RCA is used for single and usually repetitive problem but if your goal is to improve overall reliability then I suggest looking into equipment failure history and take following steps

1. Calculate MTBF( Mean Time Before Failures) by diving the span of time ( say 1 year) by number of failures
2. Using Pareto principle , figure out which failure modes are causing highest down times
3. Then rank these failure modes as per their criticality ( safety/production loss/etc) and select the ones first which are more critical 
4. Then develop a counter actions list (design improvement/Maint checks/ personnel training/logistics improvement/etc)
5. Check that if preventative checklist/task list covers the points located in no 3
6. Develop a critical spare parts list which should be readily available at store for corrective/breakdown action
7. After 3-6 months recalculate MTBF and compare it with the MTBF calculated previously (in step one) 
8. Repeat the cycle till desired reliability is achieved !

Hi! thanks a lot for you reply.

Regarding the point n°3, how do you rank the failure mode as per their criticality ?

Regards

Let’s say that I perform a pareto and discovered that I have hydraulic failure, mechanical failure, Brakes failure,  all  representing nearly 80%  of my downtimes.  How do I rank them per their (criticality safety/production loss/etc) and what are the different factors of criticality? Then do I perform an FMEA on all those three failures to select the most critical, in order to add an RCA (ISHIKIWA) on that one critical problem to implement actions ?

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

1. Brake failures 600$/year
2. Mechanical Failures 500$/year
3. 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