The employment of the inmates of any institution within the jurisdiction of the state board shall be subject to the following specific limitations:
a. Marching or transportation of convicts in irons through the public streets or to or from their places of assigned labor shall not be permitted except in case of absolute emergency;
b. Convict labor under armed guard shall not be used on public improvements in conjunction with free labor;
c. Convict labor shall not be used to take the place of free labor locked out or on strike.