What is "MTTFd"?

Factory machines use safety switches to protect human safety.
For example, it is a system that automatically stops when the door is opened.

Then, how do we determine “how reliable” the safety switch is?

That’s where the indicator “MTTFd” is used.

To put MTTFd briefly

MTTFd is a numerical value that represents the average time until a dangerous failure occurs (Mean Time To Dangerous Failure) in parts or devices in charge of safety functions.

“Dangerous Failure” here is a failure where the safety function stops working.

In case of safety switches, for example, the difference is:

• Normal: The machine stops when the door is opened.
• Dangerous Failure: Machine does not stop even when the door is opened.

What does “Mean Time” mean?

This is a slightly confusing point.

“Mean” in MTTFd signifies “the average time it takes for a dangerous failure to occur when you use many identical parts or devices.”

Please imagine that we have 100 units of the same safety switches and keep on using them respectively. There will be variation in durability as follows:

• Some will fail in 5 years
• Some will fail in 10 years
• Some will endure for 20 years

By averaging all of these numbers together, you can see “approximately how many years” it takes for a dangerous failure to occur.
That is MTTFd.

In other words, this does not mean “this product will definitely fail in X years,” but “as a tendency when used a lot, a dangerous failure will occur around this time.”

This is the reason MTTFd is called a “statistical value.”

Does the larger value of MTTFd always mean safer?

MTTFd is represented in “years,” and a larger value means a lower failure rate.

• MTTFd = 10 years → Relatively fragile
• MTTFd = 100 years → Very durable

However, caution is needed here.
Even if the MTTFd value is high, it does not mean “it never breaks.” As the number is just an average, there may still be a case of an early failure. Therefore, it is important “not to rely on just one component” in safety design.

Level classification of MTTFd

In practice, MTTFd is divided into the three levels below:

• Low: 3 years ≦ MTTFd < 10 years
• Medium: 10 years ≦ MTTFd < 30 years
• High: 30 years ≦ MTTFd < 100 years

*NOTE: If it exceeds 100 years, it is normally treated as 100 years.

Based on this classification, you can roughly decide “how reliable the part is.”

Relationship with PL (Performance Level)

MTTFd is not only used on its own, but also used in the evaluation of safety levels called “PL (Performance Level).”

PL is expressed in five levels from a to e:

• PL a: Low
• PL e: Very high

Guidelines for the required MTTFd for each PL

PL is not determined by MTTFd alone, but it is still a very important factor.

Roughly, the relationship between PL and MTTFd can be shown as follows:

• PL a to b: Achievable with Low MTTFd
• PL c: Medium MTTFd is the benchmark
• PL d: High MTTFd is frequently required
• PL e: High MTTFd is a prerequisite (and other strict conditions)

In other words, using the parts with high MTTFd is essential to aim at a high safety level (PL d and e).

Why is MTTFd alone insufficient?

Here is an important point.
MTTFd represents the “resistance to component failure,” but it is not the sole factor to determine safety.

For example:

• whether a failure can be detected immediately
• whether the same function is duplicated (redundancy)

These design considerations are also crucial.

To put it in extreme terms:

• even if the component is very strong against failure
• it is dangerous if its failure is not detected.

Therefore, PL is determined by the combination of “component reliability (MTTFd) and design ingenuity.”

Summary

The key points of MTTFd are as follows:

• Mean Time To Dangerous Failure (average time until a dangerous failure occurs)
• A “statistical value” based on a large number of samples
• The larger the value, the lower the failure rate
• Classified into Low / Medium / High
• One of the important factors to determine PL (safety level)

Finally

In the world of safety components, it is crucial to “consider safety using numerical data” rather than assuming “it is probably okay.”

MTTFd is the basic “indicator of resistance to failure.” Furthermore, when it is combined with PL, it becomes possible to evaluate safety of the entire equipment.

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