How security alarm smoke detectors work

AlarmTech - How smoke detectors work

Smoke Detectors

How do smoke detectors work?

More and more home owners are putting fire protection in with their home security systems. In addition to getting insurance discounts, the main reason is to protect the family and home in the event of a fire.

There are 2 basic types of smoke detectors, photoelectric and ionization. Photoelectric detectors use a light source, usually IR, and a photoelectric detector. These elements are arraigned inside a chamber so that under normal conditions the photoelectric detector does not "see" the light emitted by the light source. When large smoke particles enter the chamber they reflect the light on the photoelectric detector which in turn causes the alarm to activate.
Ionization detectors use a small amount of radioactive material located within a chamber that contain positive and negative electrodes. Under normal conditions the radioactive material allows electrons to flow inside the chamber, but when small smoke particulates enter the chamber they interfere with this flow, which in turn causes the alarm to activate.

Due to the basic design differences between the two detectors, photoelectric detectors are more sensitive to slow burning, low energy, high smoke fires - such as burning chair cushions - while ionization detectors are more sensitive to fast burning fires from gasoline and other similar fuels.

How to wire smoke detectors:

Smoke detectors can be wired in either a 2- or 4-wire configuration. In the 2-wire configuration, these detectors use the same 2 wires that power them to detect an alarm. The 4-wire version uses 2 of the four wires for power and the other 2 for alarm. Which wiring configuration that you or your alarm company uses will depend largely on the alarm control panel being used. If the panel supports the 2-wire configuration, then use 2-wire smoke detectors because they are easier to install and require smaller, less expensive cable.
When wiring smoke detectors it is important to fully supervise all wiring and connections, since the alarm contacts of the detector are normally open (short on alarm). In normally-open systems, a broken wire or loose connection is not detected unless you are using some form of loop supervision - like an end-of-line (EOL) resistor.

End-Of-Line resistors:

An EOL loop has a resistor located after the last detector, which causes a small amount of current to flow. The control panel monitors the loop and will alert you if the amount of current decreases (broken wire) or increases (smoke detector trip) beyond a preset level. The alarm control panel should also be programmed so a broken wire causes a trouble condition while an activated smoke detector causes an alarm. This is done to prevent false alarms caused by wiring problems.
When connecting the wires to the detector you should cut the wire and place only 1 wire under each screw terminal. If you simply strip back the insulation and "loop" the wire around the screw, the system may not be able to alert you if one of the wires come off of a terminal screw.

When using 4-wire smoke detectors, you should also install a power monitor after the last detector on each separate loop. This insures that the detectors are receiving power. This monitor is basically a relay which has its coil connected to the power wires (keeping it energized) and its contacts connected in series with the detection loop, so that if the power is removed the relay will drop out causing an open (trouble condition).

Smoke detector zones can be programmed to activate the alarm as soon as any smoke detector activates or only if more than 1 trip is detected (Alarm Verification). Verification is used to minimize false alarms and works as follows:

Smoke detector trips
The control panel automatically resets the detector
Only if a second smoke detector trip occurs within a short time, typically 60 seconds, or if the detector fails to reset, an alarm is activated

Where should they be located?

Smoke detectors should be located on the ceiling at least 4 inches from any walls, or when wall mounting they should be installed between 4 and 12 inches down from the ceiling line to the top of the detector. You should install detectors outside every separate sleeping area, on every level of the home (including basement), and inside each bedroom. You should avoid placing smoke detectors in kitchens, garages, and attics to prevent false alarms. Rate-of-rise and fixed-heat detectors should also be installed in these locations.

Powering Smoke detectors:

Smoke detectors can be powered by an internal replaceable battery, standard house 110 VAC, standard house 110 VAC with battery backup, or from a central security control panel.
If you use battery powered detectors make sure that you replace the batteries every year. Set a fixed date, Thanksgiving, New Years, etc. and replace every battery each year. If the units are powered by standard house 110 with a battery backup, those batteries must also be replaced, check the documentation that comes with the unit for the suggested replacement frequency.

Fire alarm sounders:

Since the primary purpose of a fire alarm is to save lives, you must make sure that you will be able to hear the sounders no matter where you are or what you are doing. Pay particular attention to sleeping areas- must be loud enough to wake you and your family up, workshops- where noise from tools etc. could drown out the sounders, and bathrooms- with the door closed, taking a shower, playing radio/singing.
Make sure that you know the difference between the burglary sound and the fire sound, voice sirens that "speak" in plain language eliminate this concern. Many smoke detectors have a built in horn to alert you in case of a fire, some detectors that connect to standard house 110 VAC can also be wired so that if any of the detectors is activated the horns in every unit will sound. This feature is especially valuable in multistory or large homes.

Testing smoke detectors:

You should test your smoke detectors every month. Some units have a test button, some allow you to shine a light on them, and others require that you blow smoke into them. Read the documentation that comes with your unit to determine which is the correct method for your detector. If the unit has a LED it may flash at a specific frequency to show that the unit is functioning and at a different rate or not at all if there is a problem.
Again read the documentation and periodically look at the LED. If your system is monitored make sure that you notify your security company before you begin testing.

Cleaning smoke detectors:

Like testing procedures how you should clean your detector will vary, so check the documentation. It is a good idea to clean the outside of the detector with your vacuum cleaner when you are doing your floors. This will prevent dust from clogging up the chamber and also remove any debris that may accumulate on the unit.
For additional information you can contact your security company, insurance company, or local fire department.