Emerson Exchange 365

Hi Paul,

For the short answer: it is expected that the failure rates for a transmitter operating at a consistently higher ambient temperature would be different than a transmitter operating at consistently average or lower ambient temperatures.

For some additional explanation, the FMEDA report has a few statements in relation to your question:

“Some industrial plant sites have high levels of stress. Under those conditions the failure rate data is adjusted to a higher value to account for the specific conditions of the plant… If a user has data collected from a good proof test reporting system such as exida SILStatTM that indicates higher failure rates, the higher numbers shall be used” (Section 4.2)… “The stress levels [used to derive FMEDA failure rates] are average for an industrial environment and can be compared to the exida Profile 2 with temperature limits within the manufacturer’s rating. Other environmental characteristics are assumed to be within manufacturer’s rating” (Section 4.3).

Note that Profile 2, in Appendix C, shows that this report assumes an average ambient temperature of 25 deg C. Since it is impossible to take into account all of the varying operating conditions these devices may operate in, the FMEDA report published by exida is based on average conditions and the assumption that the devices are always operated within their specified limits. The failure rates published in the FMEDA report are only valid for the Useful Life of the product, so Useful Life is certainly an appropriate consideration to take into account for any transmitter in a safety application; the Useful Life assumes that the device is maintained (and replaced when appropriate) according to IEC 61508/61511 - this includes operating within the manufacturer ratings, as you noted above.

I hope this is helpful - please feel free to reach out here with additional questions, or to me (megan.wiens@emerson.com) or Brad (see previous reply) via email.