WLAN in Hazardous Areas
05 September 2008
Wireless Data Transmission Offers the Advantage of Greater Mobility.
The transmission of data within Wireless LAN (local area networks) is becoming increasingly important because of the undisputed advantage of mobility.
Bartec MC 9090ex handheld terminals
Already there are familiar wireless applications in office computer networks. However, this technology is also being used in the process engineering industry – in the field – to control materials and processes and for the maintenance and upkeep of machinery and plants.
Typical fields of application are warehouse logistics, recipe administration in production, and the conducting of repetitive checks on systems requiring monitoring. The devices used for data acquisition are usually mobile devices that communicate wirelessly with a higher-ranking ERP system. Applications in process engineering industries often concern hazardous areas, where the risk of ignition necessitates the application of additional safety requirements to the design of the networks and mobile and stationary devices.
Operators of Ex plants were sceptical about WLAN at the beginning; after all, just a few years ago, there were hardly any hardware components on the market. Today the industry is very open to new technology and this trend is proven by the greatly increased demand for explosion–protected mobile computers.
Mobile data terminals (such as Bartec’s MC 9090ex) are used in conjunction with bar code scanners to control the flow of materials and administer recipes. Stationary operator terminals with WLAN interface are used for process control at batch plants to produce active ingredients for pharmaceuticals.
REQUIREMENTS FOR WLAN IN EX AREAS
A WLAN consists of at least one or more access points that communicate via Ethernet. The available data throughput depends on the distance between the access points. The quantity of access points needed and their positions are determined using radio measurement and taking account of local factors (building or open space). This is based on the required data throughput and the number of clients. Since usually only small quantities of data (storage locations or recipes) are transmitted, the data transfer rate in Ex areas is lower than office environments.
Apart from sparks, flames, or hot surfaces, electromagnetic waves also rank as potential sources of ignition and are subject to specific limit values. Technical report CLC/pr TR50427 or Clarification Sheet ExNB/04/160/CS describe the ignition limit values at receiver systems for continuous radio frequency sources.
More than 90% of all WLAN installations in the Ex area are found in zone 1, which makes it necessary to use explosion-protected measuring instruments when taking measurements. A practical alternative is in some countries a special permit in conjunction with a gas warning device.
DEVICES FOR WIRELESS COMMUNICATION
Two different antenna techniques are available for the access points. Where the internal solution is used, flat antennas are fitted into an enclosure with Ex d protection. The lid of the flame-proof encapsulated enclosure has a glass pane. Functionality is assured in spite of attenuation from the pane and metal enclosure.
As an alternative, two external antennas with the appropriate Ex examination can be connected. These have higher radio power.
Still another variant consists of installing the access point outside the hazardous area and the external explosion-protected antennas in the hazardous area. Electric equipment (stationary or mobile) for use in zone 1 is subject to the ATEX Directive 94/9/EC and must comply the requirements for category II 2G. The Bartec handheld devices are designed for temperature class T6 and explosion subgroup IIC.
1. IEEE 802.11, safe WLAN, directive 03103, version 1.0
2. Various radio standards under IEEE 802.11 work on two different frequencies in general. Transmission powers of 100 mW in the 2.4-GHz range and of 30 to 1000 mW in the 5-GHz field allow ranges of 10 m to 70 m in buildings and 30 m to 300 m in the open air.
3. Up to now, the 802.11 protocol has been most successful in establishing itself. It also offers compatibility with the 802.11b standard.
4. The 802.11a standard, on account of its poorer ranges and associated higher costs, is only recommendable in the case of a permanent fault in the 2.4-GHz field.
In the future all devices are to conform to the safety standard 802.11i because this is the only standard that provides reliable data transmission. At present, the great number of proprietary protocols can sometimes lead to incompatibility between different manufacturers.
Phone: +49 (0)7931 597-361
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D-97980 Bad Mergentheim
Benedikt Eckert – Mobile Computing
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