With the development of economy, the function, type, structure and application environment of architecture are becoming more and more complex. And grid ceilings are usually selected to place the ventilation ducts and electric cables of central air-conditioning system. However, due to the low fire resistance, when the ceiling is subjected to fire attack, the
supporting structure would collapse easily and combustible ceiling insulation material would be ignited, the flame will spread along the ventilation ducts so that the fire in the ceiling quickly spread. Since the grid ceilings have great fire risks, photoelectric smoke detectors must be installed in such places.