My ControlGlobal – The Great Kanduski: Best Practices in Industrial Networking RSS feed alerted me to a post, Antennas – Wireless Achilles Heel? As the title implies, it raises the question about antennas on wireless devices when installed in hazardous areas. The post opens:
While attending the WirelessHART seminar a while back we had a discussion about potential vulnerabilities and challenges with Wireless devices in classified areas and surprisingly or upon further consideration maybe not so much of a surprise, one of the real challenges faced by manufacturers is how to make the antenna explosion proof. When you think about it, an antenna is really an extension of the circuit (a wire) outside the explosion proof enclosure and hence an electrical path across the explosion proof boundary.
I ran this post by Ted Schnaare, the Director of Engineering for Emerson’s Wireless team for his thoughts. He responded:
The key to answering this question is to understand the various types of explosion protection. In the US, we use the term ‘explosionproof‘ to describe equipment enclosed in a rugged enclosure that is capable of withstanding an internal explosion and preventing the ignition of a specified atmosphere surrounding the enclosure. It is possible to design an antenna that is rugged enough to be a part of an explosionproof enclosure. Antennas of this type are often fully potted or enclosed in thick polymer radomes with tight fitting mechanical interfaces to the other parts of the enclosure (such as a threaded interface). There are at least a few of these on the market including the one pictured.
Intrinsic safety is another method used to safeguard the use of electrical equipment in a hazardous location. This term is used to describe a method of protection based on the restriction of electrical energy within apparatus and of interconnecting wiring exposed to the potentially explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects. Since no rugged high strength enclosure is required for intrinsic safety, most types of antennas which are generally passive, non-energy storing components are suitable.
Each of these protection methods has some downside when it comes to installation practice. Explosionproof equipment must generally be installed using heavy metal conduit or extra rugged cable. Intrinsically safe equipment can generally be installed with standard ordinary location wiring practices but an intrinsically safe (I.S.) barrier must be used to ensure that the electrical energy levels are maintained below safe levels. Both of these wiring practices can be costly, time consuming, and require special expertise.
One of the great things about self-contained wireless equipment is in the wiring savings. This is doubly true in a hazardous location which requires the specialized and costly wiring mentioned above. Most Emerson Smart Wireless devices are self-contained and intrinsically safe. This allows them to be installed in a hazardous location without the need for either heavy metal conduit or I.S. barriers.
Thanks for sending me this explanation to pass along to the readers of this blog, Ted!