Over the years, the typical facility accumulates the detritus of fast-track projects and various efforts to do more with less. Some misguided attempts at "efficiency" have produced gross inefficiencies, and the place is littered with "Easter eggs" that will eventually result in unplanned downtime. To head off failures (and possible catastrophe), you must make informed decisions on spending limited maintenance resources. You become informed by conducting the proper testing.
Few facilities have the expertise for this testing. However, firms with such expertise are available. How do you go about selecting such a firm and then selling the idea to the bean counters? You won't succeed by merely ordering a bunch of tests and hoping to find something. Instead, you need to make a business case for a specific program of testing.
Pick a major system (e.g., plant power distribution), and obtain the applicable standards. Among the organizations providing authoritative standards for electrical maintenance and testing are IEEE, NFPA, and NETA. Determine what gap exists between the needed tests and your capabilities for doing them. Now you know the delta and can look for a qualified testing firm.