Using Spare, Standard I/o Enhances Functional Safety While Reducing Costs And Footprint

Phil Black - PII Editor 06/03/2017

84 2 minutes read

Listen to this article

Cranes, industrial elevators and woodworking machines, as well as those countries not using international standards, can improve functional safety by capitalising on unused, standard I/O channels

By using the spare, standard I/O channels within ABB’s AC500 programmable logic controller (PLC), the functional safety of machine applications can be cost-effectively enhanced, says a new report from ABB. Using the PLC, together with ABB’s AC500-S safety central processing unit (CPU), incurs no extra control system costs by avoiding additional safety I/O modules or an increase in cabinet dimensions.

Titled “ Usage of AC500 Digital Standard I/Os in Functional Safety Applications up to PL c (ISO 13849-1)”, the report shows how the concept benefits machine builders that do not always conform to either functional safety standards, ISO 13849-1 or IEC 62061.

This includes those operating in some countries which have not embraced these standards. Machine builders in these regions can now readily increase safety of machines without significant investment, opening the opportunity to export

Various crane types, industrial elevators and woodworking machines often operate at the least demanding safety integrity level (SIL); SIL 1 for those working to comply with IEC 62061 or performance level (PL) PL c for those conforming to ISO 13849-1.

PLCs, such as ABB’s AC500-S, that are typically used in these applications, have up to 32 digital standard I/O channels, yet often not all are used. Functional safety is instantly enhanced by adding a safety CPU, in addition to the standard module, and dynamically testing the spare I/O channels.

This saves up to 30 percent of the cost of building safety in from the outset. The safety CPU is used for safety logic processing and diagnostic measures for digital standard I/O modules.

“Digital safety input channels should always be used even for functional safety applications up to PL c (ISO 13849-1),” says ABB’s product manager, Yauheni Veryha.

“However, using digital standard I/O channels is still an option if they are actually available and their reuse for functional safety functions up to PL c is needed to satisfy specific customer requirements. This could be due to limited space in the control cabinet or too high costs to add digital safety I/O modules.”

ABB’s report goes on to prove the concept of using digital standard I/O in functional safety applications and describes the methodology of proof. The results of its safety analysis confirm that the AC500 digital standard I/O channels can be used in functional safety applications up to PL c (ISO 13849-1).

However, the implementation, verification and validation of the approach in practice according to ISO 13849-1 and/or IEC 62061, remains application specific. This is because standard (non-safety) AC500 modules become involved in the execution of functional safety functions.

The responsibility for correct implementation, verification and validation of the proposed approach remains the end-user’s responsibility.

In special cases, the AC500’s digital standard I/O modules can be used in functional safety applications up to PL d (ISO 13849-1) and SIL CL 2 (IEC 62061). However, more detailed functional safety application analysis is required.