DIYbanter - Lamp ratings

Jeff wrote in message
...

"John Woodgate" wrote in message
...
I read in sci.engr.electrical.compliance that Jeff frontline_electronic
wrote (in
et.att.net) about 'Lamp ratings', on Wed, 2 Jul 2003:

Hi John, I was wondering if the higher voltages there
had any strange effects that we might not see here?

Some designs of lamp incorporate a fuse in the internal wires so that if
an arc occurs between the ends of a broken filament, the resulting high
current and temperature do not persist and cause the lamp to explode. I
should think arcing is much rarer with 120 V supplies.

A fused lamp, I would not expect that to open under
other than catastrophic conditions.
No, 120VAC designs usually will not arc without some
outside contributing factor, but more than once
I have seen an arc develope between a 5V and a 12V supply, PC traces that
were adjacent to each other and under a connector edge, over time, with
condensation
and material aging (and the possiability of outgassing and mechanical
stress) the 12VDC would arc to the
regulated 5VDC supply to the Up IC and damage the IC.
(both sources are fed constantly)
After that I now assume anything can arc, somehow.
Jeff

I had one low voltage lamp filament break and weld
itself to a short internally.(very small 3.2mm lamp)
Now one lamp in a group of many was causing the
supply to shut down and the supply feeds other
systems.... in short (no pun) who starts by looking for
a shorted lamp, not me.

I do know of this as an extremely rare event with low-voltage lamps.
'AC/DC' tube radios had the tube heaters in series and there was
sometimes a dial lamp in the chain. If the lamp failed, a high voltage
would develop across the break and would occasionally weld the whole
internal metalwork into a solid blob. So the dial light would go out but
the radio would still work. In this case, the 'arc lamp' was fed via the
resistance of the tube heaters and any additional resistance, so the
current was limited to a less than catastrophic value.

I have caused this myself with a portable 120VAC
lamp after dropping it and looking at the open filiment
I thought that maybe a light tap or violent shaking might
weld the two halves of it back together. (wrong)
This time both ends came off and both set themselves
at same points of contact, accross the smallest area
of the electrodes, blowing the breaker.
New lamp, and breaker.

--
Regards, John Woodgate, OOO - Own Opinions Only.
http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go
to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

Jeff

Fusing is common in 120 volt general service lamps. As I recall from my
days working for a lamp manufacturer, fuses are put into all 120 volt
gas-filled lamps; but are not needed in low wattage vacuum lamps since there
is a low probability of an arc being established. The manufacturers put the
fuse into one of the filament support leads so that when/if an arc is
established as the filament fails, it won't burn very long before the fuse
melts due to heat either from the current being drawn or the arc itself.
Being a thermal fuse, it is sometimes slower than dimmer circuitry or even a
circuit breaker.

TKM