Preventive maintenance focuses on preventing failures that are expected as part of the asset lifecycle. Preventive maintenance aims to attack potential failures before they turn into full functional failures by servicing or restoring equipment, or prevents failures by discarding worn out equipment before it fails. Preventive maintenance usually takes place on a time, run-time or number of cycles basis (every six months or every 6000 hours, every x revolutions, etc.) and uses some combination of experience, Original Equipment Manufacturer (OEM) recommendations and equipment history to assess (or guess) the best interval for preventive maintenance. It can be difficult to determine the effectiveness of preventive maintenance programs without allowing equipment to periodically fail. This means that in practice preventive maintenance programs often over-maintain equipment or underestimate equipment useful life in the interests of preventing potential failures. These two tasks – servicing and discarding equipment – may actually refer to the same task. For example, to maintain the function of a belt-driven pump, one might need to periodically replace the belt. In practice, this means discarding the belt at the end of its useful life. While this mode of maintenance is valuable, especially for assets that degrade at predictable rates, it can lead to unnecessary, premature or even counterproductive maintenance activities. Maintenance may take place before there is any evidence of failure, based on expected lifespan of assets or components rather than actual condition. Each scheduled maintenance overhaul or activity runs the risk of introducing human error or new functional failures into the system. As long as facilities feel comfortable with the level of spending they are incurring, and as long as failures are few and far between, preventive maintenance programs are presumed to “be working,” but this does not mean there is not room for improvement by reducing unnecessary maintenance further.