For long-term operation of the security alarm system of a critical infrastructure facility, it is important to maintain alarm systems with seismic sensors installed in controlled areas of its perimeter. The task of forming a maintenance strategy requires reliable data on the required number of backup alarm systems for a given duration of operation and the permissible value of the duration of the process of replacing a faulty alarm system. The article shows a stochastic model of the operational reliable behavior of a fragment of the security alarm complex with the installation of five alarm systems with seismic sensors in the controlled area of the perimeter. The configuration of five alarm systems with seismic sensors installed on the controlled areas of the perimeter provides the use of the majority principle with the rule {3 out of 5} to decide on the type of intruder. At the same time, the majority principle provides an effective way to detect an inoperable alarm system with seismic sensors, which is important for the maintenance of the security alarm system. Such a model is necessary for choosing an appropriate option for the maintenance strategy of the security alarm system, which will provide a given value of its reliability for long-term operation