Long proof test intervals for SIL overspeed detection systems

Proof testing is a routine action that helps ensure the integrity (SIL level) of a safety instrumented system (SIS) throughout its lifecycle. Long proof test intervals are preferable to short proof test intervals because they reduce unnecessary downtime, especially if the proof testing can be aligned with a machine’s periodic maintenance procedures.

Proof test intervals

Proof testing is a routine action that is critical to ensuring the integrity of a safety instrumented system (SIS) throughout its lifecycle. For any SIS such as a SIL overspeed detection system (ODS), proof testing must be performed at a specified interval, known as the proof test interval (PTI).

A SIS with a long proof test interval is a system that remains safe to operate over a longer period of time, or in other words, the integrity of the safety system degrades more slowly over time. This is a very important consideration for owners and operators of machinery because it means that their operational/production processes can continue to operate for longer without being interrupted for proof testing.

Read more about maintenance and proof testing for ODS here

Safety integrity degradation over time

Even though a full proof test will theoretically restore a safety instrumented system (SIS) to an “as new” condition, the integrity of any safety system always degrades over time. This gradual degradation is both an increase in the probability of failure on demand (PFD) and a decrease in the safety integrity level (SIL).

As shown in the image below, without intervention in the form of a proof test, a safety system will eventually reach an unacceptable level of PFD which results in a downgrade of the its status (for example, from SIL 3 to SIL 2).

It is worth noting that because proof testing is a manual intervention, it is prone to human error, which gives systems with a long proof test interval another advantage over those that must be tested more frequently.

In fact, if the proof test interval (PTI) of a SIL overspeed detection system is longer than the recommended maintenance intervals for the machinery being protected, then it is easy to align the proof test interval with the machine’s routine maintenance procedures without impacting the system safety integrity, thereby avoiding unnecessary downtime.

Read more about Safety integrity levels for overspeed detection system (ODS) here

Long proof test intervals

A long proof test interval is achieved primarily through a fundamentally simple yet high-quality product or system that concentrates on core safety concerns. It needs to be well-thought-out and carefully designed with minimal non-safety related functionality, that is, a lean safety system design.

Safety certification agencies such as Exida or TÜV review product design, reliability, failure rates, diagnostics features and other requirements as per IEC 61508 while certifying a product. As a result of these design and certification efforts, a SIL 3 rated safety system with a long proof test interval ensures a highly-reliable solution without any need to worry about maintenance in the short term.

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