Re: "Magnet tamper" for surface mount contact?

[Nomen Nescio <nobody@xxxxxxxxx>]

Bill said:

>I've read the specs on the balanced contacts, but don't see anything about
>how just unscrewing the magnet would trip an alarm?
>
>I see that placing an additional magnet up by the contact would upset the
>magnetic balance and then trip the alarm, or unscrewing the magnet and then
>moving it closer to the contact and taping it in place would trip the
>alarm....
>
>But I don't see where unscrewing the magnet and keeping it the same distance
>from the contact would cause an alarm.
>
>I seem to recall that there were contacts on a UL grade AA system which had
>covers over the magnets. The contact and magnet were both about 2 inches by
>4 or 5 inches and 2 inches deep. Both the contact and magnet had covers with
>tampers. If you removed the cover over the magnet, I think it would either
>drop the magnet away from the contact or place a barrier between the magnet
>and the contact. Attempting to remove the magnet from the door by removing
>the cover would cause an alarm.

On a balanced contact like Sentrol's 2707A, one simple solution is to mount
the magnet so that the magnet mounting screws are covered by the switch
when the door is in the closed position.  The switch, by the way, has an
optional tamper feature to detect removal of the switch from the door
frame.

Some brands of balanced contacts have used two magnets, one attached to the
cover of the magnet housing, one within the housing.  Since the mounting
screws are inside the housing, it becomes necessary to remove the cover to
remove the magnet housing, which unbalances the magnetic field.

- badenov