Industrial Ethernet / Intrinsic Safety / Safety Instrumented Systems / Wireless

How to do wireless for safety systems

ISA TR 84.00.08 offers some important guidelines for making it work

By Ian Verhappen, P.Eng

Continuing last month's discussion on wireless standards, there's another recently developed standard from ISA related to the use of wireless systems from ISA-84 Working Group 8. The technical report (TR) developed by Gary Laframboise and the team is analogous to the similar document developed in 2009 for fieldbus for safety systems by Working Group 6.

The scope of ISA84 Working Group 8 was to establish guidance and considerations for the use of wireless in safety applications in the process sector with application to both the sensor network as well as associated wireless backhaul. Though, like the balance of the ISA84 documents, this TR was developed for the process sector, many of the principles are relevant for many industries and sectors. This is because good design principles are application independent.

One consideration for any wireless network when compared to a wired system is latency. Latency is an important factor in safety systems because, in many cases, there's a limited amount of time to react to a change in the process where end-to-end response from input to control element becomes even more critical. Consequently, the TR emphasizes the importance of time-stamping communications to verify data latency and provide confirmation of the overall loop-response time.

Wireless signals are also affected by system layout/geography, including the environment in which they're placed because installation around a distillation unit will be different than in a tank farm or pump alley. The TR describes mechanisms to determine suitable ranges and associated best practice considerations to improve system reliability for mesh and point-multipoint network configurations. Typical security threats in each of the areas of authentication, authorization and accountability are identified, along with standard practices and measures used to manage those threats and improve overall system reliability.

Even though field sensor networks are based on intelligent field devices and a well-designed network, this doesn't mean the network has 100% availability.

TR 84.00.08 treats commissioning and the site acceptance test interchangeably with commentary and guidance provided on the initial start-up and testing of data transport, data latency, bandwidth and interfacing with other portable or temporary devices, which could impact these elements of the system as evaluation criteria during this phase of the system lifecycle.

As with any safety system, management of change is an important/critical element of assuring overall system integrity, and therefore, suggestions are made in the areas of spectrum management, adding/removing devices, configuration management including application programming, and firmware management regarding how to perform these tasks with minimal negative impact to the overall system reliability.

Even though field sensor networks are based on intelligent field devices and a well-designed network, this doesn't mean the network has 100% availability. As a result, important elements in understanding the overall health of the system are the diagnostics of the individual devices and nodes as well as regular testing of the system. When changes are identified, the associated notification of change in status of not only the devices, but also the network and spectrum communications needs to be captured. This allows them to be analyzed to determine root-cause failures, so overall system reliability will improve over time.

The document concludes with a number of key performance indicators (KPIs) for availability, throughput, capacity, latency and other related parameters critical to monitoring and predicting the health of wireless sensor networks. Having actual KPI values that can be trended provides another method of increasing overall reliability because they allow you to gauge if the situation with your system is improving, deteriorating or remaining stable. You'll expect the system to be stable or perhaps deteriorate over time without any change, however, having KPIs will allow you to confirm if system changes intended to improve response/reliability do provide intended results and are useful.

Though developed for wireless in safety applications, many items identified in the document should be considered good practices for any wireless sensor network. When they're published, if you're planning to implement wireless as part of your facility"s control infrastructure, you'll likely want to add them to your bookshelf.