16 Pa. Stat. § 4516
Promotions shall be based on merit. The county commissioners shall notify the commission of any vacancy in the police force or as firefighter or fire inspector which is to be filled by promotion and shall request the certification of a list of eligibles. The commission shall certify for each existing vacancy, from the eligible list, the names of three persons thereon, who have received the highest average in the last preceding promotional examination held within a period of two years next preceding the date of the request for such eligibles. The county commissioners shall thereupon, with sole reference to the merits and fitness of the candidates, make an appointment from the three names certified, unless they make objections to the commission as to one or more of the persons so certified. No member of the county police force or a firefighter or fire inspector shall be eligible to take any promotional examination until after serving three years in the county police force or as a firefighter or fire inspector for which such examination is held. Each member of the county police force or a firefighter or fire inspector shall have his examination mark or grade increased by an additional one-half point for each year he served on the county police force or as a firefighter or fire inspector in excess of five years but such additional points shall not exceed ten points. The board of commissioners shall have power to determine in each instance whether an increase in salary shall constitute a promotion.
16 P.S. § 4516