What Does Hold Time Mean?
Within the context of fire safety, the term “hold time” refers to the amount of time that a firefighting agent will remain within a defined space after being intentionally emitted by a fire suppression system. An understanding of the hold time of a particular suppressant system is important to ensure that the system will be able to effectively extinguish fires within the area that is designed to protect.
Hold time may refer to any fire agent that is deployed as part of a flooding suppression system; however, it usually refers to a gaseous suppressant. Flooding suppression systems, also called total flooding systems, are used for situations in which the ideal manner of extinguishing a fire is to completely smother the fire with a suppressant agent.
Safeopedia Explains Hold Time
The hold time associated with a suppression system depends on the nature of the space that the suppressant is being released within, as well as the nature of the suppressant itself. Typically, flooding systems are used to suppress fires within an enclosed space which has been deliberately designed to ensure a hold time that is lengthy enough to suppress whatever fires could potentially occur within that space.
The necessary hold time for a given space depends on the source of the fire; for instance, spaces that contain objects which are at risk of reigniting after the initial fire has been suppressed will require longer hold times so that those objects have time to cool to the point where reignition is prevented. A general “rule of thumb” hold time for ensuring that fires have been suppressed within a given space is ten minutes.
There is no single ideal method for calculating hold time within a given space. Gaseous fire suppression systems are used in a number of different workplace environments, including museums, banks, telecommunications centers, clean rooms and hospitals. Due to the significant difference in how spaces within these locations are constructed—which include differences in their ventilation, interior contents, and other variables—it is often difficult to predict an accurate hold time. Both the NFPA and ISO have released standards for calculating a predicted hold time for a given space; however, the predictions which result from those calculations often differ significantly from each other.