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Fault exclusions - an underestimated risk

Requirements for safety switch and key transfer systems


Fault exclusions - an underestimated risk



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In addition to functionality and functional safety, costs play a decisive role in the competitiveness of machines and plants. However, despite all cost constraints, the critical view of risky compromises and the associated incalculable follow-up costs must not be diminished. For example, fault exclusions for the sake of apparent cost savings involve a significant risk potential.


When developing a safety system for a plant or machine, the suitability for a specific application must be taken into account when selecting and assembling the components. This is determined not only by mechanical features, such as stability and choice of the right materials, but also by normative requirements and safety aspects, such as structure (category), reliability (MTTFd) and fault detection mechanisms (DC). However, designers and planners often come up against limits in this regard, which they can only eliminate by excluding a fault.



Why fault exclusions?


According to section 7.3 of EN ISO 13849-1, fault exclusions are a compromise between the technical safety requirements and the theoretical possibility of a fault occurring. Before considering fault exclusions in a safety system, the reasons for this should be well considered. It should be noted that in accordance with the general principles of the Machinery Directive 2006/42/EC, the state of the art must be observed.


Safety door protection

Purely mechanical safety door protection


Under certain conditions it may be necessary to exclude faults.
Fault exclusions can be based on
- the extremely low probability that these faults will occur,
- generally recognized technical experience
- and the technical requirements with regard to application and hazard.
The risk is then carried by the designer.


However, the situation is different if fault exclusions are made for financial reasons, e.g. by using cheaper components, although a technical solution exists without fault exclusion.



Consequences of fault exclusions


One should always be aware of the consequences of fault exclusions: The person who claims a fault exclusion can be held liable in the event of a fault occuring. In addition, the reasons for the fault exclusion must be listed in the documentation and enclosed with the machine or plant. This would be a clear disadvantage compared to the use of products that do not require fault exclusions.


The consequences, if an excluded fault should nevertheless occur, cannot be neglected in any case. Not only with regard to the direct consequences, but also with regard to machines and plants already delivered. This is best demonstrated by the well-known reports on recall and retrofit campaigns in the automotive industry.


Machine manufacturers who use procured components with fault exclusion should be aware that they are liable in the event of a fault!



Better without fault exclusions


Electromechanical safety door protection

Electromechanical safety door protection with release of 6 keys for the opening of wirelessly safeguarded side doors

For this reason Dold has developed a safety switch and key transfer system without compromises - SAFEMASTER STS. This TÜV-certified system, which is available in stainless steel and fibre reinforced polymer, is suitable for use in safety applications up to category 4 / PL e according to EN/ISO 13849-1 - without any fault exclusions.


This does not relieve the designer or system planner of his responsibility, but at least makes the decision easier, because with SAFEMASTER STS he is on the safe side - without fault exclusion!


In addition, SAFEMASTER STS offers further important advantages, such as its modular design and expandability and thus flexible adaptation to the user's application. SAFEMASTER STS combines the advantages of safety switches, guard locks, key transfer and command functions in one system. The modular, expandable system with the possibility of safeguarding doors wirelessly reduces installation, retrofitting and maintenance costs for the user. And with the stainless steel version, the system offers a solution that can withstand even the roughest environmental conditions.





If machine designers want to minimize the liability risk for their machines and plants, the use of components for the highest safety requirements is recommended. The components should reach the required performance level without fault exclusions being necessary. Regardless of the acquisition costs, this strategy proves to be a more cost effective option in the long run, as it protects against incalculable risks of fault exclusions and the associated possible recourse claims.


Safety switch and key transfer system SAFEMASTER STS

Safety switch and key transfer system SAFEMASTER STS for applications up to category 4 / PL e

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