Wireless for SIS?
I recently received a request from a colleague asking for more information regarding the position of the ISA 84 committee on the use of wireless in SIS applications. He had heard that the “rules” preventing the use of wireless in SIS had been relaxed. This is my reply…
There are no changes in stance of ISA 84 committee at this time (05 Nov 2013). Of course, the ISA committee on wireless is only putting together “Technical Reports”, and are not developing material that is providing normative requirements in the same scope of IEC 61508, which generally defines this field of study. Contrary to most discussion, the core standard (IEC 61508) has never disallowed wireless. The notion that wireless is forbidden was never true, it was only generally assumed based on most people’s gut reaction to the use of (at this point in time) unproven wireless systems in critical safety applications. Most of the safety communication protocols that are used by equipment vendors are “medium agnostic” meaning that it really doesn’t matter what the signal travels on because all of the safety is in the sending mechanism and the receiving mechanism. The sending and receiving equipment are equipment with elaborate and comprehensive diagnostics. Failures of wireless systems are virtually 100% detectable in a millisecond time frame, as such, safety is not an issue at all.
There are two real reasons why people don’t currently use wireless for safety (much).
No vendor (that I am aware of) has engineered and certified a wireless solution. General purpose wireless solutions are not designed in accordance with IEC 61508-2, 3, and as such are not allowed in safety applications. A safety “certified” set of equipment is not available to my knowledge. You can’t just put a cisco wireless router from Best Buy in the middle of a safety loop, it would need to come from the vendor of a complete solution.
Nuisance trips. While failures are very detectable, they generally need to result in a vote to trip. At this point in time, wireless failures are so frequent that the impact of nuisance shutdowns precludes the use of wireless systems in most SIS applications.