Designing With The Honeywell Stt700; Diagnostic Response Time; Logic Solver Inputs; Reliability Data And Lifetime Limit - Honeywell HART STT700 Safety Manual

3 — Designing with the HONEYWELL STT700

Diagnostic Response Time

The HONEYWELL STT700 will report an internal failure within 20 minutes of fault occurrence
(worst case).
The transmitter will be put to burnout output if
1. PV is not updated in 7 seconds
2. 4-20mA Output Current is not as expected in 30 seconds
3. Electronics fault is found in 20 minutes (worst case).
The transmitter needs to be power cycled in order to recover from the burnout condition.

Logic Solver Inputs

The logic solver must be configured so that the engineering range in the transmitter matches the
expected range of the logic solver.
To take advantage of the internal diagnostics in the STT700, the logic solver must be configured
to annunciate an out of band current reading (greater than 20.8 mA. or less than 3.8 mA.) in
standard configuration or (greater than 20.5 mA. or less than 3.8 mA.) with Namur configuration
as a diagnostic fault. The logic solver configuration must consider the slew time of the current
signal and ensure that filtering is used to prevent a false diagnostic failure annunciation.

Reliability data and lifetime limit

A detailed Failure Mode, Effects, and Diagnostics Analysis (FMEDA) report is available from
HONEYWELL. This report details all failure rates and failure modes, common cause factors for
applications with redundant devices and the expected lifetime of the HONEYWELL STT700.
The HONEYWELL STT700 is intended for low demand mode applications up to SIL 2 for use in
a simplex (1oo1) configuration, depending on the PFD
Instrumented Function. STT700 is classified as type B device according to IEC61508, having a
hardware fault tolerance of 0.
The development process of the HONEYWELL STT700 is certified up to SIL3, allowing
redundant use of the transmitter up to this Safety Integrity Level, depending the PFD
calculation of the entire Safety Instrumented Function.
When using the HONEYWELL STT700 in a redundant configuration, a common cause factor
should be included in reliability calculations. For reliability calculation details, useful lifetime and
SFF, see the FMEDA report.
The reliability data listed the FMEDA report is only valid for the useful life time of the
HONEYWELL STT700. The failure rates of the HONEYWELL STT700 may increase sometime
after this period. Reliability calculations based on the data listed in the FMEDA report for
mission times beyond the lifetime may yield results that are too optimistic, i.e. the calculated
Safety Integrity Level will not be achieved.

Environmental limits

The environmental limits of the HONEYWELL STT700 are specified in the customer spec sheet
as given in the STT700 Specification 34-TT-03-19.

Application limits

The application limits of the HONEYWELL STT700 are specified in the STT700 User Manual
34-TT-25-17. If the transmitter is used outside of the application limits the reliability data
provided becomes invalid.
June 2017 STT700 Safety Manual
calculation of the entire Safety
AVG
AVG
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