It’s been common for a long time that manufacturing plants and other facilities contract out some part of their maintenance work. HVAC systems, in particular, have been under this model due to the qualification requirements. Many types of electrical testing are routinely outsourced, most notably the infrastructure-related testing during the annual shutdown for that purpose.
Another practice is that of having a small core of “in-house” maintenance, whose primary job is actually downtime response and repair. They get tapped for maintenance work orders when they’re not busy fixing things. They supplement the contractor maintenance team, rather than being supplemented by it.
How much is contracted out versus how much is done in-house depends upon many things, all of which differ from plant to plant. Contractors generally provide lower cost and flexible staffing. You might occasionally need a specialist, but don’t have the resources to have your own on staff. The contractor has that specialist, and by amortizing the cost of that person over several facilities, that person is available for a reasonable cost to all of those facilities.
As great as the contractor solution is, it does have its risks and challenges for both parties.