OEE
Machine states
Why state classification decides how trustworthy the OEE is, and at what granularity states should be defined.
The machine state is the signal that says what a plant was doing at every instant. How those states are classified determines the entire Availability calculation: it is the point where an OEE becomes trustworthy, or becomes a number nobody on the shop floor recognises as true.
The categories that matter
The possible states of a machine are many and vary from plant to plant. The categories they must be mapped to, for OEE purposes, are few.
| Category | Effect on the calculation |
|---|---|
| Production | It is the run time: it contributes to Availability and Performance. |
| Unplanned stop | Availability loss: failures, waiting for material or for an operator, unexpected blocks. |
| Planned stop | Outside planned time: scheduled maintenance, shifts not planned. It does not penalise OEE. |
| Setup and changeover | Decided case by case: a scheduled changeover is a planned stop, one born of a contingency is a loss. |
Quality does not depend on states, but on Unit counts. Performance is computed only on production time: in the other states there is no speed to compare.
The error that skews everything
Classifying a failure as a planned stop inflates Availability. It is the most frequent error and the hardest to detect downstream, because the resulting number is plausible: nobody questions a high OEE.
The opposite holds too. Treating a stop the company decided and scheduled as a loss produces a pessimistic indicator, and a pessimistic indicator gets abandoned quickly.
That is why state classification is not a technical configuration: it is a shop-floor decision, to be taken with the people who know the machine.
How many states you need
Too many states make classification unmanageable and reports unreadable. Too few make it impossible to see where time is lost: the pair "on / off" says the line was stopped, not why.
A practical criterion: one extra state is worth it only if it leads to a different corrective action. Distinguishing "waiting for material" from "waiting for operator" makes sense if the two causes have different owners; it does not if the answer is the same person doing the same thing.
You usually start with four states (production, failure, setup, stop) and add more only when the analysis demands it.
It needs revisiting over time
A state classification ages. The product mix changes, shift organisation changes, operations are added that did not exist before: reviewing it every few months with the production team costs little and keeps the OEE credible. An indicator the shop floor does not recognise itself in does not get used, however correctly it is computed.