Switching Gears

04/05/2004 |

The Changing World of Perimeter Security

Every industry has its dirty little secret. The security industry's secret is that the reed switch, the mainstay of door and window perimeter sensors, can be defeated with a strong magnet. Not only that, but it can fry due to lightning or power surges, as well as break from rough handling before or during installation, or from temperature/humidity-caused swelling and shifting.

Electro-magnetic interference (EMI) from nearby power lines and cables or large appliances can disrupt its proper function as well. The truth is that reed switches are often the weakest link in perimeter security systems. A brand new switch technology is providing perimeter security on a par with highly secure government facilities, without the cost or size.

In the past, the only option has been bulky Balanced Magnetic Switches (BMSs) using multiple reeds. New high-security switches overcome the inherent weaknesses of the reed switch, while retaining the useful features such as ease of installation, smaller size, and aesthetics. "The security industry stands at a crossroads, and must now change for its own good as well as the good of its customers," says Mike Davenport, president, Nascom Inc., Vancouver, WA.

Because the reed switch is equally magnetic in all directions, its contact blades respond to any nearby magnetic field, become temporarily magnetized, and close. This means that a strong, externally positioned magnet near the switch will override the switch's own weaker magnetic field.

A further flaw of the reed switch is that high voltage from any source - such as from lightning or a power surge - can permanently weld switch contacts in a failed-close position, rendering an alarm sensor useless when it is armed. EMI from power lines, cables, or large appliances in the area can also disrupt its proper function.

Since reed switches are made of fragile glass, extreme care must be taken in handling them from initial manufacture through assembly into their housings. Even a packaged reed switch can be damaged by rough handling and installation, as well as by naturally occurring thermal expansion and contraction in doors and windows.


At the heart of the new switch technology is a tiny magnetic ball floating freely inside a small metal housing with two protruding wires. The switch remains closed only when a magnet is directly under the switch so it can pull the magnetic ball into contact with the case and the electrode closing the switch.

In fact, a strong magnet placed near the new switch technology has the opposite effect of the reed switch, as the magnetic force dislodges the ball and triggers the alarm. A single Nascom high-security switch offers commercial properties the high level of perimeter security required by the U.S. government, at a fraction of the cost and size of BMSs, which were previously required for magnetic non-defeat.

"Post 9/11, people are more aware of personal security," says Lieutenant Debra Kirk, commanding officer, Commission Investigation Division, Los Angeles Police Department, Los Angeles. "People want to do anything they can to improve personal security for themselves." Nascom believes its high-security switch will rapidly supplant the reed switch as the industry standard, as surely as the reed switch replaced the mechanical switch before it.

Del Williams is a technical writer based in Torrance, CA.


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