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Fieldbus Protection Best Practices for Fieldbus Protection

| Author/ Editor: Andreas Hennecke / Dr. Jörg Kempf

There is always a risk of faults and failures when installation technology is used for process automation, and even reliable and robust communication via fieldbus can be affected. To implement protective measures for fieldbus communication that are both effective and efficient, it is vital to pinpoint the actual level of availability and potential causes of failure.

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The tree symbolizes the multitude of connections from field instrumentation to control and management systems.
The tree symbolizes the multitude of connections from field instrumentation to control and management systems.
(Picture: Pepperl+Fuchs)

So what is the experience with availability of fieldbus installations in practice? And which potential causes are actually responsible for failures? Without knowing the answers to these questions, it is hardly possible to increase fieldbus availability long-term by adopting specific protection measures. However, availability calculations are based on descriptions, assumptions, and observations from the theory of probabilities. And here at Pepperl+Fuchs, we have discovered from many years of interaction with users that these calculations are derived from sometimes unrealistic, sometimes even plain incorrect, assumptions. As such, the results are often not a true reflection of reality either.

Discrepancy Between Theory and Practice for Fieldbus

It is often, and wrongfully, the case that only the probability of component failure (i.e., the inverse of the actual failure rate) is used to calculate availability. The probability of component failure represents the potential for random failure due to aging and wear of the component. However, this means that important systematic criteria are not being taken into account. In practice, these criteria play a decisive role in availability: When environmental influences and the mode of operation, and their effect are not accounted for in the calculation, a significant discrepancy arises between the mathematical theory and the effect in practice when it comes to process automation.

However, a brief glance at alarm and failure statistics makes it very clear that it is precisely the effects of the mode of operation and environmental conditions that are responsible for faults much more frequently than random faults that cause component failure.

How to increase Fieldbus Availability?

To increase availability long-term, it is therefore crucial to identify the typical causes of faults. As part of a long-term project fault conditions and their causes and effects on the fieldbus infrastructure were studied. The results of the project indicate that systematic failures are the most likely causes of failure in plant operation. In addition to the poor design and planning of segments or failures in the installation, typical causes of failure include:

  • Shorts circuits, contact bounce, and overloading that occur as a result of work on the segment.
  • Surges caused by lightning strikes and a reduction in the performance of lightning protection that has previously gone unnoticed.
  • Water penetration in field devices and junction boxes as a result of environmental influences or mode of operation, i.e., through heavy rainfall, condensation due to high humidity levels, use of high-pressure cleaners, and incorrect or faulty seals.

If the assessment of fieldbus technology is reduced only to competition among manufacturers to provide lower failure rates, the assessment is not accounting for maximum availability and it thus puts the user at a disadvantage.

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