engineering

MTTFd

MTTFd is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand MTTFd rather than just read about it. In short: Mean Time to Dangerous Failure. In a safety system MTTFD is the portion of failure modes that can lead to failures that may result in hazards to personnel, environment or equipment.

Key takeaways

MTTFd belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect MTTFd to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of MTTFd from memory before moving on to harder problems.

Reference excerpt

Mean Time to Dangerous Failure. In a safety system MTTFD is the portion of failure modes that can lead to failures that may result in hazards to personnel, environment or equipment. MTTFD is critical to the determination of the performance level of a safety system. ISO 13849 defines three levels of MTTFD:

ISO 13849 prescribes three methods to determine the MTTFD of a safety channel:

use the manufacturer's failure data; use the methods prescribed in Annexes C and D of ISO 13849-1 use 10 years (i.e. assume the channel has low integrity) Mean Time to Failure (MTTF) is assumed constant during the useful life period of a component. The MTTF can be calculated according to:

MTTF = 1 λ [ h o u r s ] {\displaystyle {\text{MTTF}}={\frac {1}{\lambda }}[hours]\!}

where λ is the failure rate for the component. The relationship between MTBF and MTTF is expressed as:

MTBF = M T T F + M T T R {\displaystyle {\text{MTBF}}=MTTF+MTTR\!}

where MTTR is the mean time to repair. The MTTF of a system is the sum of MTTFS and MTTFD. To understand the relationship between MTTFS and MTTFD consider the case of a switch that turns a motor on or off. The switch has two failure modes: the switch can fail stuck closed or the switch can fail stuck open. If the switch fails stuck open, the motor will never energize; as a result, the motor will not create any hazards due to its operation. In contrast, if the switch fails stuck closed, this failure can lead to a dangerous situation like for example the case where the operator needs to stop the motor, but the motor will not stop because the switch is stuck in the closed position. The failure mode where the switch is stuck in the open position is denominated the safe failure mode, whereas the stuck closed failure mode is denominated the dangerous failure mode. The likelihood of occurrence of a dangerous or safe failure may differ and is a function of several variables in the construction and design of a component. A poorly designed switch may have a higher proportion of dangerous failures (thus a lower MTTFD), whereas switches rated for use in safety circuits may very well preclude the occurrence of stuck closed failure modes (thus have infinite or very high MTTFD). Assessing the performance level of a safety system, requires knowing the distribution of the dangerous vs. safe failure modes of its components and ultimately a determination of its MTTFD.

External links "Reliability and Availability Basics". EventHelix. Nix, Doug (2017). "ISO 13849–1 Analysis, Part 4: MTTFd Mean Time to Dangerous Failure". Machinery Safety 101.

Worked examples

Example 1 — a first encounter with MTTFd

Start with the simplest possible case. Write down what MTTFd claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to MTTFd before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about MTTFd ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of MTTFd

In research: MTTFd appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses MTTFd in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: MTTFd is common in secondary-school and first-year university syllabi. It links to neighbouring topics Engineering failures, ISO standards, Reliability analysis, so understanding it makes those chapters shorter.
In everyday life: Look for MTTFd outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study MTTFd in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what MTTFd means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain MTTFd out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is MTTFd in simple terms?

Mean Time to Dangerous Failure. In a safety system MTTFD is the portion of failure modes that can lead to failures that may result in hazards to personnel, environment or equipment.

Why does MTTFd matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study MTTFd?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on MTTFd.