agent and to allow for manual interruption of the agent release sequence. If the situation is not dangerous, the sequence can be interrupted by a manual abort switch. When the detectors and control unit have been restored to their normal condition, the abort switch can be restored. The abort switch is usually designed to be held in (until the control panel is reset) so that the agent discharge cannot be accidentally impaired when the switch is unattended.

Initiating Devices

Frequently used detectors for gaseous agents are spot-type ionization smoke detectors and rate- compensated heat detectors. Factors affecting detector effectiveness, such as electrical power and air pressure, if pertinent, are supervised. One or two manual methods for release of the gaseous agent are usually provided.

Manual fire alarm devices are frequently connected to the alarm system control unit to cause immediate discharge of the gaseous agent, regardless of cross-zoning and time delays other- wise provided.

Manual devices may also be connected electrically to cause direct release of the agent, independent of the alarm system.

Direct mechanical release of agent maybe by manual actuation of a control valve.

Whether the agent release is caused by an alarm control unit auxiliary output or by an independent manual method, there should be an alarm at the alarm system control unit. Manual release of the gaseous agent usually causes an alarm by actuating a pressure switch that senses the increase in pressure in the gas line or manifold between the release valve(s) and the nozzles.

Sequence of Alarms

The normal circuit arrangement for a building alarm system to release a gaseous extinguishing agent is the same as for a building system with added features such as cross-zoning, the abort feature, manual release of agent, and other specific auxiliary functions of the alarm system. Alarm systems that release a gaseous extinguishing agent use auxiliary alarm outputs to segregate the protected area and reduce dispersion and dilution of the agent. Typical auxiliary functions are fan shutdown, door (and window) closure, and closure of air-handling system dampers. Gaseous agent-releasing alarm systems applied to computer room installations also shut down computer power at the time the agent is released to eliminate the heat source for possible electrical fires.

A typical sequence of alarm system-initiated events in a computer room installation that includes all the usual features is as follows:

Detection of fire by first detector in an area causes local and remote alarm indication, fan shutdown, door and damper closure, and other miscellaneous auxiliary functions through inter- locks with building systems.

Detection of fire by second detector in the area (cross-zoned with first detector) causes a distinctive local audible signal and initiates a time delay during which agent release and computer power shutdown may be aborted.

At the end of an adjustable delay (normally 20 seconds), assuming the release is not aborted, computer power is shut down and the extinguishing agent is released into the protected area.

INSPECTION, TESTING, AND MAINTENANCE OF GASEOUS SYSTEMS

Inspection, testing, and maintenance of gaseous fire extinguishing systems are required to be sure they are in proper operating order. Inspection and test frequencies for these systems are summarized in table 8-4.

Carbon Dioxide High-Pressure Systems

Check hoses and nozzles, cylinders, and cylinder pressure as follows:

Weekly, check that all nozzles and hand hose lines are clear and in the proper position and that all operating controls are properly set.

Semiannually, weigh cylinders and replace any that show a weight loss of greater than 10 percent. To weigh cylinders, do the following:

Loosen each cylinder support and disconnect each discharge head. Discharge heads are designed to be removed and replaced without tools.