How Can We Install Wireless Transmitters in Hazardous Areas?

Before implementing wireless transmitters in hazardous areas, it is necessary to define the area. Instrumentation commonly used in factory automation and the process industries typically requires adherence to UL Class I, Div. 1 or 2. For areas where ignitable substances such as gases, vapors, or liquids are likely to exist only under abnormal conditions, as classified under Class I, Div. 2, instrumentation rated for the area should not require an explosion-proof housing.

...For areas where these substances can exist under normal conditions, as classified under Class I, Div. 1, instrumentation must be either contained within a suitable explosion-proof housing, contained within a suitable purged housing, or rated as intrinsically safe.

...Wireless products are available with Class I, Div. 2, Groups A, B, C, and D ratings that can be incorporated into this type of hazardous location without the need for expensive explosion-proof housings, for example, our new OS2400-485 spread-spectrum wireless radios and 9xxMB Modbus I/O Products. However, if the area is classified as Class I, Div. 1 and the radio is not rated as intrinsically safe, then a special housing will need to be considered as

referred to above.

In the U.S., the National Electrical Code (NEC) covers the requirements for electrical and electronic equipment and wiring for all hazardous locations. Different approval agencies have different standards to evaluate equipment installed in hazardous locations. For example, ANSI/UL 913 is the Underwriters Laboratories (UL) and American National Standard (ANSI) standard covering intrinsically safe apparatus and associated apparatus for use in Class I, II, and III, Div. 1 hazardous locations.

...In general, in a Class I environment, you can use explosion-proof, purged and pressurized, intrinsically safe, or non-incendive techniques to install equipment in hazardous locations. Due to the nature of the performance and power requirements of radios, it is very difficult to get an intrinsically safe Class I, Div. 1 radio. However, if you install a radio in an explosion-proof enclosure, it is very difficult to get the RF signal to the antenna in a Class I, Div. 1 environment.

...We have extensive experience installing equipment in hazardous locations. By using special techniques in integrating a spread-spectrum radio and a smart multivariable transducer, our AutoGraph wireless transmitter can be installed in a Class I, Div. 1 environment. A single box will contain the flow computer and communications equipment as well as power components. Directly mounting the flow computer on the meter run with all necessary components in one box eliminates the cost of additional housings, as well as associated cabling and barriers.

Our products have been certified for use in Class I, Div. 2 areas in North America. If your application is in a Class I, Div. 1 area (or in other countries), you can wire out of the Div. 1 area to a Div. 2 area, or use a "flame-proof" or a pressurized/purged enclosure inside the Div. 1 area.

...Non-metallic pressurized enclosures are ideal for wireless applications, using a small whip antenna inside the enclosure. The non-metallic box decreases radio range, but not significantly.

...With flame-proof enclosures, an external antenna is required. The antenna is connected to the wireless unit by a length of coaxial cable. An antenna is a passive element like a wire, and requires protection against mechanical abuse like a wire. Mounting the antenna inside a heavy-duty PVC pipe is a common way of achieving this requirement. The PVC pipe reduces radio range, but again, not significantly.

Hazardous areas are increasingly being targeted for wireless communications as wire-based communications solutions can be sub-optimal due to the difficulty of getting to these areas and the costs associated with the installation. But many of these industrial environments prove difficult for RF-based solutions due to interference from motors, steel tanks in the environment, or other wireless communications such as radios.

...Recently vendors have begun to embed wireless networking solutions within their sensing and control devices, making it easier to deploy wireless transmitting/receiving devices in difficult environments. The sensing and control devices, such as temperature sensors integrated with wireless communications, are simply deployed and the wireless device identifies its location in the network and puts itself into operation.

...The most currently available, optimal wireless solutions operate in the license-free spectrum ISM band of 915 MHz and employ mesh networking algorithms for a highly robust and reliable communications network in difficult environments. With each node serving as a router and repeater, mesh networks have the ability to intelligently route data around interference and challenging RF conditions to ensure the data reaches its destination, eliminating the problems associated with point-to-multipoint "star" topologies and single-point-of-failure issues.