Blowing Light Bulb..

An interesting thing happened in the drawing room tonight.. a standard
40W incandescent lamp failed as I switched the room lights on.

What I saw was a bright flash and a red/white hot something leave the
wall lamp and travel some 8 foot to the floor.

The circuit fuse blew.

The lamp has a darning-needle sized round hole (large pin hole) in the
glass, with a rounded/melted edge. The inside of the glass has a patch
of dark discolouration opposite the hole.

Anyone ever heard of anything like that happening?

It must have produced an incredibly hot and high speed fragment to have
been able to melt through the glass quickly enough to still have the
speed to travel that distance across the room.

andrew...

Interesting -- probably a melted end of one of the lead-in
wires. Is one of them shorter and missing the end?

Some years ago I had a 60W lamp go with a bang and the glass
fell off having cracked around the neck. The glass bulb then
bounced on a hard floor and didn't break.

Paul Hovnanian P.E....

Its rare, but its possible. Most bulbs fail when the filament
evaporates, develops a weak spot and thermal stress (usually the surge
when turning on a cold lamp) finishes the job. This is a relatively low
energy event, since the arc, in series with the remaining filament
limits the current.

Its possible for the filament and/or supporting wires to fail
structurally and result in a short circuit inside the bulb. That will
dissipate quite a bit of energy by comparison and possibly trip the
branch circuit protection. The folks on sci.engr.lighting might be able
to shed some more light on this.

Victor Roberts...

It is my understanding that internal arcs are rather common
when incandescent lamps fail and that the arcs can draw
substantially more current than the filament. For that
reason incandescent lamps (at least in the US) have an
internal fuse wire that is supposed to keep the branch
circuit fuse or breaker from opening.

Adam Aglionby...

Have heard it referred to as Ballotini (sp?) fuse in the lamp pinch ,
but thought it was to protect the lamp from non passive failure as
experienced by the OP.

I suspect the object that melted through the glass was part
of the filament or support lead that was melted by the arc.

I have never seen an incandescent lamp blow a branch circuit
breaker or fuse. Terry has more experience in this area and

Adam Aglionby...

Seen plenty of larger incans,mainly theatre lamps anything from 300W
to 2K typically, destroy fuse and triac when they go.

Adam

may be able to shed more light on the subject.

TKM...

Some years ago, I worked in a test lab where such phenomena were studied to
help customers with the design of lamp circuits and, especially, electronic
dimmers. As others have said, such dimmers are often the fuse when a lamp
filament fails.

We caused the lamp to fail by using clear lamps and focusing a laser on
various parts of the filament. There may even have been a paper on the
results and I'll take a look through the old LS proceedings to see if I can
find it.

don...

Did you induce failure while the filament was hot? That will severely
limit the arc current, until the arc expands/crawls/blows-up around the
filament.

I suspect the arc could expand faster if the filament breaks from an
"end-of-life cold start failure" where most of the filament has not yet
reached its normal steady-state temperature.

Then again, as I understand it, most cold start failures have most of
the filament achieving a peak temperature most of the way to their
steady-state operating temperature.

But if you had a way to apply power and then sever the filament late in
the first half of a half cycle, then you would have an "acid test"
(test reasonably able to match or exceed the severity of *The Really
Especially Bad* filed failures).

On the other hand, I have seen some stable burnout arcs!

1. I have heard of this occurring with 120V T3 halogen lamps used in
torchiere fixtures. I have even seen one failed T3 lamp in such a fixture
that showed severe bulb blackening around the break.

2) I have seen a 60W 120V A19 with a stable burnout arc in progress. The
filament was a C-9 (the multi-supported C-shaped one with singly coiled
wire). The lamp operated 24/7 and I give slight chance that it could have
developed an unstable burnout arc, and greater chance that it would have
developed a stable burnout arc that gets broken by
cold-start-burnout-related filament movement, if it failed during a cold
start.

- Don Klipstein (don@misty.com)

But what I recall is that the fireworks inside the lamp were mainly a
function of the point in the sine wave cycle when the filament opened and
the "stiffness" of the power supply feeding the lamp. By "stiffness" I mean
the ability of the circuit to quickly increase the lamp current. Break
the filament just as the current wave was building in a circuit with plenty
of current capacity and you had a chance for some spectacular lamp failures
because of the arc that got started between the internal lamp parts. The
most powerful arcs were between the filament mounts, but more usually
between a piece of filament and one of the mounts. As Vic said, the fuse
wire in one of the leads usually limited the duration of the arc, but fuses
are not precise devices and there were often pieces of molten metal flying
around inside the bulb and embedding themselves into the glass surface.

I'll guess that in the case Paul mentions above, a gas-filled lamp is
involved and the filament arcs just at the right time and place to position
a hot pocket of gas such that the filament melts and turns into a miniature
rocket. I'm curious to know what the piece of material that hit the floor
looked like. Did it still resemble a piece of filament or was it just a
solidified blob of molten metal.

I also wonder if internal lamp arcs have the potential to be more powerful
on 50 Hz systems since there is more time for the arc to start and heat up
in a single cycle than on 60 Hz systems.

Terry McGowan