Light bulbs

DaveHerns

New Member
Why is it sometimes when a domestic light bulb blows that the circuit breaker or fuse blows ? How can a 60 watt light bulb blowing pass over 6 amps ?
 
A tungsten bulb in the process of blowing draws an arc - which is a nice freindly name for a dead short. And trust me, that's a tad more than 6 amps!

Chris
 
Another explanation is inductive kick, the sudden loss of current ie open cct filament happens at such a speed that the inductance present in the house wiring tries to compensate for the sudden loss of current draw in the filiment, the resulting spike can have a massive current componant which will take out the breaker.

Graeme
 
Dave3066 said:
Lamp or luminaire.....a bulb is something gardeners put in the ground :wink: :roll:

Dave

That would be so the Moles may have some light then :roll:



Before anyone else says it I know thier sight is impaired.................................all the more reason for some light down there :!:

Ok I've got my coat...
 
ghce said:
Another explanation is inductive kick, the sudden loss of current ie open cct filament happens at such a speed that the inductance present in the house wiring tries to compensate for the sudden loss of current draw in the filiment, the resulting spike can have a massive current componant which will take out the breaker.

Graeme

Both explanations are valid, and one doesn't go without the other. The voltage spike ignites the arc - which then is the culprit to draw the heavy current.

Vin Kohler
 
:LOL: easily fixed if you replace the 6A MCB with a 30A anti-surge job.
of course I am joking.
 
Seems a modern safety thing :? Never happened as a kid with a bit of fag packet stuffed in the fuseboard :oops:

Our toaster only has to have a large crumb in it and the trip's gone :LOL:
 
Trying to think seriously aboout this (although I'm not sure why :shock: ), I would tend towards the arc theory as Chris suggests as being the main culprit for tripping domestic circuit breakers when an incandescent lamp filament goes open circuit. From what I can remember of electrical theory, it is all down to ionisation of the gas within the lamp:-

When a tungsten filament lamp blows, it will tend to be just one spot of the filament that disintegrates and this forms a very very small gap betwen the two broken ends. Although the lamp will have been evacuated, there is till some gas inside the "bulb" - often argon (which is a "Noble" gas or inert element and usually a very good electrical and heat insulator). The voltage diffrence between the two points is enough to cause dielectric breakdown of the gas. This means that the gas atoms become ionised. When gas is ionised the atoms or particles become positively or negatively charged. This allows electron flow between atoms. In short the the gas becomes highly conductive. This current flowing through this ionised gas is better known as an arc or spark.

You then get into a sort of vicious cycle where more heat is generated and more energy is then devoted to ionising more gas aound the initial arc. The arc then grows, all the time reducing the resistance of the lamp, casing more current to be drawn and more heat and more ionisation and more arcing. So, as Chris says, you effectively get a "short" within the lamp which will cause it to momentarily draw maybe hundreds or thousands of amps.

Of course, this all happens in a fraction of a second and it's why you get a bright blue flash before your lamp "blows" and the breaker trips. Which is just as well because your 1 or 1.5mm sq cable that is used on domestic lighting circuits would soon start to melt with a few hundred amps going through it.

Posh incandesecnt lamps used to have little fuses built in them to stop the breakers tripping or fuses blowing. However, I suspect the dwindling stock of cheapo lamps that are left don't - they don't last long and are more likley to trip your breaker.
 
JVY said:
You then get into a sort of vicious cycle where more heat is generated and more energy is then devoted to ionising more gas aound the initial arc. The arc then grows, all the time reducing the resistance of the lamp, casing more current to be drawn and more heat and more ionisation and more arcing. So, as Chris says, you effectively get a "short" within the lamp which will cause it to momentarily draw maybe hundreds or thousands of amps.

Naturally the arc is alive for 1/100 sec at the most, i.e. until the next zero-crossing of a halfwave, (that is at 50 cycles/sec). By the way: That's why it is so very difficult to design circuit breakers for high tension dc-power. They would just keep an arc alive via an opened a gap.
Vin Kohler
 
Yes either of the 2 or combinations thereof, some of the cct breaker tripping comes down to what sort of cct breaker you have, whether it is a thermal or magnetic type, I tend to think the thermal type will have less problem with this sort of nuisance blowing as the time constant for the thermal content of the breaker will possibly not even notice the very short event time of the Arc failure condition whereas the magnetic systems are very fast to react to overcurrent. But then again maybe the arc that is produced on failure of the filiment can sustain it self indefinatly between the phase and neutral elctrodes...... and its only the breaker that breaks the arc.

Graeme
 
http://www.tlc-direct.co.uk/Technical/DataSheets/Wylex/WylexNsMcb.pdf
Good data sheet here for UK domestic MCBs. Gives a good explanation of exactly how they work - including the thermal & magnetic modes ghce mentions.

Now I know what I need to do this weekend - invest in some expensive timing equipment, extremely high speed video capture plus a variety of MCB types and a few hundred tungsten lamps, This should allow me to conduct some serious experinents on arc time versus MCB trip time. Or, maybe I'll just go down the pub instead? Mmmmmm....... choices :LOL: .
 
Yes conducting empirical experiments on Oats, Barley and Hops may be the go :LOL: Plasma arcs may not be conducive to good eye health whereas the temporary eye abberations caused by the application of fermented vegetable matter are only temporary and far more enjoyable. :mrgreen:

Graeme
 
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