Transmitters, communications devices and wiring systems should be arranged to meet the requirements for survivability, protection against external influences and independence.
Independent systems or redundant channels should not share multicore cables with each other or power circuits, and may require diverse routes depending upon the safety integrity level to be achieved.
Measures to protect against failures include:
- Use of fail-safe principles such as DC model (e.g. 4-20 ma loop) for analogue signal transmission diagnosis and alarm of out of range, abnormal, or fault states (such as stuck-at) with defined control system responses for both the sensor and transmitter;
- Cable selection (screening etc.);
- Protection of cables against fire, chemical attack, physical damage etc.;
- Physical separation or segregation of cables and cable routes;
- Routing in benign environments;
- Use of optical fibres to protect against electrical interference;
- Careful attention to lightning protection (BS 6651) of data links between buildings.
Use of fieldbus or other digital communication protocols in protective systems should be considered a novel approach requiring a thorough evaluation and demonstration of the safety integrity. EEMUA 189 ‘A guide to fieldbus applications in the process industry’ provides limited guidance.