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 normal position by the magnet, it is possible to defeat a "plain" magnetic
switch by placing a strong magnet near the switch, thus preventing the
contacts from actuating when the normal switch magnet is moved away (e.g.,
door opened). For this reason, the only acceptable type of magnetic switch
for most DoD applications is the balanced (or biased) magnetic switch (BMS).
BMSs are available in both surface mount and recessed configurations and in
many model and style variations. Surface-mounted models for high security
applications must be equipped with tamper alarm protection. Recessed models
are inherently protected against tampering since the actual door frame must
be attacked first to gain access to the switch components. The following
application and installation considerations apply to BMS:
a) Biased Magnetic Switch. This switch consists of one reed
switch with a biasing magnet that changes the state of the reed switch. The
actuator magnet is then placed at the correct distance to offset the bias
magnet, creating a "balanced" condition. This type of switch can be
defeated with the use of a single magnet, but the person must have the
correct size magnet, correct polarity, and must not bring the second magnet
too close to the switch. Correct installation of the actuator magnet is
essential. It could be defeated by an expert using a single magnet of the
correct size and polarity and placed in a critical location.
b) Balanced Magnetic Switch. This switch consists of one biased
reed switch and one unbiased reed switch. The second reed must be of the
correct sensitivity and position to not operate with the actuator magnet.
It must operate with the addition of a second magnet. This type switch
requires an adjustment of the actuator magnet at the time of installation
and should be periodically checked for adjustment. It could be defeated by
an expert using a single magnet that is moved into place as the door is
opened. This would require a coordinated movement of the door and magnet
from inside the protected area.
c) Preadjusted Balanced Magnetic Switch. This switch consists of
three biased reed switches and may have an optional fourth tamper reed. Two
reeds are polarized in one direction and the third is polarized in the
opposite direction. Steel is built into the switch so that mounting on
steel will have no effect during installation and the adjustment will remain
correct. The magnet housing consists of three magnets with the polarity
that corresponds to the switches and also has steel backing. The unit is
preadjusted to have a fixed space between the switch and magnet. Field
adjustments are not possible or necessary. This switch is for applications
which require the highest degree of security. The unit with three reeds
could be defeated with one of its own magnets, but not a bar magnet. The
unit with four reeds cannot be defeated with either a bar magnet or another
actuator magnet, since the tamper or fourth reed will activate when the
actuator magnet or any other magnet is brought within actuating distance.
d) Door type and fit are important considerations in BMS model
selection. Doors may tend to move due to wind or internal changes in air
pressure. If this movement is excessive, BMSs will produce nuisance alarms.
The choice between surface mounted or recessed BMS may be governed by door
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