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12
Safety Planning
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5 Safety Planning
This chapter provides information for planning a safety-related loop.
The device is not specified for a certain application. Make sure that the data provided in this
chapter is valid for your target application.
Special application-specific factors may cause the premature wear of the device and must be
taken into consideration when planning systems; take special measures to compensate for a
lack of experience based values, e.g. through implementation of shorter test intervals.
The suitability for specific applications must be assessed by considering the particular overall
safety-related system with regard to the requirements of IEC 61508.
Safety-planning must only be carried out by trained and qualified personnel. In case of doubt
contact Turck directly.
5.1 Architectural specifications
Due to architectural considerations the following characteristics are specified:
Type A
HFT 1
The internal structure is partly HFT 0 or HFT 1.
The redundancy of the circuit components was already taken into account in the probabilistic
calculations.
The failure probabilities, SFF, PFD and PFH has to be used with HFT = 0.
Experience has shown that the useful lifetime often lies within a range of 8 to 12 years. It can
be significantly less if elements are operated near their specification limits. However, it can
be extended by appropriate measures. For example, heavy temperature fluctuations could
potentially decrease the useful lifetime, as constant temperature below 40 °C could potentially
increase the useful lifetime.
For the relay outputs (cos phi=1, I=2A AC) the useful lifetimes is 8 to 12 years or 30.000 switch-
ing cycles. The relay must be protected against an over current.
5.2 Assumptions
■
Failure rates are constant for 10 years, wear out mechanisms are not included
■
Propagation of failures is not relevant
■
External power supply failure rates are not included
■
All components that are not part of the safety function and cannot influence the safety func-
tion (feedback immune) are excluded.