Many engineers work in buildings protected by fire alarm and sprinkler systems. These systems have little in common with a residential array of smoke detectors even if they are wired to operate in concert. For the curious, here is how large-scale alarm and sprinkler systems are typically setup and wired.
A commercial fire alarm system is supervised. This does not mean that there is a human at a central console monitoring it day and night. Instead, the supervisory function is automatic and electronic. The integrity of all lines, sensors, telephone ties, alarms and control equipment is monitored by means of control voltages. If one of them fluctuates, a trouble alert (not loud like a fire alarm) sounds. A description including location is reported in an alphanumeric readout that is part of the human interface.
A characteristic feature of any supervised fire alarm system is that it has a central control panel, sometimes with remote subpanels of limited functionality. There is an alphanumeric readout and keypad by which the user can set many parameters, access the system history, enter a password if required, etc.
The building or area served is divided into zones. A six-story building, for example, could have two zones per floor for a total of twelve zones. Throughout each zone are indicating devices, which include the familiar alarm heads visible in public places, as well as pull stations that can be activated by a person in the area.
Indicating devices take many other forms as well. The smoke alarm head looks a little like a residential smoke detector. Most models have a slow blinking LED, which indicates that the head is receiving power but is not conducting and thus not in alarm. An LED that is on steady, not blinking, indicates an alarm. Heads, typically arranged along a hallway or in grid pattern on the ceiling of a large room, are daisy chained, in other words, wired in parallel just like receptacles in an electrical branch circuit.
The fire alarm zone is a two-wire dc circuit, usually operating at 24 V, with a positive and a negative side, both sides isolated from ground. Red 16 AWG fire alarm cable is usually used, run inside electrical metallic tubing (EMT). This voltage originates in the control panel and powers the LEDs in the heads as well as supplying bias for the semiconductors in the heads. Some types of heads conduct across the dc lines when smoke enters the alarm chamber. False alarms can be set off by dust or flying particles in the air. Other types of heads, designed for maintenance areas where there may be dust, are sensitive only to heat.
In any event, when the indicating device senses fire, the control panel recognizes that fact by sensing the increased current in the line. Another role played by the 24-V dc voltage is to facilitate the supervisory function of the alarm system. The control panel at all times monitors the voltages at all zones as well as its own internal circuitry. A fault to ground, line-to-line short or open circuit will initiate a trouble alarm, signified by a buzzer at the control panel and message in the alphanumeric display.
You may wonder how the control panel is able to differentiate between the normal state, when there is no fire alarm, and an open circuit. The answer is by means of an ingenious device known as an end-of-line resistor. This resistor is placed after the final indicating device. Typically 1.5K Ω, its resistance is read by the control panel. The resistance dropping to zero is an indication there is an open circuit, and a trouble alert is generated.
Large commercial fire alarm systems also interact with other systems inside and outside of the building. To begin with a common example, an emergency system in alarm will also instantly slam shut fire doors, in a building so equipped. The mechanism is simple. The doors are spring-loaded to close. Normally they are held open by an electromagnet mounted on the wall. When the fire alarm control panel enters a state of alarm, the dc power to the door magnets throughout the building is interrupted, allowing the doors to swing shut. They can easily be opened manually so that occupants are not trapped.
Fire doors are made of metal with foam cores and have no glass windows. They are rated in minutes or hours to indicate how long they will withstand fire at a specified temperature.
Another building system that interacts with the fire alarms is the sprinkler array. In a dry system, if the air pressure drops below a predetermined level, the fire alarm enters the trouble state. A water-flow sensor will trigger an alarm state when, due to a ruptured head or broken pipe, the main water valve opens. In this sense, every sprinkler head is an initiating device, so the alarm system coverage is greatly extended.
The alarm system also interacts with telephones. Entering the alarm state, the control panel automatically dials predetermined telephone numbers that have been programmed in the control panel. These numbers can be for fire department, emergency call center, monitoring station or building managers. For reliability, there are usually two separate telephone lines. The system often places test phone calls monthly. If the call does not complete, the alarm panel goes into the trouble state and the alphanumeric readout alerts maintenance workers that the telephone line is faulty.
A yet more elaborate fire alarm/building system interaction is with elevators. The entire subject is called Firefighter Emergency Operations. These operations are divided into two separate phases. The intent is, in the event of fire, to remove control and accessibility of elevators from the public and to turn it over to firefighters as they arrive on the scene.
Elevator service is often essential for firefighters who approach the floor that is the scene of the fire and move in heavy equipment. To this end, when a sensor detects smoke in the lobby, hoistway or elevator machine room, it triggers Phase 1. Elevators are removed from service and building occupants are prevented from using them. The elevator automatically returns to the ground floor or, if that is the location of the fire, proceeds to a previously designated alternate floor. This protocol is known as Elevator Recall. With doors open, the car remains in place and does not respond to call buttons on other floors or within the car.
When firefighters arrive on the scene, they may need the elevator system to suppress the fire. Accordingly, they insert a firefighter’s key into a lock marked with a firefighter’s hat icon. This initiates Phase 2. The firefighters can now operate the car manually. They know not to open a door that is hot to the touch. To do so would expose them to a fiery blast. Instead, they travel with necessary heavy equipment to the floor just below the site of the fire.
The fire alarm system can also interact with other building systems, for example, by shutting off the gas supply and by taking other actions that could save human lives and protect the building.
Alarm systems also interact with building sprinkler systems. Depending upon a building’s interior finish, the only visible parts of a sprinkler system are the heads or perhaps some of the zone piping. The uninformed view is that these simply connect to the potable cold water supply. In actuality the total system is far more complex. The installed cost of a sprinkler system for a small commercial building such as an 800 ft2 restaurant can go over $100,000. The sprinkler system must be supplied by a large diameter steel pipe, often six-inch, connected to the water main, a dedicated reservoir or an elevated tank. The building’s potable cold water supply would not provide enough water flow for effective fire suppression.
There are two markedly different type of sprinkler systems – wet and dry. The dry system is far more complex and costly. In a wet system, the heads simply connect to high-capacity water distribution piping. When a sprinkler head ruptures, water floods the area served by the sprinkler. If fire should spread beyond the protected area, another head will rupture. In a well-designed installation, the protected areas overlap so that fire protection is seamless.
The dry system is necessary where there is a possibility of freezing, as in an unheated storage area or under the ceiling of an open porch. Until there is a fire, the zone pipes and heads of a dry system are filled with air, typically pressurized to 55 psi. If the air pressure drops to a preset level, usually 38 psi, a large spring-loaded rubber flipper in the valve body swings open, pushed by the higher water pressure on the upstream side, whereupon water fills the pipe and flows through the ruptured head.
The dry system is more complex, expensive and prone to false alarm. If the dry-side air pressure falls due to a long-term slow leak, the system will flood, although there will not be major water damage where the heads remain intact. Then the system has to be drained and the zone valve reset.
If condensation accumulates, the dry side may be subject to freezing, so it is typically disabled and blown dry as needed. Another disadvantage in the dry system is that there is a certain latency between when the head ruptures and the air pressure drops sufficiently for the water valve to open, until the time that the water reaches the ruptured head and actually quenches the fire.