Preventive maintenance (PM) traditionally consists of performing "PM tasks" on a calendar basis (yearly, quarterly, monthly, etc.). These tasks typically involve making physical changes, regardless of equipment condition, which often means fixing, or even breaking, things that weren't broken. Condition-based maintenance improves on this. Instead of going through a litany of transformer reconditioning work, for example, you conduct an oil analysis and do only what's needed.

Accurately assessing equipment condition allows you to predict failure modes and times. Thus, predictive maintenance (PdM) allows you to schedule corrective interventions at a cost much less than reacting to unscheduled shutdowns. But calling your maintenance "predictive" because you do more measuring these days doesn't make it predictive. Consider insulation resistance testing in cabling. When you trend the readings, you can predict a cable failure by spotting that telltale slope change.

Sometimes, management tries to "save money" by increasing the testing intervals. This usually makes the tests forensic. You discover that the insulation leaks instead of seeing it will leak if not replaced soon.