Everybody knows that you need to maintain your stuff to make sure it is in top condition; a colloquial way of saying that maintenance is a necessary component of any highly reliable system. In practice, there are three quite common approaches to maintenance of technical systems:

• If it ain’t broke don’t fix it
• Periodic inspection and maintenance
• Risk based maintenance

The first policy is the most commonly applied in private life. The only reason this is at all possible is a combination of high labor cost, relatively cheap mass production of many things, and that insurance companies allow you to do it by paying for stuff that “accidentally” breaks. For reliability, this policy is clearly the least reasonable, and it will negatively affect the reliability of your system. The following picture shows an example of the degradation that may occur when following this policy – taken from a walk in the Danish countryside.

The second policy is maybe the most commonly applied in industry – and for conscious car owners. You take your car to the mechanic once per year, or perhaps every 25000 km, for oil change and a check-up. You test the most important things, and follow up if necessary. This ensures your reliability is not negatively affected by lack of maintenance – but still the work has to be performed with high quality. Maintenance done right reduces probability of severe failures, done wrong it is outright dangerous.

Risk based maintenance uses available data to judge the risk level for your equipment – by using a combination of information gained from inspections and direct measurements. When you actively monitor the risk of failures and plan maintenance activities in accordance with real risks, you can prolong the useful life of your system while maintaining the necessary reliability.

When assessing the reliability of critical systems you should thus include both the philosophy of maintenance and the ability of the organization to execute the maintenance plan in a good way in your assessment. A simple illustration of this is shown in the below picture.