Metspitzer wrote:

It is easy to understand if you just acknowledge that your security
system is normal/ready when the window loop is closed.

Indeed.


Fire alarms are also "ready" when the zone relays are energized. This
is to allow them to fail safe.

It's the same thing. Fire alarm devices like smoke detectors, heat
detectors, duct smoke, pull-stations, whatever.... They are 'open' in a
normal situation (no alarm) and close the contacts when the device is
actuated. The big difference is wiring. Conventional fire alarms MUST
be wired in parallel, with the EOLR (end of line resistor) in parallel
with the last physical device on that zone. That way, we can detect a
disconnected wire/device and get a 'trouble' condition (instead of
ALARM) on the panel.

The reason series wiring is used for burglary is the way the house is
wired. On a "good" pre-wire job (during construction), each window and
door has a cable "home-run" to the panel. Well, most homes have an
average of 15-20 windows, 3-5 exterior doors, a couple of motion
detectors, and perhaps a monitored fire zone. Your average burg. panel
has about 8 hardwired zones. It would not make sense to put each device
on it's own zone, it would be cost-prohibitive to add the zone
expanders.

So, we identify the cables for a "set" of windows i.e. "living
room/kitchen" and wire those cables in series right at the panel. You
have to use the zones you have to work with as optimally as possible.
Because they will not be all the same type, for example: Zone 1 Fire,
Zone 2 is an entry/exit delay, Zone 3 for 'interior' devices', Zone 4 -
CO detector (or aux. device) Zones 5-8 for 'perimeter' devices and
extra interior devices.

Series wiring gives us the greatest flexibility to work 'creative'
solutions for security. Fire alarms are a whole different animal
because they MUST adhere to the NFPA 72 code, which does not permit
series wiring for a conventional class-B zone style.

I've heard a fire instructor tell us, "Fire alarms are science, security
systems are art."