More Fun With High Voltages

That can't be good

Or maybe the day after that. I’m sorry. I have so much to catch up on after Christmas, which basically punched a gaping hole in my time. People are clamouring for cartoons to be drawn and computers to be fixed. Some of them may even pay me. Various parts of my mother’s house have to be mended. Bills need to be ignored. The calendar therefore can wait. It’s what it’s good at.

For now, just a brief anecdote: I had to replace a venerable fluorescent light fitting in the kitchen which had taken to sticking at the flickering-to-come-on stage. Pretty sure it’s been in the family since the 70s, maybe longer. While I had little hope of being able to repair the thing – I barely understand how they work – it was easy to open it up in situ so I thought I’d have a look first.

Naturally I isolated it at the circuit breaker. To do that though, I had to establish which circuit it was on. As you may have seen in the photograph I took when our main breaker melted down the other week, almost none of them are marked. And as this place is an old cottage that my father spent decades gradually restoring, I could not depend on there being any rigorous scheme. It was time for adventures in ad-hoc wiring. Off goes every electrical device in the whole house.

The kitchen lights, it turns out, are on the same circuit as the immersion heater. Weird, if not quite as strange as the oven being wired to the outside light. And one breaker controlled… Nothing at all, apparently. Which is a little creepy. That one can stay switched off.

But having isolated the light (and the water heater) I undid the nuts and lowered its works down on the integral chains. Cool. Immediately I found that the ‘choke’, or ballast, was surprisingly hot. Have a look at the picture – that’s the mains wiring, which was run next to the ballast. It has very rubbery insulation which seems to have perished where it was exposed to extreme heat. It crumbled away as soon as I moved the wire. So, pretty lucky I disconnected the power before I went poking then.

It seems likely therefore that the ballast was not designed to get so hot, and that it was failing. It plays a vital – and slightly scary – role in a fluorescent. As you may be aware, these things work by applying a big voltage to a tiny amount of mercury vapour, which then glows not unlike the wire filament in an ordinary bulb. One thing that makes a vapour different though is that as soon as it starts glowing, it actually offers less resistance to the flow of current. Left to itself, it would keep getting brighter and brighter until something went horribly wrong.

Well this is AC electricity, so current flow is being reversed fifty times a second. That prevents a runaway situation occurring, Nevertheless the choke is necessary to prevent damage being done even in that brief time. Its role is to be a sort of anti-tube; the more current flows through it, the more it resists. If it’s not doing its job properly then the tube is probably getting too much juice and overheating – fifty times every second. Which would explain why two tubes had failed in fairly quick succession.

(This is at least my understanding of the situation. Perhaps Droog will be along later to tell me why I’m wrong.)

As the huge magnetic ballast was something out of electricity’s iron age, the whole device would need to be replaced. Not such bad news – fluorescent fittings are cheap enough now. I would hang the fitting back up and go to town for one.

And then I remembered the other main component of a fluorescent lamp – the capacitor. Which must be this thing about the size of an old milk bottle. Its function is to store electric charge. It was all very well turning off the power – this still probably held enough to kick me through the kitchen window.

Gingerly, I withdrew my screwdriver and backed away.

5 thoughts on “More Fun With High Voltages

  1. You’re spot on.

    -If the cable felt rubbery and crumbled away in parts then it most likely is vulcanised rubber. I’d say that places the light fitting as being much older than the house wiring, but I could be wrong. Maybe somewhere people kept using VIR decades after it was considered sensible to do so. As that diy wiki says if you find VIR you want to replace it, so your trip to get an entire new fitting is the smart thing to do. Rubber is only used these days for the outer sheath of some types of cable to keep it flexible. Better insulation is found inside. Most insulation these days can withstand to have the conductor inside it running at 90ºC without melting; rubber ran well below those levels and would bake into a crisp over time.

    -Echoing what is in one of your wiki links, the capacitor is to balance out the inductive load of the ballast. Power factor correction.

    -When replacing a fitting at your local hardware store you don’t necessarily get the best products available. If you find it and can afford a bit extra a linear fluorescent fitting (i.e. tube) can come with electronic high frequency ballasts. They are more efficient, and run at hundreds of kilohertz, eliminating 50Hz hum and flicker.

  2. Phew…

    It’s the house wiring going into the fitting that’s rubber-coated. That was done in the 80s (the fitting we brought from our previous house I believe), but it’s quite likely that wire salvaged from somewhere else was used because we were poor. To be perfectly honest, it looks suspiciously like the cord off some appliance.

    It vanishes upwards, and as the circuit is black-and-red by the time it reaches the breaker board or either of its switches, there most be a join somewhere up there. So it’s basically impossible to replace without taking down the ceiling. Bummer.

    Fortunately there was a lot of spare length, so I cut it back to where the insulation was still highly elastic and routed it so that it doesn’t come close to anything that might get hot.

  3. You see, now you’ve gone and said “phew” and that’s where you’re wrong. You’re focusing in the academic discussion and forgetting your house is wired with something you want to replace. I thought it was just the fitting but now you’ve got me worried. Out of sight but not out of mind!

    You want to keep that niggling situation in the back of your head to see if you can fix it in the very near future. If there is a joint where the cables change in colour then that is another heat point and you could find that wiring baking to a crisp on the other end of its run. Considering that you already had an electrical fire I would say don’t put that job off for very long.

    You don’t need to chase the old wire. Just figure a new way to get new wiring in and disconnect the old stuff. It will still be a disruptive job but it may be a simpler job than trying to chase every old cable.

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