A fire alarm was an alarm system wired throughout a building which would alert people inside that there was a fire in the building. Fire alarms typically had two methods of activation — manual switches placed throughout the building which people could pull in case of a fire, and sensors which would detect heat or smoke and set off the alarm automatically.
While running from the Master, the Eighth Doctor set off the fire alarm at the Institute for Technological Advancement and Research, to "liven things up". (TV: Doctor Who)
A student named Kenny set off the fire alarm at Deffry Vale High School to incapacitate the Krillitanes, who had sensitive hearing due to their bat-like forms. Unfortunately, this did not work for long as one of them merely punched through a wall and ripped out the power cables, disabling the alarm system. (TV: School Reunion)