What is MTTFd and how is it used in ISO 13849-1?
Date: 14/09/2026
MTTFd (Mean Time to Dangerous Failure) is a reliability metric used in machinery functional safety. It expresses the average expected time until a component or system experiences a dangerous failure, under defined assumptions. In ISO 13849-1 calculations, MTTFd is one of the parameters used to demonstrate that a safety-related control system can achieve the required Performance Level.
For machine designers, the key point is that MTTFd is not a safety level by itself. It is a reliability input that must be considered together with the system architecture and diagnostic measures used for the safety function.
What does MTTFd mean?
MTTFd stands for Mean Time to Dangerous Failure. It estimates the average time until a dangerous failure occurs — a failure that could contribute to a hazardous condition.
Example
If a joystick has an MTTFd of 40 years, this means that, under the stated assumptions, the average time to a dangerous failure is estimated at 40 years. The value indicates the component’s reliability contribution within a safety-related application; it does not mean that every individual joystick will operate for exactly 40 years.
đź’ˇ Good to know: The higher the MTTFd, the lower the average probability of a dangerous failure, provided the same assumptions and application conditions are used for the comparison.
Where does MTTFd fit into ISO 13849-1?
The process starts with risk assessment. For each safety function, OEM/machine designer determine the required Performance Level (PLr) by evaluating injury severity (S), frequency and/or duration of exposure (F), and the possibility of avoiding or limiting harm (P).
For a detailed explanation of this first step, see APEM’s FAQ: How to determine Performance Level (PLr) according to ISO 13849-1.
Once the PLr has been defined, the OEM/machine designer designs the safety-related parts of the control system to meet that requirement. Several design paths can lead to the same Performance Level, but the calculation typically combines system architecture, component reliability through MTTFd, and diagnostic coverage (DCavg). Depending on the architecture, common-cause failures (CCF) must also be considered.
How does MTTFd contribute to Performance Level?
MTTFd represents the reliability of the components or channels that make up a safety function. In the Performance Level calculation, it is combined with other design parameters rather than assessed alone.
Category / architecture — how the safety-related control system is structured and how it tolerates faults.
MTTFd — the average reliability of the components or channels with respect to dangerous failure.
DCavg — the average diagnostic coverage, indicating how effectively dangerous failures are detected.
CCF, where applicable — measures addressing common-cause failures in multi-channel architecture.
Higher-risk safety functions therefore generally require an appropriate combination of architecture, reliability and diagnostic measures to achieve the required PL.
How are MTTFd values classified?
MTTFd values are commonly grouped into three reliability ranges for the purpose of ISO 13849-1 calculations:
Designation | MTTFd range |
|---|---|
Low | 3 years ≤ MTTFd < 10 years |
Medium | 10 years ≤ MTTFd < 30 years |
High | 30 years ≤ MTTFd < 100 years |
Does a high MTTFd automatically mean a high Performance Level?
No. A high MTTFd improves the reliability contribution of a component or channel, but the achieved Performance Level depends on the complete safety function. Architecture and diagnostic coverage still have to be appropriate for the target PLr.
👉 Key point: MTTFd is a component- or channel-level reliability input. Performance Level is a system-level result for the complete safety function.
What is B10d and how is it converted to MTTFd?
For electromechanical components such as switches, reliability is often expressed as a B10d value rather than directly as MTTFd. B10d represents the number of operating cycles at which 10% of a population of components are expected to have failed dangerously.
Because these components operate in cycles, their MTTFd depends on how frequently they are used. For this reason, B10d is converted into MTTFd using the number of operating cycles per year:
MTTFd = B10d / (0.1 Ă— nâ‚’â‚š)
nâ‚’â‚š = number of operating cycles per year
For more details on this metric, see B10d: definition, calculation and applications.
Why does the mission profile matter?
The mission profile defines the usage assumptions needed to interpret a reliability value in the intended application. For an electromechanical component, the number of cycles per year directly affects the MTTFd calculated from B10d.
This is why the same switch can lead to different MTTFd values in two applications with different operating frequencies. The B10d value describes the component’s dangerous-failure behaviour in cycles; the mission profile converts that cycle-based information into a time-based MTTFd value for the application.
đź’ˇ Good to know: If an OEM/machine designer cannot provide a mission profile for an electromechanical product, APEM can provide the B10d value so the OEM/machine designer can complete the calculation when the application assumptions are known.
What reliability data can APEM provide?
APEM provides reliability information to support OEM/machine designer in their functional safety calculations. The type of data depends on the product technology and on whether the necessary mission-profile assumptions are available.
Product type | Typical APEM reliability data | Why |
Electronic products | MTTFd may be stated when relevant assumptions are defined | A time-based reliability value can be calculated against a defined mission profile. |
Electromechanical products | B10d is generally provided | MTTFd depends on the customer’s operating frequency and mission profile, so a cycle-based value is more directly transferable. |
For electronic products, a specific mission profile may still require a dedicated calculation. For electromechanical products, providing the actual application mission profile allows the B10d value to be converted into a relevant MTTFd value.
More information: Functional Safety in HMI Systems.
👉 APEM’s role: APEM provides component-level reliability data and technical input for the safety analysis. The OEM/machine designer remains responsible for defining the safety function, its PLr, the system architecture and the final verification of the achieved Performance Level.
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