Cost-effective explosion protection for industrial wireless networks

Wireless Ethernet in hazardous areas is on the rise with integration of explosion-proof enclosures for wireless devices.

By Kerstin Wolf September 29, 2020

Operators of process technology systems seeking to implement wireless device communication in hazardous areas have had few options to choose from when selecting suitable access points because most industry-specific models with Ex approval do not offer high-performance, central network management. Products with this feature often are not suitable for industrial environments. An option is integration of standard devices without Ex approval in explosion-protected enclosures. Compact high-frequency isolators for intrinsically safe wireless signal conversion make it possible to use standard antennae in Zones 1 and 2.

Since wireless networks became an integral component of Industry 4.0, particularly in logistics, machine maintenance and repair, and production monitoring and control, wireless communication networks have been on the rise in the process industry, too. The advantages of this technology are apparent in temporary installations, when equipping sensors and actuators in remote plants, and in process control with mobile end devices. One relevant application is asset management, where wireless measured value determination can simplify the process of monitoring the condition of production equipment and improve predictive diagnostics.

Industrial wireless standards

Different wireless standards meet industrial requirements and offer different advantages depending on the intended purpose. For many static sensor/actuator connections with low data volumes, sub-GHz or 2.4 GHz technology such as WirelessHART, ISA100.11a Wireless Systems for Automation, LoRaWAN from LoRa Alliance or Low Energy Bluetooth are ideal. For communication with separate production units or mobile devices, technology such as 3G, LTE, NB-IoT or, in the future, 5G are more suitable.

For fast, broadband data transmission between mobile participants over limited terrain, however, powerful Wi-Fi solutions in accordance with the IEEE 802.11 Ethernet protocol series of standards are the solutions of choice. They enable the interruption-free transfer of network participants within from one access point to another within production areas.

Wireless challenges in hazardous areas

However, since all wireless-capable equipment, such as Wi-Fi access points, mobile wireless components and radio frequency identification (RFID) readers and antennae, always represent potential ignition hazards according to applicable explosion protection directives, suitable ignition protection that includes the interfaces is required to enable use in hazardous areas. Electromagnetic fields cannot, as a rule, ignite an explosive mixture as long as the ignition source does not work with a transmission power of several hundred watts.

However, there is a risk the fields could induce currents in insufficiently EMC-protected metal items or electronic switches and these currents could cause sparks to form. As a result, the international standard IEC EN 60079-0 defines limiting value requirements for wireless signals in hazardous areas. As well as compliance with limiting values in normal operation, this standard requires a malfunction inspection for devices in Zone 1, which accounts for the explosion hazard posed by short circuits, shunts or interruptions.

Conventional explosion protection, high-frequency signals

While the production industry has a wide range of suitable network components to choose from, process technology plant operators, who need to take explosion protection into account when looking for suitable equipment, face unique challenges. The wireless components required for operation in Ex Zone 1 are mostly protected by special enclosures.

To use standard Wi-Fi devices without special approval in hazardous areas, they need to be installed in a flame-proof enclosure or an enclosure encapsulated using an overpressure; this enclosure must meet the requirements of the Ex d or Ex p type of protection.

However, this does not address the issue of antennae because most encapsulated enclosures of this kind are made of metal, which blocks the majority of the electromagnetic radiation from the antenna. With a directional antennae, RFID readers or motion detectors can be positioned in Ex d housings behind a glass pane.

With omnidirectional radiation, a limited number of Ex certified external antennae have been available. The designs, which are determined by the need to provide explosion protection, do not include the standard plug connectors that are needed, making installation and maintenance more difficult. But to guarantee that the wireless connection functions reliably, using an antenna suitable for the application is essential.

Safely connect any industrially-capable antennae

Based on a comprehensive range of enclosures and suitable HF isolators, solutions for use of conventional Wi-Fi hardware in Ex Zones 1 and 2 are available to accompany conventional antennae. To safely encapsulate potentially hazardous Wi-Fi access points in Zone 1/21, the explosion protection expert has produced flame-proof enclosures with project-specific dimensions for devices from manufacturers like Cisco and Aruba with Ex d type of protection.

A separate connection chamber with Ex e protection enables CAT cables or fiber optics to be connected easily and quickly. This is also enabled by the Ethernet terminals and FO splice cassettes for use in Zone 1. The enclosures have been tested and certified for global use have intrinsically safe bushings that allow the external connection of any industrially-capable antennae. Compact, high-frequency isolators are available for frequency ranges from 150 MHz to 8 GHz; these isolators support Wi-Fi and a range of other wireless standards such as Bluetooth, LoRaWAN, WirelessHART and ISA100.11a. The high-frequency isolators prevent low-frequency signals from being transmitted in case of an error and ensure the external antennae only receive signals at an intrinsically safe level in accordance with Ex ia IIC. Such solutions are easier to install and handle because the wireless devices and external antennae don’t need to be connected using inflexible wires without plug connectors. Instead, they use preassembled connecting cables that can be connected and disconnected when necessary. An even easier option is attaching the antenna to the enclosure bushing.

Using standard IT wireless devices in hazardous areas

Many companies already use Wi-Fi networks in administration and logistics departments. Since the advantages of these wireless networks are greatest when they are standardized throughout the company and can be managed centrally, IT specialists typically recommend using consistently coordinated hardware. Another hurdle is avoiding additional work and expenditure caused by changing supplier or by changes to maintenance strategies – a transparent aim. As a result, many users are keen to use identical devices in their production areas when extending their Wi-Fi network. IT devices generally do not have Ex approval; in the past, this has meant these devices had to be additionally protected by an Ex d enclosure for use in Zone 2.

New enclosures for Zone 2

For installations in Zone 2, operators previously had to use enclosures and configurations almost identical to those used in Zone 1, incurring the same costs and requiring the same installation work.

Restricted breathing enclosures are plastic housings designed with Ex nR type of protection and restrict the ingress of explosive atmospheres to a safe extent. They have a separate Ex ec protected connection chamber that minimizes the effort involved in connection like Zone 1 housings.

By protecting IT equipment with a separate connection chamber, the user only needs to use the connection chamber. The plastic enclosures are available in a range of sizes. In comparison with Ex d solutions, they are up to 70% lighter and significantly easier to install. Enclosure material enables installation of wireless modules with internal antennae such as many Wi-Fi and RFID readers.

Enclosure advances extend wireless use

Protective enclosures and connections are available for antennae for wireless installations in hazardous areas in Zones 1 and 2.

Kerstin Wolf is press information officer, R. Stahl; Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media and Technology, mhoske@cfemedia.com.