In this paper we investigate the reliability of shared protection systems such as one for N (1:N) that can be applied to access network node devices. We describe the effectiveness of such a protection system in a quantitative manner. The mathematical analysis gives closed form solutions of the availability and MTTF (Mean Time to Failure) perceived by an arbitrary end user. We also describe the effectiveness of priority switching control in shared protection systems. These analyses provide some useful information for the design and control of not only the access network node devices but also other general shared protection systems that are subject to service level agreements (SLA) involving reliability measures.