Re: comfort fitting

["the_home_automator" <db@xxxxxxx>]

--- In ukha_d@xxxxxxx, "Frank Mc Alinden" <armagh@b...>
wrote:
> Hi Des
>
> > Whats a 24 hr loop for?
> >
>
> This is used for Panic buttons (hold up situation) equipment
tampers.. eg
> bell box ..industrial alarms.....fire doors (which normally kept
closed)
> etc...

And just for completion here - your insurance company definition of
an alarm system will be "it complies with BS4737", and one of the
specfic requirements in this standard is tamper protection for all
items of the entre system.

The traditional method for UK wired alarms is to have two loops to
each contact assembly (or PIR or whatever), one of which is armed
24x7 so if the wirnig is interfered with it is detected.  Thats why
UK alarms use four core security cable, or six core for things that
need 12v, like a PIR.

The Americans achieve the same result, but do it dfferently; they use
an end of line (EOL) resistor across the cntact, so that even hen the
contact is open, there is a measurable resistance.  Trouble is,this
method doesnt detect shorts on the wire, so by using a second
resistor in series with the cotact you can detect the four states
needed, namely open circuit, short circuit, contact open, contact
closed.

UK fire alarm practice universally use end of line resistors, its
just intruder alarms that are diferent.

David.



http://www.automatedhome.co.uk
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