A Question on the Use of a Risk Matrix to Match Maintenance Strategy Mix to Risk Level


Hello Mister Mike, I hope that you are well

I am interested in the two matrices you provided in your book entitled “Plant and Equipment Wellness”, the first one is related to criticality levels, while the second one attributes each criticality level to a maintenance strategy. I have noticed that a maintenance strategy mix can reduce risk in two different levels, for example: Preventive Maintenance occupied two distinct levels (low and medium).

Knowing that from my literature review each criticality level is attributed to a specific maintenance strategy, I would like to understand more about the reasoning behind your classification.


Best regards,

PhD Student, Industrial Risk Assessment

Dear NEHK,

Thank you for your interest in the Plant and Equipment Wellness concepts. Since the Plant and Equipment Wellness book was written the methodology has matured. It is now called the Plant Wellness Way (PWW). It is used in the Plant Wellness Way Enterprise Asset Management (PWWEAM) methodology, as described in the book titled, Industrial and Manufacturing Wellness (IMW). Industrial Press is its publisher.

The risk matrix shown in your email was generated from the requirements of ISO 31000 Risk Management Guidelines. Every operation needs to develop and calibrate their own risk matrix specific to their situation and circumstances. When the situation or circumstances change, so too must their risk matrix be recalibrated for it to truly represent their current reality and the consequential risks.

The table that allocates various maintenance strategy mix to risk levels derives from the core philosophy behind Plant Wellness Way EAM methodology. PWWEAM philosophy requires pre-emptively eliminating the root causes of equipment failure, as justified by the economics of the situation and circumstances.

When an operating asset failure event will produce Extreme risk, which, if it occurs, has massive economic losses beyond the capacity of the operation to comfortably absorb, it is unacceptable that the event happens. Risk elimination for such scenarios requires a comprehensive re-engineering of the asset to design-out the possibility of the failure cause arising.

If High risk results from an equipment failure, the PWWEAM aim of asset lifetime health and wellness leads to preferentially adopting failure cause design-out, and where that is not done, then to adopt the entire Precision Maintenance (PrM) strategy for component stress minimisation. When component stress is minimised its failure-free service life, i.e., its reliability, is maximised.

Because Precision Maintenance stress reduction practices minimize stress in components and thereby maximizes parts’ reliability, it is universally applicable for reducing operational asset risks. Hence, in the maintenance strategy mix allocation table PrM is preferentially used everywhere for all risk levels—it is a reliability strategy that works in all places, on all physical assets.

But most operations limit themselves to using a mix of Breakdown Maintenance (BM), Preventive Maintenance (PM/PvM), and Predictive Maintenance (PdM), also called Condition Monitoring (CM) to address their operating assets’ risks. Those strategies can be useful when used in scenarios where the consequences of failure have Low and Medium risks.

If the loss from equipment failure is small, and the event rarely occurs, then BM strategy may be justifiable. Though not the best choice, as PrM would reduce the likelihood of failure much lower.

Preventive Maintenance is defined as the replacement of equipment parts before they fail. It is a failure prevention strategy, not a failure cause elimination strategy, like PWW is. When used properly, preventive maintenance effectively reduces frequency of breakdowns because parts are renewed before they fail. Failure is not meant to happen with PvM strategy. But evidence from around the world shows that breakdowns still occur even when PvM is used. Because it is an untrustworthy strategy it is limited to Low-risk scenarios.

Predictive Maintenance monitors the condition of components in equipment. By observing the changes in the state of equipment parts it aims to detect the start of future failures. Knowing a part is on the way to failure allows rectification work to be well planned and prepared for the least maintenance cost and disruption to the operation. The ability to forecast failure through condition monitoring makes PdM a useful strategy for medium-risk scenarios. Though again, evidence from around the world shows that breakdowns still occur even when PdM is used. So PdM used alone is not a comprehensive solution to protect against equipment failure.

The first diagram at the top of this article is a risk matrix overlayed with maintenance strategy options. It aims to show that in PWWEAM a maintenance strategy mix is intentionally used to get lower operational asset risks. When selected based on the risk level of each failure, different maintenance strategies, PvM, PdM, Precision Maintenance, design change, and even breakdown maintenance, complement each other.

PWWEAM permits use of any maintenance strategy mix that is effective at removing the causes of failure, while being economically acceptable to the operation. PWWEAM is not limited only to the use of maintenance strategies to maximize reliability. It considers the whole asset life cycle and allows the use of every means in all phases of the life cycle to eliminate the true root causes of physical asset failure. Maintenance strategies make a small part of the PWWEAM continuum that organisations can use to get world class asset reliability.

The drawback of BM, PvM, and PdM strategies is they allow failure causes to exist. They accept equipment failures as allowable events and not deviant events. They do not require you to act to prevent the root causes. Whereas, in PWW, failure is abhorrent, and its causes are proactively eliminated from everywhere in the life cycle. PWWEAM is a shift in physical asset owners and users thinking and philosophy. Those Operations Managers, Asset Managers, and Maintenance Managers who adopt PWWEAM expect ZERO FAILURE. Its achievement is by any means possible throughout the lifecycle that ensures the continual health and wellness of an equipment item all its service life.

You are welcome to make contact by email if you have any questions on the above.

All the best to you,

Mike Sondalini
Plant Wellness Way EAM System-of-Reliability
23 September 2022