fire alarm vs warning bell

fire alarm

noun
  • An electromechanical or electronic bell, klaxon, chime, horn, speaker, strobe light or other device which warns people in a building of a possible fire or other condition requiring evacuation. 

  • The sound or other warning made by a fire alarm. 

  • An emergency response to a fire by the fire department with manpower and equipment. 

warning bell

noun
  • A bell rung to warn people of a danger. 

  • A problematic or alarming aspect. 

How often have the words fire alarm and warning bell occurred in a corpus of books? (source: Google Ngram Viewer )