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Lucien Nunes

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The lighting on my boat includes about thirty T5 8W 12" tubes in conventional 12V fittings of good make (mainly Labcraft) with HF inverters, taking one, two or three tubes as required. For over 25 years these have worked more or less flawlessly; the only outright failure was when water got into a fitting and blew the inverter transistors, which were easily replaced. A year or so ago there was a spate of tube failures, often with their ends blackened and overheating and melting the plastic housing. I put this down to a bad batch of cheap Crompton-branded tubes that had been installed over the previous few years and replaced them all with Sylvania 6000-hour 400lm white 535 tubes. Now they are failing again, not overheating this time but one or both ends blackening / low output / flicker / poor starting.

This time, I suspected the fittings, so I took one down that had been working perfectly before developing one badly blackened tube end, and tested the output capacitors of the inverter. These should prevent any assymmetry in the drive waveform creating a DC component in the tube current that would cause mercury and eroded cathode material to migrate. Disappointingly, the capacitors (and inverter) seem perfectly OK. It might be happening more on the fittings that rely on emission-heating of the cathodes; there are no filament windings on the inverter transformer, the two pins at each end are paralleled and a high open-circuit voltage is used to strike the tube cold. However, the other type of fitting does not get as much use so it's difficult to compare their behaviour.

What is happening here? Are current production tubes shorter-lived than they used to be and the 6000 hours is now rather optimistic? Is some change in manufacture (e.g. reduced mercury content) making them unsuitable for emission heating? Have Sylvania gone downhill and should I put in GE or Philips or what? I could convert the lot to LED but I don't like the LED drop-in replacements for these fittings very much, nor the T5 LED tubes which tend to be either expensive or give poor light quality. Ultimately I'll probably fit my own variable colour-temp (dual-white) LED tape inside these fittings but I'm kind-of busy at the moment and would prefer to spend 20 minutes swapping the tubes one last time, getting a couple of years out of them and then doing the lot properly.
 
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Is the fixed wiring solid core or stranded?
I ask only because I had a boat where the stranded wire deteriorated over time due to the constant salty atmosphere and absorption, whereas an older boat I had was wired in solid singles and after 40 years saw no problems. Thus, while I can't offer a solution to your query, I am extremely interested in the outcome.
I am shortly due to rewire a small sailing boat, and will probably use LEDs throughout. The system will be powered by a leisure battery (golf-cart style) and I'll probably use 1mm t&e internally. The masthead and navigation lights will be in "marine grade" flexible...but I haven't decided what, yet.
 
Cables with solid conductors are prohibited for power circuits by many marine regulations because of the risk of vibration-induced fracture. I have seen T+E used and it probably survives perfectly well much of the time, but is likely to be non-compliant. Other than houseboats which fall under the scope of BS7671, it will be the requirements of ISO 13297 and 10133 that apply to craft like this (which enable compliance with the Recreational Craft Directive.)

Vehicle-wiring cables to ISO 6722-3 and -4 are often specified and that is what my boat is mainly wired with, but Tri-rated is popular too or other class 5 fine-stranded conductor with suitably rated insulation. I have a couple of ELV DC circuits in T+E but those are 6.0 and 10.0mm² so at least it's a class 2 stranded conductor, plus they are in conduit. A good way to minimise salt corrosion is to specify tinned copper and there are tinned versions of Tri-rated available. Another good way is not to go to sea, which is often more trouble than it is worth :)
 
Thanks for that info Lucien. I believe the regs for inland waterways are quite extensive, but have never researched those as my seafaring is done on the high seas...
There are some crazy regs about gas installations too, but that's another story.

However, I heartily agree that sometimes going to sea is far more trouble than it's worth!
 
There are some crazy regs about gas installations too,
A lot of that is due to the way lpg ,seeks out the lowest level.
(vs Natural gas just floating away upward !)

Had wondered if some volts had been lost and the 8W lamps been under-run. ( or cold conditions /less heating -more thru drafts )
-getting stingy with oxides of barium and strontium ?-- for green-ness !
 
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I remember as a teenager I once asked an electrician why tall office blocks left their fluorescent lights on when the workers had gone home. He said it was because by leaving them on the tubes lasted longer and it was cheaper to burn the electricity than burn the cash to replace ones as they failed randomly. It was the turning on event which shortened their life.

Prompted by this memory I did a quick google 'high failure rate of T5 tubes' and turned up this:

How long do T5 lamps last before they burn out? | What are T5 Lamps? | T5 Fluorescent Systems | Lighting Answers | NLPIP - https://www.lrc.rpi.edu/programs/nlpip/lightingAnswers/lat5/pc8.asp

and see 'Maintenance and Costs' paragraph in:

EcoSpecifier: Technical Guide 5: High Performance Fluorescent Lighting - http://www.ecospecifier.com.au/knowledge-green/technical-guides/technical-guide-5-high-performance-fluorescent-lighting/

I suppose by the very nature of a cold cathode electron emitter the cathode will gradually disappear due to bombardment/erosion by positive ions produced from the electrons emitted by the cathode and accelerated by the electric field - Townsend avalanche effect. Cold cathode electron emission is stable enough for a fluorescent tube when for each electron emitted there is enough positive ions created which bombard the cathode to cause another electron to be emitted to continue the process. So as the cathode disappears through erosion the probability of this happening decreases until the process is no longer self-sustaining. I think cold cathode tubes operate at a higher voltage (HF ballast drive) which makes me wonder whether the erosion I mentioned happens at a higher rate than when they are operated at a lower voltage (magnetic ballast drive) thus causing a shorter life.
 
I agree that cathode erosion is a key factor and might break open a tube examine it, but it's not clear why it now seems to occur faster. Cold emission only really occurs at startup, you can see thermionic emission established in a second or two with corresponding increase in arc current. I wouldn't say the number of starts is excessive, their consumption is low so I do tend to leave them on.
 
A further thought - I doubt but you would know - that the HF inverter output is stabilised to avoid changing light output with variation in the nominal 12V dc input. There is quite a step up in voltage from 12V to HF ac - high gearing say 1:10 (don't know actual figure) - so a 1V increase in the 12 V input will cause perhaps a 10V increase in HF output voltage so from 120 to 130V which may cause the tubes to age faster. Have you replaced your boat batteries and /or its charger or perhaps fitted PV charging which would have raised the average voltage fed to the T5 fittings?
 
There's no voltage regulation; it's a simple self-oscillating inverter with a pair of bipolar transistors in push-pull. But the supply voltage is pretty well controlled during the evenings when the lights are most likely to be in use, either 12.25-12.75 off-charge, or 13.8V on float charge. None of this has really changed, although perhaps a greater fraction of the use recently has been when on float. Bulk charge at 14.5V is likely to happen when underway during the day with less use of the fluorescents.
 

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