No policeman employed by a city of the second class A shall be required to be on duty for more than forty hours in any calendar week, except in emergency cases for the suppression of riots or tumults or the preservation of the public peace in times of riot, conflagration or public celebrations. The existing salary or compensation of any policeman shall not be diminished because of the reduced number of hours of duty prescribed by this act.
53 P.S. § 30531