Maintenance as a main process?

Your business is the production of electricity.

Sure, your company needs preventive maintenance. Now, considering preventive maintenance a process or not is up to your organization. There is no technical answer, it is a management answer. Working with some organizations I advise to consider preventive maintenance as an autonomous support process, working with some other organizations I advise to consider preventive maintenance as an activity inside the production process. It is up to the organization to determine the processes needed for its QMS. You should consider the context of the organization and the application of risk-based thinking to determine the level to which processes need to be detailed.

thank you first for quick answer and please. For the choice o main or support process, can we proceed by the number of worker ( for us we have more worker in maintenance than production) also a relevant interested party, namely our tutelage, imposes on us as a main objective the availability of the power plant and not on the production of electricity.
so can we put maintenance as main process?
thank you

Yes, you can consider maintenance as a main process.