Ballotini Fuses

"Me too". I've never seen this before either. A Google search on "Ballotini" and "Fuse" turned up references to glass beads. Even consulting the erratic wisdom of en.wikipedia.org turned up nothing. I've never seen this statement on decorative lamps I've purchased in Canada.

I do know that some lamps have a fusible element built into them to prevent trouble if the lamp develops an internal short; though it may be unlikely, sometimes vibration (or a manufacturing defect) will allow the lamp filament leads or supports to touch and short out. The lamp's built-in fuse I guess saves having to replace the branch circuit fuse (or reset the branch breaker) - you'd rather have one lamp go out over the dining room table than find your way in darkness to the household panel.

Bill

Thanks very much for the reply. I also did the Google search with the same result, though it did turn up a large page of what seemed to be standard legal disclaimers which had some reference to Ballotini ( I couldn't be bothered to read it all - it was a large tightly written page - aren't all legal things).

Your comments about the internal fault protection sound as if they hold water and are totally reasonable, so I think I'll run with that one for the time being.

Thanks for the reply.

Best wishes from the UK

Pilgarlick

Try the manufacturer and let us all know

Newsey

I must say I've never seen that happen, and I'd say it's virtually impossible given the spacing on 240V bulbs.

What happens very frequently IME is the element itself breaks down (expansion/contraction causing distortion) to a point where the coils short out to one another causing a low resistance. This usually causes a bright blue flash and trips a breaker. A real PITA is when it happens to a bulb connected through a dimmer and it takes out the triac.

I would say about 75% of my household bulbs have tripped the breaker when they failed, and on examining them there's been no obvious shorts other than rather poorly-looking filament.

I recently had a mains halogen bulb fail, which have very fine and close-wound filaments. Naturally when it failed it took out the dimmer and tripped the breaker and lit the room like a strobe light!

Dave

Ballotini are tiny glass spheres, often used in reflective paint used on road surfaces. However, they are also used inside fuses where they help quench the arc, hence more rapidly interrupting the current flow when the fuse blows.

The problem is that the lamp can flash-over internally when the filament breaks. The mechanism is that as the filament breaks, a small spark jumps the gap. This forms a plasma through which the current continues to flow. The plasma has a lower impedance than the filamant, so it grows in length, with the two ends of the arc running in opposite directions up the filamant until it reaches the lead-in wires. Now you have an unballasted discharge lamp, whose current draw is limited only by the supply impedance; this is basically a short circuit. This all happens in the order if a millisecond, and may be familiar to you as a flash and pop sound which sometimes happens when a lamp dies.

I would speculate this is probably more common on 240V supplies than on 120V supplies, but I have no evidence to back that up.

120V incandescent lamps are usually have an internal fuse. See page 3 of "Incandescent Technology": *=4001

Yes, the 240V incandescent types sold here in the UK tend to have them as well, but typically they don't blow fast enough to stop the breaker tripping! Aren't fuses marvellous!

Dave

This sounds a bit like the old 'silver-sand' fuses we used to use. A sliver fusible link, but the cartridge filled with a fine silica sand. When the fuse blows, the arc melts the sand forming a glass 'slug' that interrupts the arc. Gives remarkably high interrupting rating in a small package.

FWIW, a friend in the training department at work maintains all the overhead projectors used in the class rooms. These are modern digital projectors with a 'timer' for lamp life. When the lamp life timer reaches 1500 hours, it shuts off the lamp regardless of light output. These bulbs *look* incandescent in nature, but are some exotic version that cost $450 a piece.

Because of their high cost, my friend decided to just reset the timer on one once and 'run till failure' of the bulb. Got an extra 300 hours, but the 'failure' was catastrophic. Bulb shattered while in use and filled the inside of the projector with glass from the envelope and blew out the circuit board that powers the bulb. (i.e. there was a *good* reason for the 'timer')

Anyway, sometimes when he replaces the bulbs that have reached their timer limit (he never again 'just reset the timer' ;-) the glass envelope around the bulb shows signs of melting and drooping. Apparently as the inside surface gets coated with evaporated filement material, they get much hotter (and light output diminishes).

daestrom

The enquiry was politely and expansively explained by various contributors and I can well do without uninformative smart-ass comments such as yours, as can all newsgroups. When you have nothing to say then say nothing.

Pilgarlick

They're usually metal halide IIRC, ie discharge bulbs with no filament. The prices charged for spare projector bulbs are extortionate IMO, and when choosing a brand it's wise to check the price of spare bulbs as they do vary considerably.

The timer isn't just there to prevent catastrophic failure, apparently the brightness diminishes and the colour temperature of the bulb shifts to an unpleasant yellow at the end of its life which adversely affects the image quality.

Dave

I've not had a filament take out a CB in a *long* time (nor a dimmer, in fact). I have a bunch of 300W and 500W halogens, as well as a housefull of smaller ones. As slow as fuses are, CBs are slower.

Depends which country you are in and on your supply impedance. The magnetic (fault current) element of EU MCB's is significantly faster than fuses used in electricity supply (unless your supply impedance is too high such that the prospective short circuit current is not high enough to trigger the MCB's magnetic element). If you are in a country with uses only thermal elements inside CB's, then a fuse is going to be faster.

Halogens are less likely to flash over has the bulb operates at a much higher internal gas pressure and flashover just doesn't happen in the power ratings of lamps you find used at home. However, when higher power halogens do flashover, they explode.

housefull of

Current-limiting fuses aren't slow, they must clear within 1/2 cycle. But a fuse built into a lamp won't be a current-limiting type.

Bill

I have seen miniature 120 V incandescent lamps short out and damage their driving circuitry quite regularly. I used to work at a steel mill where the scrap aisle crane scale used a score-board display composed of dozens of 6-watt lamps. Due to the heavy vibration and an unfortunate choice of lamp style, the filament support wires would touch and take out the triacs driving the lamp array.

Now that's unusual in my experience. I've been swapping burnt lamp bulbs in my home for decades and not once have I had a breaker trip on a lamp failure. Could this be the one advantage of 120 V domestic wiring over 240 V? True, it does take us much longer to boil a kettle...

Bill

This should be clearly visible in a lamp. I've taken apart many 120 V lamps in my school days (there was something about a 1000 watt clear mogul base lamp carefully eviscerated of all its internals and filled with water that delighted my childish mind) ; but never noticed a fuse element.

Interesting - this momentary plasma explains the very bright flash I've noticed when a lamp burns out. My family is tired of hearing me complain that I might have saved that lamp if I'd only waited 1/120th of a second to switch it on.

Bill

I'm pretty sure that whatever British Standard covers our GLS lamps, it must require fuses in the lamp base because all such lamps have them. The ballotini fuses are wire ended glass tubes, perhaps 10mm long and a few mm diameter in the lead-in wires, but not in the low pressure gas-fill part. Ballotini fuses are quite rare though -- normally it's simply done by having a thinned section of lead-in wire (again not in the low pressure gas-fill part). Fuses always seem to be fitted to both lead-in wires. They can be seen in the clear glass GLS lamps.

I don't recall seeing fuses in 300W and upwards GES (mogul) lamps here either. (Yes, I took them apart too, and carefully cut round the neck to remove the pear shaped glass bulb. My school squash court used to use the 300W and 500W ones.)