How Hard Wired Smoke Detectors Work

A smoke detector is an important safety device in homes, offices and other enclosed structures. It can save lives especially when fires occur during the most unexpected times. Thousands of people are killed every year as a result of suffocation from thick smoke caused by fires.

Smoke detectors in homes can either be the hard wired or the battery-powered units. Most states, however, require the installation of hard wired smoke alarms which are more reliable. This type is powered by 120-volt electricity in the home and is interconnected with other smoke detector units in the house. They are normally wired into the electrical system of the home. Most homes install several units with each room and hallways having a separate alarm to make sure that all members of the family are warned in case a fire occurs.

Hard wired smoke alarms also have a battery backup. This is its added feature which ensures continuous safety for the occupants of a home even when there’s power outage. But homeowners should make sure their units have batteries that work. Batteries need to be replaced at once every year.

Wiring the detector

Smoke detectors can be wired either in the four-way or two-way bus. The two-wire bus is the latest method with two special terminals available in the security panel for attaching the detector. All detectors in this arrangement are wired parallel to the two-wire bus with the last unit getting a so-called end-of-line resistor. From the security panel, current flows down to the two wires. It is also the security panel that checks if any of the detectors have tripped off.

In the four-way bus arrangement, two wires are attached to the resistor while the other two carry DC voltage to the smoke detectors. Except for the last unit in the chain, each detector will have eight wires attached to it. The smoke alarm units in this type of arrangement are open switches and there’s possibility that power can get lost anywhere in the wiring chain.

Basic types

Smoke detectors are generally of two types – the ionization and photoelectric. Each has its own advantages and disadvantages. The ionization smoke alarms are the most commonly used. They are fast in detecting flaming fire that moves quickly and are cheaper in price. This type consists of an ionization chamber and a source of ionizing radiation usually a radioactive americium. The photoelectric types are not that efficient in detecting thin smoke which means that it may take some time before it could send off alarms. These are also bigger in size compared to the ionization units but have a longer lifespan and low maintenance requirement. In this type of detector, a light beam is used such that when the light is blocked by smoke, the alarm sounds off. In other types, light is spread by smoke particles to a photocell which results in the sounding of the alarm.

Also important to take into account is the relative humidity and temperature in the area where the smoke detectors are installed. Smoke alarm units with higher humidity and temperature ranges are ideal to prevent false alarms.