Almost no one in the industry associates wireless with safety, in particular safety systems, even though wireless is often used with these types of devices for diagnostics and maintenance. Wireless is certainly used with such devices as gas detectors and personnel monitors, communicating back to security or a staffed location to track man-down situations.
ISA 84, “Instrumented Systems to Achieve Functional Safety in the Process Industries,” addressed wireless and safety as far back as five years ago with Technical Report (TR) ISA TR84.00.08:2017, “Guidance for Application of Wireless Sensor Technology to Non-SIS Independent Protection Layers.” Safety practitioners understand that an underlying principle of safety systems is independent protection layers, so this TR provides useful information about using wireless in safety infrastructures.
The TR addresses wireless technology-based fixed, portable and traveling sensors. They're used in independent protection layers to provide a risk reduction factor of less than or equal to 10 (non-SIS IPL), and establishes guidance and considerations for their utilization in the process sector. The use of wireless technology for higher risk reduction factors, such as a safety instrumented functions (SIF), has been excluded from consideration in the scope of this technical report.
The guidance provided in the document is applicable for periods of time when the sensors are connected to the network and covers the following topics:
- Wireless sensor network design: layout, availability, security, purchasing and commissioning;
- Operations and maintenance: spectrum management, network performance, power and security; and
- Management of change.
One of the main concerns with wireless networks is their integrity. Typical network management techniques include the following defense mechanisms to counter radio interference:
- Direct sequence spread spectrum,
- Channel hopping,
- Multi-path routing, and
- Channel blacklisting.
Though ISA TR84.00.08 was prepared under the auspices of the ISA-84 safety standards committee because it was concerned with availability and reliability of networks, this roughly 30-page document is a very good guide of things to consider for any wireless network installation.
Norwegian research organization SINTEF, in its 2015 report, A26762 “Wireless Instrumentation for Safety Critical Systems,” came to several conclusions regarding the application of wireless safety.
It found that modern wireless techniques, such as ISA100 and WirelessHART, are capable of handling safety data. ISA100 can be used “as is,” whereas WirelessHART will need to announce safety features in the official standard.
Main performance limitations inherent to wireless SIS are packet loss and low data bandwidth. These limitations can be overcome with consideration of the system's physical environment.
A serious challenge for battery-powered applications is energy consumption at high update rates. Rapid update cycles require the radio and sensing circuitry to be active for prolonged periods of time and will drain the batteries. Short battery replacement cycles will tend to negatively affect the usability of the safety system.
Both ProfiSafe and CIP Safety support wireless communication networks.
The black channel concepts of IEC 61784-3 used for safety fieldbuses should also be applicable to wireless because the intent of black channel is to separate the message from the media or how it's transmitted between the two end points. Of course, if you're mindful of the items describe in the ISA TR84.00.08 document, this will certainly help provide the reliable backbone for a black channel implementation.
About the author: Ian Verhappen
Ian Verhappen is the senior project manager of Automation at Willowglen Systems Inc. He can be contacted at [email protected].