On Mon, 21 May 2012 15:29:24 -0700 (PDT), klem kedidelhopper
wrote:


As you see, in that I was speculating whether, if the reed's magnetic
polarity had become reversed, that would solve the puzzle. But thinking
about it for a few minutes that still doesn't make sense! It would explain
my bench test after removal. But it shouldn't have worked in the alarm
circuit, and it did. The exception was that it sometimes got 'stuck'. So
that on re-entry it didn't trigger the delay warning as it should, until I
gave it a gentle tap.

BTW, it's only about 2 or 3 years old - I've been replacing these
regularly!

--
Terry, East Grinstead, UK

Typically on a loop circuit we use a normally closed reed contact.
Admittedly the terminology is downright confusing but very simply put
the reed is open when not in close proximity to the magnet. Therefore
the alarm control panel is looking for the loop to open, (door open),
and then an alarm will be generated. Most panels can also be
configured to have a zone be NO or normally open. So what that means
is when the magnet is in close proximity to the reed the circuit is
open. Removing the magnet from the reed will then provide a short on
the loop. A "closed loop" by definition is a supervised loop.

Inaccurate. See NFPA 72.

What I
don't like about open circuit detection is that if someone were to cut
the wires one at a time to the loop, the panel would never "see" it.
Open loop circuits are generally used for 24 hour temperature
monitoring or holdup (panic circuits). Lenny