POP!

[bigclive]

So why do lamps fail with a pop and a flash? And why do they go all sooty inside and take out the breaker when they do fail like that?

The reason is because when a filament fails some tungsten is vaporised at the failure point and effectively turns the lamp into an unballasted metal halide discharge lamp as the electricity conducts directly from the filament supports through the metal vapour inside. This leads to a sudden high current situation which has an avalanche effect of liberating more tungsten into the globe. This will either trip a breaker or blow clear after a moment, but the vaporised tungsten will then deposit on the inside of the lamp turning it metallic black.

If the lamp is on a dimmer then there's a good chance that older dimmers may go dead short as the triac fails and/or have their internal fuse blown.

This effect is more common with smaller lamps which is why pygmy lamps tended to fail in this manner and is why some of the GU10 style compact tungsten halogen sources fail with a bang.

Next bit of trivia... Why does a series connected string of fairy lights stay on when one blows?

 

[Andy™]

Next bit of trivia... Why does a series connected string of fairy lights stay on when one blows?

something about they short circuit, and each remaining lamp then gets a little higher voltage drop accross it

 

[ianmacd]

Next bit of trivia... Why does a series connected string of fairy lights stay on when one blows?

They don't always and hardly ever used to.

 

[jl-heating]

they dont stay thats why u have to change all the bulbs till u find the broken 1

 

[boltonsparky]

Buy LED fairy lights :coat

Is it something to do with the electricity conducting through the gas in the bulb thus still making a circuit?!?! I have no idea... just guessing!

 

[spinlondon]

The reason the lamps go sooty, is that the soot is made up of Tungston, which is vapourised from the element each time the lamp is switched on.

Fairy lights now have a system where when the lamp blows, a short circuit is produced to keep the other lamps lit.

 

[bigclive]

Yes the failed lamps DO go short circuit (sometimes) but HOW do they do it.

I want you guys to guess before I tell you.

Do you want a clue?

 

[Andy™]

isnt it something like when it blows, there is a high voltage over lamp & another part (an parallel to lamp), and this high voltage makes something go short circuit? or am i talking *? maybe google will find the answer...

 

[spinlondon]

I believe there is a little man in the lamp, and when the lamp blows, he quickly attaches a jumper cable.

They are fairy lamps.

 

[bigclive]

isnt it something like when it blows, there is a high voltage over lamp & another part (an parallel to lamp), and this high voltage makes something go short circuit? or am i talking crap? maybe google will find the answer...

Spot on. If you look through the lamp you'll see a thin wire wrapped around the filament support leads just above the glass bead. It;s a wire with what I guess to be a thin oxide coating on it that has a high resistance at 12V but when the lamp goes open circuit the voltage rises to the full mains voltage and breaks down the insulation on the wire causing it to short out and complete the circuit.

The fuse lamp does not have this wire (except in some very dodgy cheapo sets!) so that it can break the circuit if too many lamps fail and go short circuit.

As previously mentioned, sometimes the defective lamp doesn't go short circuit and then you have to start swapping lamps. Maybe a quick 500V blast from a megger would sort that out.

The 24v sets are notorious for the shunt not working.