Apologies for the late response.
Criticality factors depends on the type of machines and process. However, it is based on the risk of financial loss, safety consequence, loss of reputation etc. \
As a general rule, Risk= probability of failure X consequence of failure
In terms of Plant machinery,
During say one year time, Risk ($/yr) = Failure frequency (per year) X Cost consequence ($)
Now consider the example below :
- Now say you have 5 mechanical failures and financial loss is around 100 $ each failure, so using the formula above the risk is 500 $/ year
- You have 1 electrical failure and financial loss is 80$ each failure, so the risk is 80 $/year
- You have 3 brake failures and each failure costs you 200 $ each failure, so the total risk is 600$ /year.
Now you can rank the failures by the associated financial risk
- Brake failures 600$/year
- Mechanical Failures 500$/year
- Electrical failures 80$/year
Notice that number of brake failures (3) is less the number of mechanical failures (5) but due to the highest financial impact, it should be taken care of first.
Now if your improving your preventative maintenance, having some brakes in the store , getting better quality brakes , training your people for replace the brake quicker in case of failure etc etc you can eliminate nearly half 600/1180 of the financial risk.
After wards, follow the same principle on the rank below i.e., Mechanical failure(s)
Complete and thorough RCA is difficult and time consuming. I suggest do it on the most critical first and then the next and so on