Mean time to dangerous failure (MTTFd)

Free guide to maintenance metrics

What is mean time to dangerous failure (MTTFd)?

MTTFd stands for mean time to dangerous failure. It is the calculation of how often a piece of equipment or safety system fails in a way that threatens the safety of workers.

Because it only considers dangerous failures, MTTFd is not the same as mean time to failure (MTTF). For example, let’s say a contact needs to be open to trigger a safe stop on a machine. MTTFd would only include failures where the contact remained closed, while MTTF would include all failures, regardless of the contact’s position.

MTTFd is also similar to another safety metric: Time between workplace incident/accident. Where time between workplace incident/accident accounts for the people aspect of a facility, MTTFd looks at safety from an asset perspective.

What is failure?

To understand how to calculate MTTFd, it’s crucial to know what failure is and how it differs from a fault.

Failure is a single event. It is the point in time that a piece of equipment no longer produces exact and required results, like the moment a car’s tire is punctured. The tire continues to lose air and eventually goes flat. The car can still operate, but not the way it’s supposed to, all because of that moment of failure.

A fault is a way to describe the state of an asset after failure. Using the example of a car, the vehicle’s flat tire would be the fault.

Types of asset failure

There are several types of failure. Understanding what separates failure from dangerous failure is important for calculating MTTFd

  • Safe failure: A failure that results in a safe state and does not produce a hazardous environment. For example, a contact that fails in the open position, triggering a safe stop, is considered to be a safe failure.
  • Dangerous failure: A failure that results in an unsafe, hazardous state. This presents a risk to people and may result in damage to equipment or other assets. For example, a safety sensor might fail, but the machine might still be running.
  • Detectable dangerous failure: A potentially dangerous failure that is detected by testing or monitoring and is then repaired to a safe state. For example, you can use monitors to measure if a certain dangerous gas was present in the air.
  • Undetectable dangerous failure: A potentially dangerous failure that is not detected by testing or monitoring. For example, when a contact fails in the closed position, the command to shut off the equipment will fail, and the equipment will keep running.

How to calculate MTTFd

Calculating MTTFd starts with knowing a little about MTTF.

MTTF measures the average lifespan of a non-repairable asset, from the time it begins operating to the point of failure.

MTTF is calculated by dividing the number of operational hours for a group of assets by the total number of assets.

For example, you have three identical fans. One runs for 30 hours before failing, while the second runs for 21 hours, and the third runs for 27 hours. That’s a total of 78 hours.

The MTTF for these fans would be calculated by dividing 78 by three for a total of 26 hours.

To calculate MTTFd, divide the total number of operational hours for a group of assets by the number of dangerous failures that occur among those assets.

For example, let’s say there’s a group of five machines that ran for a total of 1,000. The MTTF would be 200 hours. However, only two of these machines experienced a dangerous failure. To find the MTTFd, divide 1,000 by two for a total of 500 hours. Therefore you can go through more than two lifespans of these machines before worrying about a dangerous failure occurring.

Benefits of MTTFd

There are several health and safety benefits of knowing MTTFd:

  • Monitoring MTTFd helps maintenance teams focus on potential safety hazards, helping to prevent accidents
  • Knowing the MTTFd of your organization’s assets can help you build a comprehensive safety training program
  • Keeping track of MTTFd may be key to passing audits and staying compliant with certain regulations
  • By calculation dangerous failure rates, it’s possible to find the expected lifetime of a safety system, which helps facilities fine-tune and forecast inventory purchases
  • MTTFd helps maintenance managers establish maintenance schedules for safety systems and safety equipment

MTTFd: A key component of a healthy maintenance team

MTTFd is a crucial element in maintaining functional safety in an organization. With a better understanding of the types of failures that occur, and the rates at which they happen, organizations can develop maintenance schedules that are optimized to keep equipment, and people, working safely.

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