On 10 Nov 2006 06:52:54 -0800, "Mark" wrote:


w_tom wrote:
Kurt Ullman wrote:
Thanks for the reply. I think the diagnosis will take more time than
replacement, and the string is low enough on the tree that it will only
a minor pain to change it out.

A string of 25 8 watt bulbs would draw less than 2 amps. Two strings
in series would not blow out that 5 amp fuse. Clearly there must be a
short in that string. Load would approach excessive if three strings
were connected through that same 5 amp fuse.

And there is a REAL reason for the fuses. The bulbs are all low
voltage and wired in series

Possibly some very old ones. Modern C7 and C9 are 120V bulbs. I have
some made over 30 years ago that are definitely parallel. disconnected
one and use it separately (Rudolph's nose), something that would not
be doable with series lights. Also, some plug-in night lights use C7
bulbs, another indication they're not series.

and when a single bulb fails open all the
lights would go out EXCEPT they have a special device that when a bulb
fails open the full voltage is applied across the failed bulb and then
it shorts itself. So then that light is off but all the others can
continune to work, with a slightly higher voltage since one bulb has
failed.


That describes the way miniature lights are SUPPOSED to work. They
often don't. There's a $5 tester that can be used on those (you'll see
120V al the way from one end of the series to the defective bulb).

So you should be able to see where this is going.. if enough bulbs
fail, they will all begin to fail and when they all short, you will
have one big short, hence you need a fuse that is rated to protect the
wire that the string is made out of, not the 15 or 20 Amps that your
breaker is rated at..


Depending on the severity of the short, the insulation on the wires
may melt before the breaker trips.

I've seen 30A fuse-protected 120V circuits in a mobile home.

Mark
--
45 days until the winter solstice celebration

Mark Lloyd
http://notstupid.laughingsquid.com

"God was invented by man for a reason, that
reason is no longer applicable."