The paper deals with two-unit cold standby Programmable Logic Controller (PLC) system with regular inspections, repairs and various types of maintenances by regular visiting repairmen at industry itself or at service station. From the literature, it is observed that in the past, considerations were depicted regarding the hypothetical industrial cases. It is felt that a practical industrial case may be taken up, in which practical side of the unit will be studied with utmost care and slightness. For this purpose, we have taken PLC as a unit. Here regular engineers are appointed; expert repairman can also be called upon, if required. Besides, PLC can be sent to authorized service station if required. To go deep into the matter, two types of maintenances—preventive and corrective maintenance—can be consulted. Also, various reliability parameters of a system are studied and plotted graphically.

Researchers like Yamashem (1980), Govil (1981), Murari and Goyal (1983), Attahiru and Zhao (1998) had worked on the reliability models which were hypothetical and the databases used in them were of abstract nature. Several others have assumed the values of failure and repair rates while making the graphical study, i.e., real data has not been taken for failure and repair rates. Further, Malik (1992), Tateja et al. (1992) and Naveen (2002) considered industrial databases for analysis of models. Duhan et al. (2004) used a case of biscuit industry in NCR region for modeling of PLCs located at various locations in the plant.

Since the plant is closed on Sundays and two hours daily at the lunch time, preventive maintenance can be done by the regular engineer. Whatever fault arises, it is firstly inspected by the regular engineer. If there is any possibility for him to get it repaired or replaced, he starts the corrective action, otherwise an expert repairman from the concerned industry is called upon. Further, there is possibility that the machine (PLC) may be sent to the authorized service station if not repaired at the plant site.

Electrical and Electronics Engineering Journal, Programmable Logic Controller (PLC), Laplace transform, Corrective maintenance, Preventive maintenance.