Fire Alarm Systems
By David Herres
Fire alarm systems are at least an order of magnitude more challenging to understand than homeowner-type smoke alarms, even if individual smoke alarms within the home are wired together to work in concert. Fire alarm work must not be taken for granted because if a system should fail to respond to a smoke or heat event, humans could perish in a fiery inferno. The recent fire at a 24-story high-rise apartment building in London demonstrates how quickly a fire can spread- and its deadly consequences. It is a little less dramatic, but the possibility of false alarm also presents hazards that should be recognized. Recurring false alarms make for complacency so that the occupants may disregard the real one and also divert fire department resources from real emergencies. To ensure their reliability, fire alarm systems must be installed and maintained by trained technicians.
The defining element in a fire alarm system is the control panel.
The fire alarm control panel is the heart of the system, where technicians program and interact with initiating devices and indicating appliances as required.
If the control panel is located in an office or maintenance center, building management can quickly respond to alarm or trouble signals, but additional annunciator panels may be required in location(s) accessible to emergency responders. The purpose of an annunciator panel is to allow the emergency responders to quickly identify the location of the fire and provide information on the status of any interconnected fire safety systems.
The control panel contains a central processing unit, an alphanumeric readout that displays the system status (normal, trouble, or alarm), and responds to queries and programming instructions entered on a keypad. The control panel also contains back-up batteries, terminals for connecting conductors to remote devices, etc. This control panel can range in size from a small wall-mounted box set at eye level, to a heavy floor-to-ceiling steel enclosure for large occupancies with many zones.
The control panel is a highly intelligent unit with lots of functionality. A large part of its job is to coordinate interaction with other systems inside and beyond the confines of the building. The alarm system interacts with elevators so that occupants are not brought to a floor that is impacted and firefighters arriving at the scene can manually control the lift from inside the car. Additionally, the control panel places the system in alarm if the sprinkler system is activated. In essence, every sprinkler head is an alarm initiating device.
Where the fire alarm system is monitored by a central supervising station, if the system goes into alarm, a call is automatically placed to the designated supervisory station. Typically, two dedicated phone lines are required for monitoring and alarm transmission. The control panel makes test calls periodically to check the phone lines and goes into the trouble state, indicated by an audible signal and alphanumeric report at the control panel, if either of these lines is down. Where fire alarm systems are not monitored by a supervising station, a sign is typically required at each manual fire alarm box that reads, WHEN ALARM SOUNDS-CALL FIRE DEPARTMENT.
The remote elements of the fire alarm system fall into two major categories:
- Initiating devices, including alarm heads and duct sensors that, detecting the presence of heat or smoke, perform the function of placing the system into alarm status. Fire alarm heads and pull stations are initiating devices that automatically or manually report to the control panel, which responds by placing the system in the alarm state.
- Indicating appliances, including horns, strobe lights, chimes and bells, serve to alert occupants and building personnel to the presence of fire.
These sensors and loads are wired in parallel, daisy-chain style, just like feed-through receptacles on a branch circuit, with each conductor terminated to a separate terminal on the device. Neither line is grounded, although for line integrity and ease of maintenance, it is often recommended that all zone wiring be run in electrical metallic tubing (EMT), which serves as an equipment-grounding conductor and provides physical protection. Alarm heads are designed to conduct only when fire is detected, placing the system into alarm status. How, you may wonder, can the system differentiate between normal operation and an open circuit? The answer is by means of an end-of-line resistor, typically 4.7K ohms, which is continuously read by the control panel when the line is intact.
Regulatory statutes for fire alarm work differ widely. Some states mandate one or more levels of licensing for fire alarm professionals, while others make no mention of this type of work. New Hampshire falls into neither category with an optional licensing program that will become mandatory down the road. In all cases the local authority having jurisdiction or local fire department should be contacted if a fire alarm system is to be installed, modified, or removed from operation. Installation requirements for fire alarm systems are found in several documents:
NFPA 101 Life Safety Code, the ICC International Building Code, and the ICC International Fire Code specify which occupancies are required to have fire alarm systems. NFPA 72 National Fire Alarm and Signaling Code lays out system design requirements, including location and spacing of heads and pull stations, testing and maintenance procedures, minimum performance requirements and operational protocols.
NFPA 70 National Electric Code in Article 760, Fire Alarm Systems, specifies wiring requirements, both for power to the control console and zone wiring to initiating devices and to indicating appliances and phone lines used for automatic calling. Other fire alarm functions including guard’s tour, sprinkler water flow, elevator capture and shutdown, door release, smoke doors and damper control, fire doors and fan shutdown are covered by Article 760 where these functions are controlled and powered by the fire alarm system.