Layman Explanation of MTTR (Mean Time To Repair or Replace)

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The term Mean Time To Repair (or Replace) is used in maintenance management for expressing important information related time required for repairing a faulty piece of equipment.

In today’s article, we will explain MTTR in such a simple way that even a layman would understand it by the end of this article.

Appreciating the Complexity of MTTR

Mean Time To Replace or Repair (MTTR) is the average time taken to restore a machine or piece of equipment to its normal operational state after it has become unavailable for operation due to a fault or failure.

In cases where replacement is carried out instead of repair (imagine replacing the axle of an automobile because it can’t be fixed), the “R” in the end of “MTTR” represents Replace instead of “Repair”.

For example, if in a course of a month, a heavy-duty pump broke down three times. The first time it took 20 minutes to restore it back to service. The second time it took 45 minutes and the third time it took 120 minutes to restore the pump, the Mean Time To Repair (MTTR) for the pump would be the average of 20, 45 and 120 i.e., 48 minutes.

While it may sound simple, the devil is in the details.

When we said it took 20 minutes to repair the pump and restore it back to the service, does 20 minute include the time it took for the technician to reach site?

Does it represent the time it took to diagnose the fault or just the time for the repair? For instance, if a seal had ruptured, does 20 minute include the time it took to find the ruptured seal by the technician? Or is it just the time of removing the old seal and replacing it with new one?

Does it include the time it took for a helper to get a new seal issued from the store?

Does it include the time it took to test the pump after the repair had been performed?

To know what exactly is reflected by the metric Mean Time to Repair/Replace (MTTR), it is necessary that you know the answer to above questions. Only after that will you be able to understand why this metric even exists? What benefit does it provide?

Understanding Time To Repair (TTR) – The Key to Understanding MTTR

Let us define what is termed as Time To Repair (TTR) in maintenance management. Once you understand what is Time to Repair (TTR), understanding Mean Time To Repair (MTTR) will become a no brainer (because it is just the mean or average value of TTR).

So, what is the Time To Repair (TTR) in a maintenance organization?

The timer starts from the moment maintenance department has been informed of an equipment going out of service. If the operations made the call to maintenance at 12:05 pm that the pump has gone down, the timer starts from 12:05 pm.

The maintenance department will come into action. It will take some time for them to assign the work to a maintenance supervisor who will choose a technician from his team to go and check the pump.

Let’s say the technician reached site by 12:15 pm i.e., 10 minutes. So, the first segment of Time To Repair (TTR) includes the time it takes for the maintenance department to get a technician to reach the site.

Now, the technician cuts the power to the pump from its main circuit breaker and observes the Lock Out/Tag Out measures.

He checks the pump to see what’s wrong with it. He tries to identify whether the fault is mechanical or electrical. He checks any sign of leakage or pressure build up. He may check if any pressure sensor has tripped the pump.

He finally diagnosis that that the roller bearing of the pump’s motor has jammed up. Let’s say he reaches to this conclusion by 12:30 pm i.e., 15 minutes after reaching the site. So, the second segment of Time To Repair (TTR) includes the time it takes for the craftsman to diagnose the fault.

What does he need now? He needs a new bearing. He goes to the spare parts store and gets a new ball bearing issued. He reaches back the site at 12:40 pm. So, the third segment of Time To Repair (TTR) includes the time it takes for arranging the spare part needed to repair the equipment.

Now, the technician begins to disassemble the pump. He removes the faulty ball bearing and installs the newly issued ball bearing. He assembles the pump back by 01:00 pm. So, the fourth segment of Time To Repair (TTR) includes the time spent on actual repair (commonly referred to as Wrench Time).

What’s left? – The technician needs to test the pump to make sure it is okay before returning it to service. He turns the main circuit breaker back on. He runs the pump for 5 minutes during which he checks if there is any abnormal noise or vibration.

The test is completed by 01:05 pm. So, the fifth segment of Time To Repair (TTR) includes the time spent on testing the equipment after a repair job has been completed.

The pump that had gone down at 12:05 pm was returned to service at 01:05 pm. Therefore, the Time To Repair (TTR) is 60 minutes.

Conclusively, Time To Repair (TTR) is actually equipment downtime. It starts from the time operations makes the first call to maintenance and ends the moment maintenance gives the equipment back to operations.

To summarize, Time To Repair (TTR) is a sum of five main times:

  1. Time taken by maintenance department to get a technician onsite to check the equipment.
  2. Time taken by technician to diagnose the fault.
  3. Time taken to arrange faulty spare parts.
  4. Time taken to perform the repair.
  5. Time taken to test the equipment after the repair.

MTTR = Mean of TTR

Mean Time To Repair or Replace (MTTR) is nothing but the average of Time To Repair (TTR) of all the repair jobs performed on a piece of equipment in a week, month, year or any other selected time.

Furthermore, it is important to emphasize that it includes only such repair jobs that are performed when the equipment goes down and repair jobs restores it into service. Such repair jobs which are of a non-critical nature such as repairing a minor leakage in a pump is not counted.

To make it even more clear by an example, imagine the same pump we discussed above goes through three more critical repair jobs in the same year. Time To Repair (TTR) for each of these three jobs were 70, 80 and 90 minutes respectively.

The Mean Time To Repair (MTTR) for the pump would simply be the average of 60, 70, 80 and 90 minutes i.e., 75 minutes.

The important question now becomes, what benefit does calculating Mean Time To Repair or Replace (MTTR) do?

How MTTR is Used for Improving Maintenance?

The Mean Time To Repair or Replace (MTTR) gives a tangible and quantitative measure for maintenance managers to understand gaps and available opportunities.

Remember the five components of Time To Repair (TTR) we discussed a few paragraphs earlier?

The maintenance manager can look at each of the components of Mean Time To Repair or Replace (MTTR) to identify trends and opportunities for improvement:

  1. Time taken by maintenance department to get a technician onsite could be reduced by implementing a Computerized Maintenance Management System (CMMS) that cuts the phone calls or radio set communication and allows faster communication between operation and maintenance teams as well as between maintenance supervisors and the technicians in his team
  2. Time taken by technician to diagnose the fault could be reduced by imparting training. An experienced and skilled technician will be able to diagnose the fault quicker.
  3. Time taken to arrange faulty spare parts could be reduced by streamlining the supply chain.
  4. Time taken to perform the repair could be improved by providing better tools to technicians along with training on how to use them. There is a reason Formula-1 pit stop is idolized in maintenance world.
  5. Time taken to test the equipment after the repair could be decreased by coming up with improved testing procedures.

Once any of the improvements are implemented, the maintenance manager would observe if Mean Time To Repair or Replace (MTTR) improves the next year (or next month).

In this way, Mean Time To Repair (MTTR) allows maintenance departments to improve maintenance efficiency and track improvements in the maintenance process.

References & Further Reading

1. "Reliability Engineering Handbook" by Dimitri Kececioglu (Link).

2. "Maintenance Planning and Scheduling Handbook" by Richard (Doc) Palmer (Link).

3. "Maintenance Engineering Handbook" by R. Keith Mobley (Link).

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