There are two reasons why preventive maintenance (PM) procedures should be well-designed work procedures.
First, let’s consider the impact human error can have on an organization. The table below shows the probability of human error. According to the chart, a person working independently, under stress, and without a well-defined work procedure is 30,000 times more likely to commit an error than someone who is working with a well-designed procedure and with a team.
Furthermore, when we make errors every day and become accustomed to these errors then these problems become institutionalized losses, as we have made them part of our daily expectations. Well-defined work procedures are intended to eliminate as many of these as possible.
Description |
Probability |
General rate for errors involving very high stress levels |
30% |
Complicated non-routine task, with stress |
30% |
Supervisor does not recognize the operator’s error |
10% |
Non-routine operation, with other duties at the same time |
10% |
Operator fails to act correctly in the first 30 minutes of stressful emergency situations |
10% |
Errors in simple arithmetic with self-checking |
3% |
General error rate for oral communication |
3% |
Failure to return the manually operated test valve to the correct configuration after maintenance |
1% |
Operator fails to act correctly after the first few hours in a high stress scenario |
1% |
General error of omission |
1% |
General error rate for an act performed incorrectly |
0.3% |
Error in simple routine operation |
0.1% |
Selection of the wrong switch (dissimilar in shape) |
0.1% |
Selection of a key-operated switch rather than a non-key operated switch (EOC) |
0.01% |
Human performance limit: single operator |
0.01% |
Human performance limit: team of operators performing a well-designed task |
0.001% |
Human Error Rates
Source: A Guide to Practical Human Reliability Assessment, by Barry Kirwin
The second reason well-defined work procedures are so important pertains to manpower utilization, namely wrench time. The more comprehensive the job packet, the more efficient the work. There is a lot of value to be gained by an organization for getting the wrench time up.
For example, if an organization has 60 technicians, 15% of the 60 technicians (9) should be working on PM at any given time. If the wrench time for this group of technicians is 29% (North American average) and through better procedures and more complete job packets, the wrench time was shifted to 55% (considered by many to be the upper limit of possibilities), then the organization would effectively realize an 89.65% increase in productivity without hiring any additional personnel.
Some craftsmen have a negative reaction to this line of logic. They believe that doubling the output means they must double their efforts, and the doubled effort is expected at no additional increase in compensation. If these were the only elements in the equation, one could see this as a demotivator. However, more efficient work means easier work.
If I can get better results with less effort, I get an increase in efficiency. The less effort part comes from not looking for parts, not waiting to get a machine down, not waiting for a permit, and not wondering how to do the job or having to invent a way to do the job. A well-constructed job packet contains all these things; therefore, the technician expends less effort while getting more work accomplished. This is wrench time logic that creates a significant portion of the business case for improved PM procedures.
Download this report to learn the #1 law you should know about PM, the first question you should ask to evaluate your PM program, and the real truth about Preventive Maintenance.
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