"For some unknown reason, the sprinklers in the photograph were installed within just a few inches of one another. While it may seem like this is providing an extra amount of protection, the installation does not comply with the requirements of NFPA [National Fire Protection Association] 13, 'Standard for the Installation of Automatic Sprinklers'. When a sprinkler system is designed, sprinkler placement is selected based on the sprinkler type, the occupancy hazard classification and potential obstructions to heat collection or water discharge. NFPA 13 and the sprinklers' listings provide strict rules for the sprinkler installer to follow. In this case, the problem is the sprinklers are installed too closely to one another. If one sprinkler were to operate, the water spray from it likely would fall upon the other sprinkler and prevent its heat-sensitive element from operating. This is a condition known in the fire protection industry as 'cold soldering.'"
Coffee Break Training - Fire Protection Series No. 2008-45
United States Fire Administration: http://www.usfa.fema.gov/