The Anatomy of a false alarm
Sep 1, 1999 12:00 PM, Steve Filippini
People working in the alarm industry will generally agree that the number one cause of sleepless nights and premature hair loss is the evil and dreaded false alarm. A false alarm can be defined as an alarm condition that resulted from something other than a break-in, fire or an actual emergency situation. There are several causes of false alarms. The customer may forget to disarm the system before opening a protected door or window, or he may pass in front of an armed motion detector in the middle of the night. The customer may forget to secure a door or window, and a gust of wind could open it while the system was armed. Momentary or extended AC/Battery power loss to the main security system or peripheral devices may generate a false alarm. Substandard installation practices - splices not taped/soldered or properly capped off, loose wires at the control panel's terminal strip, system wiring sharing the same routing holes used by the electrician, incorrect wire size for the ap! plication, incorrect equipment u sed for the protected area, and damaged wiring. There is even the possibility of defective alarm equipment.
Let's look at how to approach, identify and avoid false alarms. Every customer that has an active alarm system will, at one time or another, accidentally trigger the alarm. An alarm system should be customized and installed in a way that does not change the normal lifestyle of the customer. If the process of arming and disarming the system flows with everyday activities, you should not have too many problems.
Arming and disarming
If the customer enters and exits the home through a garage door, then locate the system keypad near that door. If the keypad needs to be located some distance away from the door, program your exit delays to the customer's ability to enter the arming code and leave the home in time before the delay expires. One of the biggest causes of false alarms when arming the system is when the customer forgets a wallet or purse and searches for it as the seconds count down. All of that can be averted with adequate delays in arming time.
The entry delay is just as important to consider as the exit delay. Allow a client enough time to walk to the keypad and enter the disarm code. Customers will enter the home with bags of groceries in both arms and walk to the kitchen to drop them off before disarming the system. Sometimes they make it in time, some times they do not. Again, allowing enough time will certainly help to decrease the number of false alarms.
Pets and children
I am a firm believer that if there are cats in the home, you should not use interior motion sensors. Cats will climb on anything. If there are dogs in the home, you might get away with a special "Pet Alley" lens designed specifically to allow a dog to pass underneath the motion sensor's field of detection.
Some manufacturers of motion sensors claim their products will know the difference between a dog and an intruder. I believe the deciding factor is body mass and generated heat. I have heard from some colleagues that they work, but others are a little skeptical. My suggestion is to follow the installation instructions carefully, and report any problems you encounter with the company that makes the system.
With children, the challenge, as described by a child to an adult, is to be able to leave the premise undetected. I design systems to keep people out, not in, and I have worked on a few systems where a child bypassed the window contact by either providing his own magnet or shorting the pair of wires leading to the window contact itself. Recessed door and window contacts make it almost impossible for children to tamper with detection loops.
Damaged or exposed wiring
Protection circuits are routed from the point of opening to the main control panel. There's always a chance something may lessen the integrity of the cable. Rodents are notorious wire eaters. They gnaw through the rubber insulation and leave bare copper wires to short out. Exposed wires are the result of other things as well. Pulling a cable through an attic opening may cause a long and sporadic tear in the cable if the cable is dragged across a sharp burr or bent nail tip. Staples from a wire staple gun may hit a nail during its path into the wall and curl back into itself, impaling the cable as it goes. Splices are an issue as well. Tape and solder is always the preferred method. If you skip the solder and just tape the splice, the constant expanding and contracting of the metal wires due to temperature changes will eventually loosen the tightest, twisted connection. Another thing to keep in mind is the natural oxidation that occurs with exposed wires. Oxidation will coat th! e outside of a wire and build up a resistive material that limits the current flow in a circuit. As the wire resistance builds, your chance for a false alarm builds with it. Splice caps are OK to use provided you know how to use them correctly.
Power outages
Security systems are usually powered by a step-down, plug-in transformer. These transformers will take a 120 VAC input and reduce it to an 18 VAC to 24 VAC output, depending on the power requirements of the security system. The 18 V to 24 V source is wired to the control panel AC input where it is sent through an AC/DC rectifier. This rectifier converts the AC voltage input to a DC voltage output. Almost all security systems on the market today run off a 12 VDC power source. In the event of an AC failure, 12 V back-up batteries that are wired into the control panel will take over and keep the system running. These batteries, typically Gel-Cell or sealed lead in nature, will provide the normal system anywhere from 4 to 12 hours of back-up power. When a system is first installed, it is advised that the back-up battery be allowed to charge for at least 24 hours to ensure a full charge before testing. Most of the power problems that plague systems are either brown outs (slight dip! s in power that last for just a few moments) or black outs (full power outages that may last minutes or hours). The back-up battery will keep the system running uninterrupted until AC power is restored. Of course, this requires that the duration of the outage lasts no longer than the battery's charge. If the system shuts down during minor AC failure, check the integrity of the battery fuse, if one is present. If the fuse is all right, connect your volt meter probes to the back-up battery with the security system still wired to it. With the volt meter set to DC volts, you should measure 12.5 V to 13.8 V depending on the system. Now, unplug the step-down transformer from the wall receptacle. The voltage reading on the meter will drop to around 12.3 V to 12.6 V. Watch the meter reading for at least two minutes. The reading should not drop below 12 V. If it does, you need to check the security panel's charging circuit or replace the back-up battery. Back-up batteries will usually need to be replaced every three ! years.
Smoke detectors
Somewhere in the book of life, I believe it clearly states that all fire alarms, false or actual, shall become audible in the middle of the night. Smoke detectors are probably the most important devices your security system can have. What you want to avoid is having the fire alarm trip for reasons other than an actual fire. There are two common types of smokedetectors in the home - ionization and photoelectric. The ionization detector in ternally emits a particle radiation pattern that collides with air particles. These ion particles, when mixed with particles of combustion, send the detector into an alarm condition. Photoelectric detectors have an internal LED (source) that will send the detector into an alarm condition whenever smoke particles either block out or refract into the receiving LED's lens, which may vary from detector to detector. I usually instruct customers and field technicians to vacuum out any dust that may collect in the smoke-sensing chamber of the detecto! r. If you are lucky, you will al so suck out any spiders or bugs that may be living in the chamber. A final, obvious cause of false alarms is the practice of placing smoke detectors in or near kitchens.
Defective devices
Defective devices may include door/window contacts, motion sensors, carpet mats, vibration sensors and smoke/heat detectors. A sensing device that begins to generate false alarms may eventually fail altogether. At that point, it is easy to locate and replace. If your security system is properly zoned out and broken down by points, it is much easier to isolate and correct a problem. If you combined several points and forgot to label the wiring, you may be there awhile. I used to measure the loop resistance for each point/zone and write it down inside the control panel. That way, if a point/zone was suspected of failure, I would compare loop resistance with earlier readings to see if there was a significant difference. Points of protection are nothing more than a pair of wires tied together at one end with some sort of sensing device wired in series. Loop resistance should not be more than 5 V to 10 V. I would begin to worry when the resistance climbed higher than 20 Vto 30 V. D! o not forget to take into consid eration any end-of-line-resistors that may be part of the circuit.
False alarms result in needless response from the emergency agencies. Most cities will give you anywhere from one to three freebies before they charge you. The more false alarms that occur after the charges begin, the higher the fine. If the customer does not feel secure or confident about the security system, he will not stick around for long, and word of mouth sells more systems than advertisements. You will know when a customer has reached that point of peak frustration when he greets you at the door with the phrase, "Fix it or pull it." Do not do what I once did and start cutting wires out of the control panel.
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