We propose in this work, a new approach based on a probability study to choose the standby Programmable Logic Controller (PLC). Indeed, the solution of standby PLC was adopted by the majority of the industrial companies faced to automated production management and scheduling. This solution proved its efficiency by guaranteeing the continuity of functioning of the equipment of production managed by PLC; the latter can be automatically replaced by another in the case of failure. Unfortunately, this solution has shown its limits. Several anomalies have been identified by the automation specialists. The most known are the not starting up of the standby PLC for various causes. The new developed approach allows not only incrementing the number of standby PLC but also choosing the best one in the replacement task. This choice is based on a new probability approach. Simulations are performed through T.I.A Portal platform.