The Equation to Measure Ratio of Emergency Maintenance Work to All Maintenance Work

To measure proportion of ‘emergency’ maintenance work you first need to unambiguously define what is ‘emergency work’. Once you can categorise maintenance work as ‘emergency’ you can calculate percent ‘emergency work’ in the whole maintenance work order population you are interrogating.



Dear Mike,

In our plant we defined emergency work as reactive maintenance work type which is not planned or scheduled, but is due to unexpected failures for which urgent corrective maintenance is required. We segregate the maintenance work orders into job priority, with the period specified to execute the work order as follows.

  • Priority 1 WO – To be done within 24 hrs due to plant stoppage or safety reason
  • Priority 2 WO – To be executed within 5 days, due to plant downtime from asset failure
  • Priority 3 WO – To be done in 30 days, which by nature are scheduled preventive and corrective action work orders

Of these three maintenance work order priority types, we consider P1 and P2 as Reactive or Emergency Maintenance Work and want to control the percent of this work over all other work order types. Our target is to make this emergency maintenance KPI below 20%.

My question is, which is the correct formula to calculate this ratio, out of three given below?

  1. As the ratio of total number of reactive work orders executed to total work orders executed
  2. As the ratio of total repair hours of reactive work orders to total work orders repair hours (Repair Time)
  3. As the total man hours spent on reactive work orders divided by the total man hours utilized on all work order types

Thank you for sharing your valuable time for me.



Hello Madhava,

When you want to measure emergency maintenance work percent you first need to define all the maintenance work type categories you will use to classify your maintenance work orders. By allocating work orders a type you can later determine the proportion of work orders in each category. You can further analyse each category to identify their related costs spent from your maintenance budget.

The definition used for Priority 1 is not what I would call emergency work. To me, work orders done within 24 hours to prevent failure are still corrective maintenance. Maintenance done to repair what has already failed is breakdown maintenance. It is my experience that maintenance ’emergency work’ is usually defined as emergency breakdown maintenance work. This is work where production stops, or is drastically curtailed, due to an equipment failure or an operational incident. If production can continue at near normal rates with SFIRP risk of harm to anyone or the environment, then you do not have an ’emergency’ (SFAIRP – So far as is reasonably practicable). There may be broken equipment which you will classify as a breakdown, but it is not an emergency breakdown with its accompanying disasterous production consequences.

One unfortunate aspect of using ‘percent emergency work’ as a maintenance KPI is that the KPI does not help you prevent the emergency work; it is a lagging indicator. Your real challenge is to create equipment reliability. Highly reliable equipment will surely slash the volume of emergency work orders in a company.

To me the most important emergency maintenance measure is the total moneys lost to the business because of each unwanted production stoppage. The percent of emergency maintenance work is interesting, but I would much rather know what the production emergencies requiring maintenance truly lost the operation. Once I have a financial value for the losses from emergency breakdowns I can develop a business case to remove the causes of those failures and make the plant more reliable.

Of the three measures you listed for percent emergency maintenance work, the most meaningful equations would be 1 and/or 3. Using Equation 1 identifies the proportion of all work orders in the emergency category. Equation 3 indicates the proportion of all man-hours spent on emergency work orders. I do not like Equation 2 as it is restricted to measuring just the maintenance crew hours. In reality the total hours lost to emergency work includes all the time spent by management, supervision and personnel from the engineering, reliability, maintenance and operations groups caught up in the event.

It is unlikely that the ratios from each equation will be the same since one involves counting work orders and the other requires tallying all the hours everyone spent on emergency work orders. For example, if ten work orders are issued and completed in one day, where 8 of them were planned and scheduled and required no production involvement, and two were emergency work orders. The maintenance crew has a supervisor and five maintainers on a standard 8-hour shift, which provides 48 hours of maintenance manpower a day. One emergency work order took a total of 20 hours (two maintainers x 8 hrs each, maintenance supervision x 2 hours, operator x 2 hours) and the other took two hours (maintainer x 1 hour, operator x 1 hour). Using Equation 1 gives an emergency work order ratio for the shift of 20%. Using Equation 3 the ratio is 22/51, which is 43% (with 3 hours of operator time added to the 48 hours of maintenance crew time). Both ratios are necessary to help you understand what happened in the maintenance department that day.

Equation 3 requires instilling the dedication of collecting all man-hours spent on emergency work and for all work. In many companies collecting all the man-hours by labour category spent doing each maintenance work order is too hard to do, or it is considered to be unnecessary. In such cases you are left with Equation 1—the count of work orders classified as emergency work compared to the count of all work orders. Thus in many companies Equation 1 is the only possible way to implement proportionate measurement of emergency work.

But as you saw above, using only Equation 1 provides an incomplete understanding of the impact of emergency work on a company. Its use skews the truth of where your man-power is really used. The low value from using Equation 1 presents an unrealistic picture of the real maintenance performance. When you are given a single ratio of emergency work for a company it is vitally important to know which form of the equation was used.

Make Preventive Maintenance the Highest Priority

I found it interesting that your Preventive Maintenance work orders are classified as Priority 3; to be done sometime in the next 30 days. I think that is part of the reason you have so much emergency work—the PM’s are done too late to address the failure causes. If I were you I would make all PM’s also Priority 1.

This change will reap great operating profit-making rewards for you in the next six months and forevermore. When you make PM work a ‘first priority’ you spot equipment problems early and address them before they breakdown. You proactively restore and renew equipment by replacing aging parts, and those suffering from excessive stresses, before they fail. Your plant and equipment will go without breakdowns for longer—your PM ‘first priority’ strategy will help get you the higher reliability you want. The emergency work orders will decrease; the Priority 2 work order category, to be executed within 5 days, will increase for a while; eventually the Priority 3 category, to be done in 30 days, will become a large proportion of your maintenance work.

Do you see and understand how putting PM work orders at the highest priority will improve your future reliability and lower your maintenance costs?

It will be necessary that you dedicate people to do the PM restoration work orders on the day they fall due. You still must act when emergency work arises, but do the emergency work orders with a separate crew. Leave the PM crew alone to do their work. One final piece of advice—put your most accurate and precise maintainers in the PM restoration crew. Recognise them when they do best-in-class, top quality refurbishment work. Let them use their higher skills and abilities to make your plant and equipment run reliably.

I hope you will test the above ideas for yourself.


All the best to you,

Mike Sondalini
Managing Director
Lifetime Reliability Solutions HQ


PS. If you require advice on industrial asset management, industrial equipment maintenance strategy, defect elimination and failure prevention or plant and equipment maintenance and reliability, please feel free to contact me by email at

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