Multiple-alarm fire
Fires are sometimes categorized as one-alarm,
two-alarm, three-alarm fires, or higher. The number of
alarms correlates with the level of response by local authorities,
with an elevated number of alarms indicating increased commitment of
resources. The term multiple-alarm is a quick way of
indicating that a fire was severe and difficult to contain. This
system of classification is common among both fire departments and
consequently news agencies
The initial dispatch is referred to as the first alarm and
is typically the largest. Subsequent alarms are calls for additional
units as needed, usually because the fire has grown and additional
resources are needed to combat it or the incident is taking long
enough that firefighters on scene need to be replaced due to
exhaustion. The number of alarms doesn't necessarily indicate the
size or the severity of the actual blaze so much as the size of the
incident and how long and hard the fire department had to
work to control it.
The units dispatched on the first and subsequent alarms
depends on what resources are available in the area and so changes
from department to department. Requests for units from outside
jurisdictions normally don't occur in urban areas until elevated
alarms are reached (third and above), but depends on the location of
the incident and the condition of the authority having jurisdiction
at the time of the incident.
History
The system of classification comes from the old tradition
of using pull stations to alert the local departments to a fire in
their area. The "box" would send a message to all local stations by
telegraph that there was a fire, indicating the location as a
number: (station area) - (box number), e.g. 11-2. Fires are still
dispatched as "box alarms," following this tradition, with maps
broken up into a grid of "box areas."
In some systems the maximum alarm is a five alarm fire, which has prompted chili cooks to name their most incendiary concoctions "5 Alarm Chili".