125w Fluorescent Tube on an 85w Ballast

[Gigagator]

Hi all,

First off, I'm not after a lesson on the obsolescence of fluorescents. I know all that, this is largely just a curiosity question.

I have a quite old 8ft T12 fluorescent fitting that has a 'quickstart' ballast designed for 85w tubes.

I have one 100w tube which only very occasionally ignites.
However, I have an abundance of 125w tubes and they ignite every time - pretty quickly too, which I was surprised at.

My question and curiosity is, what damage if any will running a 125w tube do to the 85w ballast long term?
Would it cause a failure of the ballast and does damage occur only when the switch is flicked and it tries to ignite, or is it causing damage the whole time the tube is lit?

Thanks in advance for the information.

Regards

 

[JohnW2]

My question and curiosity is, what damage if any will running a 125w tube do to the 85w ballast long term?

Others may have differing thoughts, but I wouldn't really have expected any damage.

I imagine that the 85W ballast would probably limit the current through the 125W tube to much the same as it would have been with an 85W one (i.e. what it is designed to carry). I would think that the greatest 'risk' would be that the 125W tube would not work (satisfactorily or at all) with the (higher impedance) 85W ballast - but you clearly don't have that problem.

Kind Regards, John

 

[ericmark]

A wire wound ballast is very different to an electronic ballast. I see no problem with a wire wound ballast, it starts by delivering a high strike voltage, and then acts like a resistance limiting the current through the tube. The problem with the wire wound ballast was they were very voltage dependent, I found this out the hard way.

I had to light some corridors with no natural light on the building of Sizewell 'B' power station. The idea seemed simple, 16 amp 110 volt and 58 watt 110 volt fluorescent fittings, so approx ½ amp each, so 25 units seemed to be well within the system, but no it tripped the MCB, so armed with a clamp on ammeter I went to find out why tripping, and recorded 25 amp, no wonder it tripped the 16 amp MCB.

So next into the workshop, and do some tests, the lamp was really a 230 volt fluorescent with an auto transformer to step up, the step up auto transformer had two options 110 or 127 volt, changing to the 127 volt tapping reduced the amps, but the last 5 lamps did not like starting, so first 20 set to 127 volt and last 5 at 110 volt and total draw dropped to 15 amp, which was fine.

The lesson learnt is one can't read the wattage on the tube and think this is what they really draw.

However the HF or electronic ballast is a completely different beast. This seems to auto adjust for small voltage variations. And a 58 watt tube often only draws 55 watt, and has a higher output, longer life, and compares favourably with LED and there is in general no advantage going to LED rather than using the fluorescent other than governments deciding mercury is more dangerous to arsenic, are they really sure about this?

However it still means can't really answer until we know if magnetic or HF ballast, and with the latter it really does depend on the make, so hard to answer your question.

 

[333rocky333]

8 foots were an odd fitting, they came in a few versions, 100w seemed to be the most temperamental, they also eerimented using 2 half chokes, meaning each tube used 2 chokes and twin units had series capacitors to eliminate strobing.
They I recall were quite problematic if the wrong tube or starter fitted.
The 85 even had its own starter and some brand fs125 starters were not always suitable for a 100w fitting.
But if yours strike then happy days.
As John says the chokes limit the current, this will be stamped on the choke, when I have time I have a book somewhere with each chokes ratings.
The worst it will likely do is over or under current the filament, if it was severe it would blow and cause tube failure but its likely minimal difference 0.0? Amps

Oddly Tridonic only have the 85w T12 on there website

I have a brand new boxed twin 100w with tubes in my garage if any one interested
@ericmark
Don't recall a HF 8ft fitting ever existing they were always switch start