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Countdown Timers

CO₂, Halon, and Halon substitutes such as FM-200 are considered gas-based systems. All gas systems should use a countdown timer (both visible and audible) before gas is released. This is primarily for safety reasons, to allow personnel evacuation before release. A secondary effect is to allow personnel to stop the release in case of false alarm.

CO₂ cannot be breathed in high quantities and is deadly. While Halon and Halon replacements are designed to be breathed in normal concentrations, they are still more dangerous than water (minus electricity).

Note
Water is usually the recommended fire suppression agent. Water (in the absence of electricity) is the safest suppression agent for people.

Sprinkler Systems

All sprinkler systems should be combined with a fire alarm that alerts people to evacuate the premises in case of fire. Safe evacuation is the primary goal of fire safety.

Wet Pipe

Wet pipes have water right up to the sprinkler heads: the pipes are “wet.” The sprinkler head contains a metal (common in older sprinklers) or small glass bulb designed to melt or break at a specific temperature. Once that occurs, the sprinkler head opens and water flows. Each head will open independently as the trigger temperature is exceeded. Fig. 4.39 shows a bulb type sprinkler head.

Fig. 4.39 Bulb sprinkler head.

The bulbs come in different colors, which indicate the ceiling temperature that will trigger the bulb to burst and open the sprinkler head. The colors used are orange (135°F/57°C), red (155°F/68°C), yellow (175°F/79°C), green (200°F/93°C), and blue (286°F/141°C). NFPA 13: Standard for the Installation of Sprinkler Systems describes the color conventions used for these sprinkler heads. See https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=13.