Each candidate for promotion in the fire department who first has received a passing grade in a promotional examination, shall be entitled to a seniority credit to be added to officer's examination grade. The seniority credit shall be applied by adding one point for each year of active service the officer has had with the department to the total numerical score of correct answers in the examination; provided, that the seniority credit shall in no case exceed ten percent of the examination grade achieved by the officer.