Coffee Break Training: Automatic Sprinklers: Sprinklers Beneath Ducts and Other Obstructions [open pdf - 475KB]
"Control mode sprinklers are intended to prevent fire spread by cooling the atmosphere and wetting combustible materials so they do not ignite. (See Coffee Break Training FP-2007-4.) While sprinklers generally are installed at or near the ceiling, there are conditions, called 'obstructions,' that may prevent discharged water from reaching the intended target. In National Fire Protection Association 13 (NFPA), 'Standard for the Installation of Sprinkler Systems', obstructions are defined two ways:  Continuous Obstruction. An obstruction located at or below the level of sprinkler deflectors that affects the discharge pattern of two or more adjacent sprinklers.  Noncontinuous Obstruction. An obstruction at or below the level of the sprinkler deflector that affects the discharge pattern of a single sprinkler."
Coffee Break Training - Fire Protection Series No. FP-2013-10
United States Fire Administration: http://www.usfa.fema.gov/