In article ,
"dennis@home" writes:


"Andrew Gabriel" wrote in message
...
In article ,
Alan writes:
In message , "dennis@home"
wrote

the theory was that you wanted to catch burglars in the property not
scare them away.

Isn't the aim to deter burglars from a domestic property hence the
prominently displayed external bell box.

Yes - that's the first aim.
Burglar alarms don't 'catch' burglars. Police used to temporarily
supply silent alert systems where they wanted to catch someone, but
if you aren't the police, you don't want to encounter a burglar,
you just want them to scarper empty handed as fast as possible.

I think the real reason it was done the way it was was because if you alarm
a locked door (i.e. the periphery) the alarm will never go off because the
bugler will find a weaker spot. The sensors were always fitted to internal
doors that were likely to be used by a burglar.

Now PIRs are so cheap you don't need to protect the periphery at all.

All external doors are always protected.
They are the most common form of burglary entry/exit.
This also enables alerting when attempting to set the
alarm that an external door isn't closed (sometimes
it includes a check that the door isn't only closed,
but also locked, although that's unlikely to be added
in a domestic installation).

If you don't protect the perimeter, the alarm can't
trigger until the burglar is inside, which is already
later than you want.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]