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Just as croc-infested moats might be crossed by a medieval pole-vaulter, today's perimeter alarm contacts also have limitations. The experienced security


Feb 1, 2001 12:00 PM,
Steve Filippini

Just as croc-infested moats might be crossed by a medievalpole-vaulter, today’s perimeter alarm contacts also havelimitations.

The experienced security installer can stand back and look atthe entire residence through the eyes of an intruder. The key is toknow where the weaknesses are and compensate for them.

These days, very few residences are protected byalligator-filled moats and iron portcullises. “Castles”in today’s world have security systems that are much subtler but noless effective.

Generally, security systems are made up of two lines of defense:the perimeter and the interior protection. The perimeter protectioncovers all external points of entry from the front door to thebasement windows. The types of contacts in security systems vary inmodel and style. Most often, the system relies on a glass-encased,micro-reed switch. The contact must be aligned with a magnet toachieve a closed-circuit connection. If the person invading yourcastle slides open a window, the contact loses the magnetic fieldthat holds the circuit closed, and the alarm sounds.

If the perimeter is protected, why would you need to employinterior protection devices? Just as croc-infested moats might becrossed by a medieval pole-vaulter, today’s perimeter alarmcontacts also have limitations. If, instead of opening the window,a burglar smashes through the glass and climbs over the sill, hewill have defeated the system. One method used to catch abroken-window intruder is the window bug, or vibration sensor. Inmy experience, these sensors are prone to false alarms. Andalthough they are better looking than window foil, they, too, havefaded into the background.

Breaking the pane is just one method of getting around perimeterprotection. I have seen wooden doors with the bottom half cut andkicked through. One home had a portion of the back wall removedwith a sledgehammer. My favorite example of illegal entry was thetime a burglar cut a hole in the roof and lowered himself down intothe living room. More about that one later.

Knowing that contacts can only do so much, a good installer willadd interior protection devices. The experienced security installercan stand back and look at the entire residence through the eyes ofan intruder. The key is to know where the weaknesses are andcompensate for them. I usually start with the front door and workclockwise. Evaluate each room: Will a burglar want to pillage it?If the room has an expensive home entertainment system or computer,it is a target. Each window is a potential entry into the home andshould be considered. Here is a quick look at some methods anddevices used to secure homes in the past.

Trip wires were popular in the beginning. A small, wall-mountedclip with two large ball bearings that almost touched would beinstalled in a hallway, about a foot off the floor. A copper clipinserted between the ball bearings completed a closed circuit. Athin wire attached to the clip was strung across the hall andsecured to the opposite wall. Anyone walking through would pull theclip away from the ball bearings and set off the alarm. This workedvery well — in poorly lit hallways with an impatientintruder.

Next came carpet mats. Two large sheets of plastic with severallong copper strips, layered one on the other, spanned the protectedarea. The pairs of copper strips were slightly curved away fromeach other and separated by a thin piece of foam. Anyone walking onthe mat would force the two copper strips together, setting off thealarm. There are several reasons why I disliked this method.

The carpet mat was normally open and relied on good, solidconnections with the circuit wire. If the wire were to break, thealarm system would not know it, thus defeating the device. Earlyalarm systems did not supervise normally open circuits and weresubject to failure. In addition, the carpet mat needed to beinserted under the carpet. This required pulling up the sides ofthe carpet and sliding the sheet between the carpet and the padbeneath it. Long hallways were difficult: one needed to pull upboth sides of the short, wall-to-wall piece of carpet and snakepull wires across to pull the mat back through.

If you accidentally bent the copper strips, the carpet mat wouldbecome extra sensitive, resulting in false alarms. Some carpet matswere located in open areas of a room, while others were installedon individual staircase steps. The customer needed to be aware ofthe mats’ placement to avoid setting furniture on them. A chair legcould trip the device, disabling the device or system.

I also disliked carpet mats because my fingers were mysteriouslydrawn to the vicious carpet tacks. My pinky finger was infected somany times from nicking it on the sharp tacks, I was worried I wasgoing to go through life with a permanently swollen digit.

The alarm industry soon developed motion sensors that picked upmovement inside the protected area. The first models available werebased on microwave technology. A transmitting sensor or sensorswould emit a microwave signal that bounced off a distant object andback into a receiving sensor. Anyone moving around the room woulddisrupt the return signal and set off the alarm. There were twoproblems with this technology. In homes with central heating or airconditioning, as the warm or cold air left the registers, themicrowave sensor would pick up the disruption and set off thealarm. Drafts would also trip the alarm if they moved plants orobjects within the sensor’s range of protection. The second problemwas that dust collected in the transmit/receive sensor. If enoughparticles were present, the device would emit a very high-pitchedfrequency. This frequency would result in customer headaches andangry canines.

Another method of interior protection relied on line-of-sightphotoelectric cells. The PECs traveled in pairs, one being atransmitter, the other a receiver. The transmitter would be locatedat one end of the protected area with its receiving counterpart atthe other end, tied together by a photoelectric beam. Anyonewalking through the beam would interrupt the reception of thissignal, tripping the alarm system. The installer had to be carefulwhen aligning the two sensors. Each device was equipped with amirror that reflected the signal out to its partner. They had to“see” each other in order to be properly set up. If themirrors were not set correctly, a weak beam created the potentialfor false alarms. I used to fan three fingers in front of thereceiver to see if it remained in a non-alarm condition. If thedevice tripped, I would attach a strobe light to each side. Theflashing light aided in finding the centerline of the path of thebeam.

Today’s interior devices are often based on passive infraredtechnology. The human body emits IR radiation. Security deviceswere developed to sense and react to it. As the IR radiation entersthe lens of the PIR, the level of radiation received is measuredagainst external and internal sensitivity settings. That’s thesimple way to look at it. The broader picture also considers thesurrounding environment. The PIR, when first powered up, looks atthe walls and nearby objects and remembers their IR emissions. Whensomeone or something enters the field of coverage, it carries aconcentrated 10° temperature difference from its surroundings.The PIR device reacts to it.

As with all devices that rely on environment settings, take carewhen placing them around the home. Do not, for example, aim themnear kitchens. Ovens and dishwashers can emit rapid temperaturechanges that can be mistaken for an intruder. Also avoid aiming atthe fireplace. Sudden flare-ups can set off a false alarm.

You may wonder how these devices react in homes that are in verywarm or cold climates. The PIR, when measuring its surroundings,will accept slow changes in temperature. It’s the rapid changes itwon’t ignore.

As time moved on, someone combined the earlier microwavetechnology with the current workings of the PIR. The intent was tocapture the movement of an intruder while using temperature changesas the verifying factor. Although this new combination reduced thefrequency of false alarms, there are still installation pitfalls.Aiming the device at entry/exit points of protection is still notadvisable. You run the risk of the interior device tripping beforethe delayed zone has a chance to react to its being opened,resulting in an instant alarm instead of a delayed one.

Additionally, installers must avoid aiming the device atheater/cooler registers and vents or at outside windows. In thelatter case, a halogen headlight from a passing car or directsunlight into the lens could set off the sensor. Moreover —and I have said this before — if you have a dog or cat, youshould never use an interior motion/PIR sensor. Although themanufacturer may state that it is okay, past experience has taughtme to be wary of such claims. I had a customer swear her dog wastoo old and moved too slowly to set off her PIR. I explained thatunless her dog was dead, he was capable of setting off the alarm.That resulted in a proverbial “trip to the principal’soffice.”

Technology is always trying to top itself. More vendors haveembraced the challenge to develop a better interior trap and toreduce the possibility of a false alarm. The latter is always oursecondary goal; protecting the customer is the first.

By the way, the fellow who lowered himself through the roof ofthe home was apprehended despite his careful planning. It seems hewas unaware of the new guard dogs the homeowner had purchased onlydays before. I can only imagine it looked like something out of aBugs Bunny cartoon. The intruder stood in the center of the room,afraid to move while the dogs growled and snarled until thecustomer returned home. I love my job, and that’s one of thereasons why. There is always something to look back on and laughabout.

Steve Filippini is a senior security technician with 20 years ofexperience in the security installation industry. He can be reachedat [email protected].

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