MTTFd Calculation and B10d Testing Services

Jan. 24, 2011
The machinery safety standard ISO 13849 is driving the need to determine either MTTFd values or B10d values for equipment and components. MTTFd is the mean time to dangerous failure, and this value is needed for all equipment that is part of a safety-related control system, in accordance with ISO 13849.

This standard also applies to components that do not have a safety rating, so are not certified to a certain performance level. This may be a simple relay or contactor—whatever ends up in a safety-related system. Therefore, it is increasingly important for manufacturers of such components to be able to provide MTTFd values.

MTTFd can be determined directly for electrical or electronic components by doing either a parts count or a Failure Mode and Effects Analysis (FMEA). For electro-mechanical, hydraulic, and pneumatic components that have a cycling operational profile, it is more difficult to provide an MTTFd value because the value depends on how often the device is used over time (i.e., how often for example a switch is activated). For these devices, a B10d value is needed. B10d is the number of operating cycles after which 10 percent of a population of a component will have failed dangerously. With the B10d, the MTTFd can then be calculated.

We Can Help

UL can assist you in obtaining an MTTFd for any type of component. We can do the MTTFd determination via parts count or FMEA. The latter is more accurate and may provide more optimistic values, but is more labor intensive. We also offer B10d testing for MTTFd determination. This is done through destructive lifetime testing, the methods for which are provided by the following standards:

  • IEC 61649: Weibull Analysis.
  • IEC 60947-4: Low-voltage switchgear and control gear. Part 4-1: Contactors and motor-starters, Electromechanical contactors and motor-starters. Annexes B and K.
  • IEC 61810-2: Electromechanical elementary relays, Part 2: Reliability. Draft IEC 61810-2-1: Electromechanical elementary relays, Part 2-1: Reliability – Procedure for the verification of B10 values.
  • There is no requirement that a third party be involved in providing MTTFd values—a manufacturer can self-declare—but, the resources needed to provide this may prove problematic, being outside of the manufacturer’s core competency. Additionally, value is provided through the credibility of using a third party to validate MTTFd claims.

    For more information on UL’s MTTFd calculation and B10d testing services, please contact:

    Kevin Connelly
    [email protected]

    Or go to the web:

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