"Spot-type heat detectors-those that are installed on a specific ceiling location and protect an area of listed spacing- may not be suitable for all applications. For example, if an electric utility wanted detection in its cable trays, or a wood processor on its conveyor, or even a bulk flammable liquid storage operator around its tanks and valves, spot detectors would not be suitable. In those cases, and similar unique situations, a line-type heat detector may be an appropriate selection. Line-type heat detectors are installed in a looped, continuous configuration from the fire alarm control panel, throughout the protected asset or premises, and back to the fire alarm control panel."
Coffee Break Training - Fire Protection Series No. FP-2012-28
United States Fire Administration: http://www.usfa.fema.gov/