In article ,
Andrew Gabriel wrote:
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.

My thoughts too. When DIYing where time isn't a problem it is perfectly
possible to fit sensors to doors and windows that don't show at all. Wired
ones that is - dunno about wireless types. All my ground floor external
doors and windows are so protected.

--
*Husband and cat lost -- reward for cat

Dave Plowman London SW
To e-mail, change noise into sound.