On Sun, 25 Mar 2012 07:53:23 -0400, "Stormin Mormon"
wrote:
Well, Bob Haller said he had 15 amp thermals, and occasional 20 amp inrush.
Why would that need to be "current split evenly"? Supposing they are
carrying 15 amps and 5 amps. What's the problem?
A 15A thermal fuse will not blow on a 20A inrush. It needs some time
to melt the wax inside the fuse and blow. I'm not sure how much is
required to blow it up reliably, but a short circuit across the line
should do the trick.
The problem with unequal current distribution is that the fusing
current is not predictable. However, you're partially correct. If
one fuse blows at some current, the full current load will transfer to
the other parallel fuse, which will then have enough current to blow.
The problem is that if it takes one minute for the first fuse to blow,
it will probably take more than an additional minute to blow the 2nd
fuse because of the higher series resistance which is what causes the
unequal current distribution in the first place. In effect, such a
derangement extends the time it takes for the power to be cut, and a
fire prevented.
If the thermals are properly placed, they would both melt open when the
device overheated.
That's a big if. Presumably, if the heating element was one
contiguous device, with no possibility of a partial short to the metal
water tank, there would be no thermal gradient across the water tank.
However, if the heater was a series of heating elements, distributed
in some artistic pattern across the bottom of the water tank, and only
a partial short occurs, the resulting thermal gradient will cause one
thermal fuse to be much hotter than the other, especially if the water
tank is empty.
And suppose one thermal fails from old age? Then, Bob is back where he was
when he started.
From: UL 60691 Pg 17
NOTE 2 For reasons of safety, it should be made clear in the
documentation that a THERMAL-LINK is a non-repairable item
and that, in case of replacement, an equivalent THERMAL-LINK
from the same manufacturer and having the same catalogue
reference should be used, mounted in exactly the same way.
This might explain why every pot warmer I could find has the plastic
base solvent welded shut, or metal base riveted together, to prevent
(or at least discourage) thermal fuse replacement.
However, in the case of the coffee machine, UL contradicts itself:
From: UL 1082 Pg 26
18.1 If an appliance is provided with a thermal cutoff, it
shall be secured in place and shall be so located that it will
be accessible for replacement without damaging other connections
or internal wiring. See 50.6.
So, one spec says it should be un repairable, while another says that
it should be accessible for replacement. Toss a coin.
Incidentally, if the thermal fuse blows, one should consider asking
why it blew. It isn't always old age or crappy quality. There may be
an intermittent or obscure fault causing it to blow.
I don't see the big problem.
Look harder and you'll see. What you're suggesting is not in itself
unsafe. It is possible to run parallel thermal fuses and still have
it perform its intended function. However, it's much safer to put
them in series.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558