bit

I presume they mean that the 'ballast' will go on generating ~1kV (and potentially getting hot) until the tube strikes - so, just in case it doesn't start 'immediately' (with ~1kV,in 'cold cathode' mode),it almost certainly will start one the filament(s) have had a chance to warm up, thereby 'sparing' the ballast.

So as per the OP .. Could that create a shock risk from the exposed pin or pins to earth via your body if the opposite tube end was in and connected

Indeed, with one like the Wikipedia described. If there's ~1kV knocking around until the tube strikes, you wouldn't get me changing the tube 'live' (even though I often would with a 'conventional' fluorescent). It might even strike 'through me'!

Kind Regards, John