The Anatomy of a false alarm
Sep 1, 1999 12:00 PM,
People working in the alarm industry will generally agree that the numberone cause of sleepless nights and premature hair loss is the evil anddreaded false alarm. A false alarm can be defined as an alarm conditionthat resulted from something other than a break-in, fire or an actualemergency situation. There are several causes of false alarms. The customermay 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 thenight. The customer may forget to secure a door or window, and a gust ofwind could open it while the system was armed. Momentary or extendedAC/Battery power loss to the main security system or peripheral devices maygenerate a false alarm. Substandard installation practices – splices nottaped/soldered or properly capped off, loose wires at the control panel’sterminal strip, system wiring sharing the same routing holes used by theelectrician, incorrect wire size for the ap!plication, incorrect equipment used for the protected area, and damaged wiring. There is even thepossibility of defective alarm equipment.
Let’s look at how to approach, identify and avoid false alarms. Everycustomer that has an active alarm system will, at one time or another,accidentally trigger the alarm. An alarm system should be customized andinstalled in a way that does not change the normal lifestyle of thecustomer. If the process of arming and disarming the system flows witheveryday activities, you should not have too many problems.
Arming and disarming
If the customer enters and exits the home through a garage door, thenlocate the system keypad near that door. If the keypad needs to be locatedsome distance away from the door, program your exit delays to thecustomer’s ability to enter the arming code and leave the home in timebefore the delay expires. One of the biggest causes of false alarms whenarming the system is when the customer forgets a wallet or purse andsearches for it as the seconds count down. All of that can be averted withadequate delays in arming time.
The entry delay is just as important to consider as the exit delay. Allow aclient 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 walkto the kitchen to drop them off before disarming the system. Sometimes theymake it in time, some times they do not. Again, allowing enough time willcertainly 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 useinterior motion sensors. Cats will climb on anything. If there are dogs inthe home, you might get away with a special “Pet Alley” lens designedspecifically to allow a dog to pass underneath the motion sensor’s field ofdetection.
Some manufacturers of motion sensors claim their products will know thedifference between a dog and an intruder. I believe the deciding factor isbody mass and generated heat. I have heard from some colleagues that theywork, but others are a little skeptical. My suggestion is to follow theinstallation instructions carefully, and report any problems you encounterwith the company that makes the system.
With children, the challenge, as described by a child to an adult, is to beable 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 thewindow contact by either providing his own magnet or shorting the pair ofwires leading to the window contact itself. Recessed door and windowcontacts make it almost impossible for children to tamper with detectionloops.
Damaged or exposed wiring
Protection circuits are routed from the point of opening to the maincontrol panel. There’s always a chance something may lessen the integrityof the cable. Rodents are notorious wire eaters. They gnaw through therubber insulation and leave bare copper wires to short out. Exposed wiresare the result of other things as well. Pulling a cable through an atticopening may cause a long and sporadic tear in the cable if the cable isdragged across a sharp burr or bent nail tip. Staples from a wire staplegun 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 andsolder is always the preferred method. If you skip the solder and just tapethe splice, the constant expanding and contracting of the metal wires dueto temperature changes will eventually loosen the tightest, twistedconnection. Another thing to keep in mind is the natural oxidation thatoccurs 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 thewire 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.
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 to24 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 issent through an AC/DC rectifier. This rectifier converts the AC voltageinput to a DC voltage output. Almost all security systems on the markettoday run off a 12 VDC power source. In the event of an AC failure, 12 Vback-up batteries that are wired into the control panel will take over andkeep the system running. These batteries, typically Gel-Cell or sealed leadin nature, will provide the normal system anywhere from 4 to 12 hours ofback-up power. When a system is first installed, it is advised that theback-up battery be allowed to charge for at least 24 hours to ensure a fullcharge before testing. Most of the power problems that plague systems areeither brown outs (slight dip!s in power that last for just afew moments) or black outs (full power outages that may last minutes orhours). The back-up battery will keep the system running uninterrupteduntil AC power is restored. Of course, this requires that the duration ofthe outage lasts no longer than the battery’s charge. If the system shutsdown during minor AC failure, check the integrity of the battery fuse, ifone is present. If the fuse is all right, connect your volt meter probes tothe back-up battery with the security system still wired to it. With thevolt meter set to DC volts, you should measure 12.5 V to 13.8 V dependingon the system. Now, unplug the step-down transformer from the wallreceptacle. The voltage reading on the meter will drop to around 12.3 V to12.6 V. Watch the meter reading for at least two minutes. The readingshould not drop below 12 V. If it does, you need to check the securitypanel’s charging circuit or replace the back-up battery. Back-up batterieswill usually need to be replaced every three !years.
Somewhere in the book of life, I believe it clearly states that all firealarms, false or actual, shall become audible in the middle of the night.Smoke detectors are probably the most important devices your securitysystem can have. What you want to avoid is having the fire alarm trip forreasons other than an actual fire. There are two common types of smokedetectors in the home – ionization and photoelectric. The ionization detector internally emits a particle radiation pattern that collides with airparticles. These ion particles, when mixed with particles of combustion,send the detector into an alarm condition. Photoelectric detectors have aninternal LED (source) that will send the detector into an alarm conditionwhenever smoke particles either block out or refract into the receivingLED’s lens, which may vary from detector to detector. I usually instructcustomers and field technicians to vacuum out any dust that may collect inthe smoke-sensing chamber of the detecto!r. If you are lucky, you will also 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 inor near kitchens.
Defective devices may include door/window contacts, motion sensors, carpetmats, vibration sensors and smoke/heat detectors. A sensing device thatbegins to generate false alarms may eventually fail altogether. At thatpoint, it is easy to locate and replace. If your security system isproperly zoned out and broken down by points, it is much easier to isolateand correct a problem. If you combined several points and forgot to labelthe wiring, you may be there awhile. I used to measure the loop resistancefor 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 resistancewith earlier readings to see if there was a significant difference. Pointsof protection are nothing more than a pair of wires tied together at oneend with some sort of sensing device wired in series. Loop resistanceshould not be more than 5 V to 10 V. I would begin to worry when theresistance climbed higher than 20 Vto 30 V. D!o not forget to take into consideration any end-of-line-resistors that may be part of the circuit.
False alarms result in needless response from the emergency agencies. Mostcities will give you anywhere from one to three freebies before they chargeyou. The more false alarms that occur after the charges begin, the higherthe fine. If the customer does not feel secure or confident about thesecurity system, he will not stick around for long, and word of mouth sellsmore systems than advertisements. You will know when a customer has reachedthat point of peak frustration when he greets you at the door with thephrase, “Fix it or pull it.” Do not do what I once did and start cuttingwires out of the control panel.