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 you left it. If you leave home with a $2,500 personal computer on the desk in your office, you expect it to be there when you return. If an alarm system is needed to keep the public honest, so be it.
If you take the concept one step further, you are now looking at protecting life as well as property. Fire detection devices are the most important investments you owe your customer and their family. You do not need a fancy, high-priced home automation and security system to protect them from the dangers of fire. What you need are smoke detectors located strategically throughout the home. Smoke detectors are inexpensive, easy to install and easy to maintain, but before I begin looking at the world of smoke detectors, there is one point I cannot stress enough. Smoke detectors are always the first thing the homeowner takes for granted. If you keep that in the back of your mind while reading through the rest of this article, you may come away with a better sense of respect for that little white circular plate that blinks its red light at you every 40 seconds or so.
Selecting the right fire detector for the right job is not always simple. There are several things to take into consideration. For example, most people do not realize that there are two types of smoke detectors on the market today. The more common of the two is the ionization smoke detector, which is sold in almost all retail stores across the country. The other type of smoke detector is the photoelectric, which is more expensive and is installed by the home automation and security technician. Before you can properly 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 two electrically charged plates. There is also a small radioactive source used for ionizing the air between the two plates. The radioactive source is typically Americium 241. The radiation emitted from this device, to the best of my knowledge, has not caused any household pets to glow in the dark. Nevertheless, there is usually a warning that comes with each device that asks that you not dismantle the detector because there is a source of radiation inside. Without getting too technical on the operation of this type of detector, I will try to explain how it works. The air between the two plates consists of molecules, and the radiation emitted from the source is slapping the electrons off of those molecules. When the molecule loses enough electrons, it becomes a positively charged ion. The electrons that were 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 equal number of positive and negative ions. As long as there is an equal balance of ions between the two plates, everyone is happy, living inside what is commonly referred to as the sensing chamber.
When a fire occurs, tiny particles of combustion float up into the sensing chamber of the detector and start to combine with the ions floating around inside. The particles of combustion will then combine with enough ions to create an imbalance that will cause the detector to go into an alarm condition. Simple enough, except the sensing chamber can be misled into thinking that there are particles of combustion floating around when there are not any present. Atmospheric pressure and humidity level changes are two causes of a false alarm. Dust, critters and condensation can also wreak havoc with a sensing chamber. One way to compensate for some of the causes of false readings is to use a detector with two sensing chambers. Still, if enough of the foreign material works its way into the sensing chamber, you will 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 a sensing chamber just like its ionization cousin, except the method of detection is different. You are probably familiar with doorbells in liquor stores and small convenience stores that sound when someone enters through a doorway. That sound is caused by someone walking between a pair of photoelectric cells. These are also the same devices that shine across your automatic overhead garage door to prevent small children from being crushed as the door is lowering. The same concept is used for this type of smoke detector, except that there are two different methods of detecting particles of combustion.
The first method is called light obscuration. The LED inside the sensing chamber is shining its light into a photosensitive receiver (photo-diode is one example). When particles of combustion float into the chamber, they block the LED from fully shining into the receiver and cause the detector to go into an alarm condition.
The second method is light scattering. The LED inside the sensing chamber is shining its light away from the receiver. When particles of combustion float into the chamber, they reflect the light from the LED into the receiver, 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 your customer, you should be aware that both the photoelectric and the ionization smoke detectors are available in most retail stores that offer fire protection devices. The consumer-installed smoke detectors are usually powered off of a 9 V battery that needs to be routinely replaced. The battery should be alkaline or similar to ensure a life span of at least one year. If you happen to exceed the one year mark, the detector will nag you to death with a quick, metallic "dit" sound. When my travels to our security branches included service calls to the local customers, I would always be amazed at how many battery operated smoke detectors had the 9 V battery removed to stop false alarms or "dit" from disturbing their sleep. I would be further amazed at how many actual smoke detectors had been removed by the customers and stuffed into the hall linen closet for whatever reason without having the battery removed. The house would then be possessed by the dreaded haunted beep emitting from the walls, quick and piercing, always evading the most persistent hunter.
The next question is simple: "Which type of smoke detector is best suited for your customer's needs?" The ionization detector is best suited to detect fast, flaming fires. The photoelectric detector is best suited to detect slow, smoldering fires. Both become overly sensitive when excess dust and critters invade the sensing chamber, and both will detect the same types of fire but at different response times. In either case, installation instructions should be read carefully before the first mounting screw is used.
Installation
Having covered consumer-purchased and installed detectors, it is time we moved up to the professionally installed home fire systems. The method of detection is the same, but the installation process is different.
Most of the home automation and security systems on the market today rely on 12 VDC smoke detectors. The detectors can be either ionization or photoelectric, and they may or may not have a built-in heat-sensing detector. A heat detector is designed to sense and respond to flash fires in the immediate area of the detector. There are two primary types of heat detectors. The first is rated at 135 Degrees F (57.2 Degrees C), the other at 199 Degrees F (92.8 Degrees C). Location will dictate which one you will use. There are also heat detectors that do not have smoke detectors attached to them, but I will cover those later.
There are also two types of 12 VDC smoke detectors - two-wire or four-wire devices. Not that one is better than the other, their use is usually dictated by the premise unit that happens to be providing the alarm-sensing intelligence. Regardless of which one you use, there is one rule that must always be followed: All heat and smoke detectors that share the same protection 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 to be aware of all of the fire circuit's wiring at all times. If at any time there is a break in any of the fire loops, the premise unit needs to produce an audible indication alerting the customer to investigate and correct the problem. The standard response to a broken wire is to initiate a "fire trouble" condition. The standard response to a shorted wire that provides the fire-sensing circuit is to initiate a "fire alarm" condition. If you do not wire your fire-protection circuit as described above, you run the risk of having a critical wire breaking and disabling any detection of a fire that may be spreading throughout the home. If you do not know how to wire your fire circuit in a fully supervised manner, refer to the premise unit's installation manual. It should be fully documented with pictures or drawings.
To fully supervise a fire loop, you need to be able to detect a break in the fire-sensing loop as well as the wires that provide the 12 VDC power source to each device. Some alarm premise units require only an end-of-line resistor (EOLR). The resistor is located at the end of the fire-sensing loop. Any break in the fire-sensing loop will produce a fire trouble condition. Other premise units require an end-of-line module (EOLM). The module is also located at the end of the fire circuit, but it monitors both power and loop integrity, which is another reason why you should never T-tap your fire-loop wiring.
The cable used for a fire alarm circuit must meet industry standards. Most of the fire circuits that I have worked with require either a two or four conductor, AWG#16, fire-approved wire. The cable is usually red and is designed to burn more slowly than average wire.
Now that you have completed the ground work, and you know what to use and why, we need to determine where these devices should be located. It's easy to figure it out in a new construction. Building and fire codes will require the builder to locate 110 V powered smoke detectors outside each and every bedroom throughout the home. Some locations also require a detector located inside each bedroom. The detector must have a built-in audible 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? Start with the basics. Each and every bedroom should have a smoke detector located in the immediate vicinity. Smoke detectors should be located in long hallways, at the top of stairways, and above drop ceilings, basements and lofts. Smoke detectors must be located at least 4 inches (102 mm) to 12 inches (305 mm) away from walls (if mounted on the ceiling) and ceilings (if mounted on walls). Air currents do not change directions at 90 Degree angles; they curve around corners. Never replace a 110 V powered smoke detector with a 9 V or 12 V model. It is illegal and in violation of fire and 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 fire department knocking on the door. If your customer is not home, they knock with a big axe. Attics and garages are dust and dirt magnets and attract critters from all walks of life. If the area in which you want to locate a heat detector is always warm or hot, you may want to consider using the 199 Degrees F. model. Other areas that do not produce heat (closets and cabinets) should have the lower rated 135 Degrees F models installed.
Until a few years ago, it was easy for the security and home automation technician to determine if the fire device that was giving the homeowner problems was store bought or professionally installed. If it produced the metallic "dit" at any time, it was store bought. We were more than happy to replace the 9 V battery at a service charge of $55. Boundaries were established, and life was rosy. Then, the wireless smoke detector was introduced. It looked, smelled and felt store bought. It used a 9 V battery. It even "dits" late at night. It also possessed a low-powered transmitter to send an alarm message to the premise unit. For the installer, it was an answered prayer. If you cannot get a wire from one point to another, plug in a battery and slap it up on the ceiling. For the customer, it was magic. For the service technician, it was one more device you needed to carry on your truck.
Wrapping up
We are down to testing and maintaining the fire devices. Most store-bought detectors have a mechanical push button located on the head of the detector. Press and hold the button until the detector shrieks at you to let go. That usually means that the detector is functioning. Professionally installed detectors usually require someone to hold a test magnet up to a marked location on the head of the detector. Some technicians go out and purchase canned smoke, which is all right as long as the smoke spray does not leave a residue inside the sensing chamber. Do not light a cigarette and blow smoke into the sensing chamber. Particles of ash might linger inside the sensing chamber and cause a false alarm later. Do not place burning materials under the detector; You are in the business to detect fires, not start them.
Keep the smoke detector free of dust and critters by either blowing out the sensing chamber using compressed air or vacuuming around the outside of the sensing chamber with a shop vac. The alarm technician has been trained on the 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 V batteries yearly. Do not let them assume they have fire protection. Verify it. ClichAs often have truth behind their origins. Here is a truth to live by: "The life you save may be your own".
Acceptable Use Policy blog comments powered by Disqus
