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THE BASICS of fire protection

The goal behind any security system is to keep your stuff right where you left it. If you leave home with a $2,500 personal computer on the desk in your

THE BASICS of fire protection

Feb 1, 2000 12:00 PM,
Steve Filippini

The goal behind any security system is to keep your stuff right where youleft it. If you leave home with a $2,500 personal computer on the desk inyour office, you expect it to be there when you return. If an alarm systemis needed to keep the public honest, so be it.

If you take the concept one step further, you are now looking at protectinglife as well as property. Fire detection devices are the most importantinvestments you owe your customer and their family. You do not need afancy, high-priced home automation and security system to protect them fromthe dangers of fire. What you need are smoke detectors locatedstrategically throughout the home. Smoke detectors are inexpensive, easy toinstall and easy to maintain, but before I begin looking at the world ofsmoke detectors, there is one point I cannot stress enough. Smoke detectorsare always the first thing the homeowner takes for granted. If you keepthat in the back of your mind while reading through the rest of thisarticle, you may come away with a better sense of respect for that littlewhite circular plate that blinks its red light at you every 40 seconds orso.

Selecting the right fire detector for the right job is not always simple.There are several things to take into consideration. For example, mostpeople do not realize that there are two types of smoke detectors on themarket today. The more common of the two is the ionization smoke detector,which is sold in almost all retail stores across the country. The othertype of smoke detector is the photoelectric, which is more expensive and isinstalled by the home automation and security technician. Before you canproperly select the device best suited for your customer’s needs, however,you need to understand the differences between the two.

Ionization smoke detectors

These detectors have an ionization chamber that consists of twoelectrically charged plates. There is also a small radioactive source usedfor ionizing the air between the two plates. The radioactive source istypically Americium 241. The radiation emitted from this device, to thebest of my knowledge, has not caused any household pets to glow in thedark. Nevertheless, there is usually a warning that comes with each devicethat asks that you not dismantle the detector because there is a source ofradiation inside. Without getting too technical on the operation of thistype of detector, I will try to explain how it works. The air between thetwo plates consists of molecules, and the radiation emitted from the sourceis slapping the electrons off of those molecules. When the molecule losesenough electrons, it becomes a positively charged ion. The electrons thatwere slapped away find other molecules to which they can attach themselves.As they do so, the molecule becomes electron heavy and, therefore,negatively charged. This process is constant, and it creates an equalnumber of positive and negative ions. As long as there is an equal balanceof ions between the two plates, everyone is happy, living inside what iscommonly referred to as the sensing chamber.

When a fire occurs, tiny particles of combustion float up into the sensingchamber of the detector and start to combine with the ions floating aroundinside. The particles of combustion will then combine with enough ions tocreate an imbalance that will cause the detector to go into an alarmcondition. Simple enough, except the sensing chamber can be misled intothinking that there are particles of combustion floating around when thereare not any present. Atmospheric pressure and humidity level changes aretwo causes of a false alarm. Dust, critters and condensation can also wreakhavoc with a sensing chamber. One way to compensate for some of the causesof false readings is to use a detector with two sensing chambers. Still, ifenough of the foreign material works its way into the sensing chamber, youwill experience false alarms.

Photoelectric smoke detectors

There is no radiation in these detectors, just a light emitting diode (LED)looking for a partner to shine on. The photoelectric smoke detector has asensing chamber just like its ionization cousin, except the method ofdetection is different. You are probably familiar with doorbells in liquorstores and small convenience stores that sound when someone enters througha doorway. That sound is caused by someone walking between a pair ofphotoelectric cells. These are also the same devices that shine across yourautomatic overhead garage door to prevent small children from being crushedas the door is lowering. The same concept is used for this type of smokedetector, except that there are two different methods of detectingparticles of combustion.

The first method is called light obscuration. The LED inside the sensingchamber is shining its light into a photosensitive receiver (photo-diode isone example). When particles of combustion float into the chamber, theyblock the LED from fully shining into the receiver and cause the detectorto go into an alarm condition.

The second method is light scattering. The LED inside the sensing chamberis shining its light away from the receiver. When particles of combustionfloat into the chamber, they reflect the light from the LED into thereceiver, which, in turn, causes the detector to go into an alarm condition.

Because you should be kept up to date of what options are available to yourcustomer, you should be aware that both the photoelectric and theionization smoke detectors are available in most retail stores that offerfire protection devices. The consumer-installed smoke detectors are usuallypowered off of a 9 V battery that needs to be routinely replaced. Thebattery should be alkaline or similar to ensure a life span of at least oneyear. If you happen to exceed the one year mark, the detector will nag youto death with a quick, metallic “dit” sound. When my travels to oursecurity branches included service calls to the local customers, I wouldalways be amazed at how many battery operated smoke detectors had the 9 Vbattery removed to stop false alarms or “dit” from disturbing their sleep.I would be further amazed at how many actual smoke detectors had beenremoved by the customers and stuffed into the hall linen closet forwhatever reason without having the battery removed. The house would then bepossessed by the dreaded haunted beep emitting from the walls, quick andpiercing, always evading the most persistent hunter.

The next question is simple: “Which type of smoke detector is best suitedfor your customer’s needs?” The ionization detector is best suited todetect fast, flaming fires. The photoelectric detector is best suited todetect slow, smoldering fires. Both become overly sensitive when excessdust and critters invade the sensing chamber, and both will detect the sametypes of fire but at different response times. In either case, installationinstructions should be read carefully before the first mounting screw isused.


Having covered consumer-purchased and installed detectors, it is time wemoved up to the professionally installed home fire systems. The method ofdetection is the same, but the installation process is different.

Most of the home automation and security systems on the market today relyon 12 VDC smoke detectors. The detectors can be either ionization orphotoelectric, and they may or may not have a built-in heat-sensingdetector. A heat detector is designed to sense and respond to flash firesin the immediate area of the detector. There are two primary types of heatdetectors. The first is rated at 135 Degrees F (57.2 Degrees C), the otherat 199 Degrees F (92.8 Degrees C). Location will dictate which one you willuse. There are also heat detectors that do not have smoke detectorsattached to them, but I will cover those later.

There are also two types of 12 VDC smoke detectors – two-wire or four-wiredevices. Not that one is better than the other, their use is usuallydictated by the premise unit that happens to be providing the alarm-sensingintelligence. Regardless of which one you use, there is one rule that mustalways be followed: All heat and smoke detectors that share the sameprotection loop must be wired in a daisy-chained, fully supervised manner.

Why the emphasis? This rule is violated more often for the lamest reasons.The concept of a fully supervised loop is simple; the premise unit needs tobe aware of all of the fire circuit’s wiring at all times. If at any timethere is a break in any of the fire loops, the premise unit needs toproduce an audible indication alerting the customer to investigate andcorrect the problem. The standard response to a broken wire is to initiatea “fire trouble” condition. The standard response to a shorted wire thatprovides the fire-sensing circuit is to initiate a “fire alarm” condition.If you do not wire your fire-protection circuit as described above, you runthe risk of having a critical wire breaking and disabling any detection ofa fire that may be spreading throughout the home. If you do not know how towire your fire circuit in a fully supervised manner, refer to the premiseunit’s installation manual. It should be fully documented with pictures ordrawings.

To fully supervise a fire loop, you need to be able to detect a break inthe fire-sensing loop as well as the wires that provide the 12 VDC powersource to each device. Some alarm premise units require only an end-of-lineresistor (EOLR). The resistor is located at the end of the fire-sensingloop. Any break in the fire-sensing loop will produce a fire troublecondition. Other premise units require an end-of-line module (EOLM). Themodule is also located at the end of the fire circuit, but it monitors bothpower and loop integrity, which is another reason why you should neverT-tap your fire-loop wiring.

The cable used for a fire alarm circuit must meet industry standards. Mostof the fire circuits that I have worked with require either a two or fourconductor, AWG#16, fire-approved wire. The cable is usually red and isdesigned to burn more slowly than average wire.

Now that you have completed the ground work, and you know what to use andwhy, we need to determine where these devices should be located. It’s easyto figure it out in a new construction. Building and fire codes willrequire the builder to locate 110 V powered smoke detectors outside eachand every bedroom throughout the home. Some locations also require adetector located inside each bedroom. The detector must have a built-inaudible alarm that sounds when smoke or a fire is detected.

What if the home on which you are working is more than 30 years old? Startwith the basics. Each and every bedroom should have a smoke detectorlocated in the immediate vicinity. Smoke detectors should be located inlong hallways, at the top of stairways, and above drop ceilings, basementsand lofts. Smoke detectors must be located at least 4 inches (102 mm) to 12inches (305 mm) away from walls (if mounted on the ceiling) and ceilings(if mounted on walls). Air currents do not change directions at 90 Degreeangles; they curve around corners. Never replace a 110 V powered smokedetector with a 9 V or 12 V model. It is illegal and in violation of fireand building codes.

A heat detector located in the kitchen, garage, hot water closet,unfinished attic and utility room will add to your customer’s protection.Do not place a smoke detector in your unfinished attic, garage or kitchen.Kitchens are an obvious no-no. One piece of burnt toast will bring the firedepartment knocking on the door. If your customer is not home, they knockwith a big axe. Attics and garages are dust and dirt magnets and attractcritters from all walks of life. If the area in which you want to locate aheat detector is always warm or hot, you may want to consider using the 199Degrees F. model. Other areas that do not produce heat (closets andcabinets) should have the lower rated 135 Degrees F models installed.

Until a few years ago, it was easy for the security and home automationtechnician to determine if the fire device that was giving the homeownerproblems was store bought or professionally installed. If it produced themetallic “dit” at any time, it was store bought. We were more than happy toreplace the 9 V battery at a service charge of $55. Boundaries wereestablished, and life was rosy. Then, the wireless smoke detector wasintroduced. It looked, smelled and felt store bought. It used a 9 Vbattery. It even “dits” late at night. It also possessed a low-poweredtransmitter to send an alarm message to the premise unit. For theinstaller, it was an answered prayer. If you cannot get a wire from onepoint to another, plug in a battery and slap it up on the ceiling. For thecustomer, it was magic. For the service technician, it was one more deviceyou needed to carry on your truck.

Wrapping up

We are down to testing and maintaining the fire devices. Most store-boughtdetectors have a mechanical push button located on the head of thedetector. Press and hold the button until the detector shrieks at you tolet go. That usually means that the detector is functioning. Professionallyinstalled detectors usually require someone to hold a test magnet up to amarked location on the head of the detector. Some technicians go out andpurchase canned smoke, which is all right as long as the smoke spray doesnot leave a residue inside the sensing chamber. Do not light a cigaretteand blow smoke into the sensing chamber. Particles of ash might lingerinside the sensing chamber and cause a false alarm later. Do not placeburning materials under the detector; You are in the business to detectfires, not start them.

Keep the smoke detector free of dust and critters by either blowing out thesensing chamber using compressed air or vacuuming around the outside of thesensing chamber with a shop vac. The alarm technician has been trained onthe proper method of dismantling, cleaning and rebuilding the smoke heads.The customer does not have to deal with it.

Finally, have your customer test their fire system weekly. Replace all 9 Vbatteries yearly. Do not let them assume they have fire protection. Verifyit. ClichAs often have truth behind their origins. Here is a truth to liveby: “The life you save may be your own”.

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