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Intrinsically Safe Ethernet for the Hazardous Area

Fri, 08/29/2008 - 7:36am
A comparison of existing explosion protection and intrinsically safe Ethernet applications

By Andreas Hennecke Marketing Manager – Fieldbus Technology Pepperl+Fuchs

Fast communication based on Ethernet is standard in the automation industry and results in a highly synchronized production operation.

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Table 1: Name of Ethernet Networks
Explosion protection is one hindrance for wide application of Ethernet in process automation where existing methods are protected installation, wireless LAN and fiber optics. Intrinsically safe Ethernet now offers high speed in combination with the live – work benefits and simplified maintenance procedures of intrinsic safety where the connection and disconnection of cabling is possible in the hazardous area of a process plant without a hot work permit. This paper compares existing methods for explosion protection and intrinsically safe Ethernet with a practical view on the advantages, disadvantages and use cases of each method.

Introduction

The process industry is known for their less dynamic processes compared to factory automation. For standard measurements such as flow,
Figure 1: Operating terminal communicates via Ethernet in the hazardous area.
Figure 1: Operating terminal communicates via Ethernet in the hazardous area.
level, and temperature, modern fieldbus technologies such as PROFIBUS and FOUNDATION Fieldbus are available with their low transmission rates compared to Ethernet. These fieldbus systems combine power and data transmission on the same cable with explosion protection. When compared to Ethernet, however, these fieldbus systems operate at relatively slow speeds.

The demand for higher data transmission rates exists for selected applications. These applications involve operating terminals and infrastructure for information technology, as well as apparatus for analysis and other multifunction units with large amounts of data to transmit. Due to the high density of information in these applications, a significant increase in transmissions capacity is required. Large I/O units such as Remote I/O can be located in the hazardous area. Modern day DCS systems will bring Ethernet-based applications into the field. New standards have been established such as PROFINET or FOUNDATION Fieldbus HSE (High-speed Ethernet). As a side benefit, these protocols enable peer-to-peer communication in the field. For high-speed fieldbus systems, explosion protection is a basic demand. This allows connections to be opened without the need for a hot work permit.

High data rates are transmitted via shielded or unshielded twisted pair via Ethernet. Development of transmission rates and their respective names are listed in Table 1: Names of Ethernet Networks. Ethernet is defined in IEEE 803.2. The most common standard in industrial automation is "Fast Ethernet" with 100 MBit/s. For the purpose of this article, the name Ethernet is used for all speeds used in automation technology.

Figure 2: Ethernet isolator integrated into mobile apparatus.
Figure 2: Ethernet isolator integrated into mobile apparatus.
In practice, three solutions, each with distinct benefits and drawbacks, have established themselves for process automation. Three media types are available regardless of the transmission rate.
• Standard multi-conductor cable, with or without shield
• Glass fiber optics ?• Radio transmission via wireless LAN ?The following requirements are typically taken into consideration:
• Explosion protection – working on running machinery and equipment without a hot work permit.
• Appliance permanently installed in process automation.
• Mobile devices that must be moved within a hazardous area.
• Temporary installations of appliances and machines that must be modified or relocated at varying intervals of time.
• Specialized knowledge needed for installation, commissioning and maintenance.
All solutions are described with their specific benefits. Table 2 at the end of the document gives an overview and comparison of attributes, both technical and methodical for physical layer solutions for Ethernet.

Increased Safety

This is the classic option for Ethernet in the hazardous area. There are special requirements to be considered for planning and during installation. The routing of explosion-protected Ethernet networks must be in compliance with IEC 60079-14 and meet several requirements for the electrical installation. In general, the network
Figure 3: Remote I/O with Ethernet interface
Figure 3: Remote I/O with Ethernet interface
cable shall be safeguarded against unintentional interruption or connection. Plus, the routing shall be designed such that the cable cannot be damaged by mechanical influences or aggressive atmospheres. This is a reliable technique in data transfer. Grounding and shielding protect the network from electromagnetic interference. The engineering hardware suitable for industrial environments is available to a large extent. This type of installation is meaningful, in particular for permanently attached machine-mounted accessories like Ex e-devices, measuring equipment and operating terminals, where work is not normally performed during operation.

Wireless LAN

In the case of wireless LAN, there is no wiring because only the stationary transmitting and receiving stations, the access points, can be mounted in the hazardous area. This is thus the most straightforward method for the installer. Function and explosion protection are assured by a suitable product design. Ex em or Ex q protected housings with intrinsically safe antennas are normally used. The software support makes wireless LAN easy to handle. Planning and expanding a wireless LAN network always includes "illuminating" the radio set-up by conducting measurements. This is due to the fact that the maximum range in buildings is limited considerably by concrete walls, especially when these are interspersed with metal structures. Large-size metal objects like vessels, tanks and pipes are impenetrable by radio waves. Full coverage for a large production plant requires access points to be installed in the required density. Considerable costs are incurred by this requirement.

Data transmission can become unreliable, especially where there are several stations using the same frequency. The transmission rate drops because all the stations using the same frequency must share the same data transmission rate. Reflections cause standing waves and this can lead to gaps in the coverage.

Glass Fiber Optics

Optical cables have low mechanical strength and resistance to damage compared to screened/shielded copper cables. Glass fiber optics provide galvanic isolation. The energy required for communication is extremely low and this allows realization of intrinsically safe explosion protection "Ex is op". Glass fiber optics are immune to EMI. The high reliability of glass fiber optics during normal operation have
Figure 4: Wireless station for industrial use
Figure 4: Wireless station for industrial use
been demonstrated. This means there are distinct benefits for the installation technology. (1) Connections may be interrupted by live work during operation; (2) ground/potential compensation, screening/shielding and surge protection are not needed.

Glass fiber optics have limitations. In particular, specialized knowledge is needed for the installation, and for completing the connections. The connectors are often installed on site and precise adjustment is typically required. The connector must be attached in the center of the glass fiber optics, the diameter of which is only 50 µm. Any contamination that can be trapped during installation between connector and cable can impede the communication. A clean and dust-free environment must be ensured on site. Hot-melt adhesive or instant glue are often used for securing the connection; however, none of these methods are suitable for use in the hazardous area. In practice, this often leads to sharp kinks directly at the connector that in turn can cause the glass fiber optics to break.

Costly and complex measuring techniques are then needed for the verification and failure localization work. Connector engineering, converters and even the glass fiber optics are all cost-intensive in comparison with using copper wiring.

Intrinsically Safe Ethernet

Intrinsically safe Ethernet combines the advantages of installing standard cables with explosion protection. Two galvanically isolated converters (Ethernet isolators) constitute the end points of a copper-based, intrinsically safe Ethernet for hazardous areas. The Ethernet isolator can be installed as an associated electrical apparatus in zone 2. In this example, the isolator acts as a "Pass Thru", i.e., the data is passed through without any buffering. The data transmission rate of 10 or 100 MBit/s is automatically recognized.

Alternatively, an Ethernet Isolator is installed in the safe area or can be located in the hazardous area in the connected apparatus that typically is designed as pressurized (Ex p) or flameproof (Ex d). The benefits:

The connection can be established or interrupted while the equipment is running without a hot work permit.
• No special measures for protection are needed during routing. The installation is straightforward and cost-effective.
• Neither hard grounding nor potential compensation is needed because galvanic isolation exists.
• No further boundary conditions have to be considered during planning.
• No specialized knowledge is needed for installation, commissioning and maintenance. This makes cabling and any modifications straightforward while the equipment is running. The data transmission characteristics are the same as those for screened/shielded cable routing. Intrinsically safe Ethernet is particularly suitable for the following three application cases:
• Equipment and machines where mobility and flexibility are required or optimal.
• Temporary installations.
• Applications where the risk of cables breaking is high, e.g., moving parts of equipment.

Conclusion

Intrinsically safe Ethernet adds an alternative solution that is simple to apply for network topologies in the hazardous area. It combines proven and well-known technologies. Because it is configuration free and intrinsically
Table 2: Comparison of physical layer solutions for Ethernet relevant to process automation
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Table 2: Comparison of physical layer solutions for Ethernet relevant to process automation
safe with galvanic isolation, system designers, installation teams and maintenance personnel can work with this technology without the need for special knowledge on Ethernet technology. Based on cost-effective standard components, Ethernet can be applied "flexibly" in the hazardous area. During construction and operation of temporary installations or while operating mobile apparatus, users benefit from the simplified demands on installation technology. Standard cabling and accessories reduce operating cost.

Ethernet moves into process automation including the explosion hazardous area. Open standards such as PROFINET and FOUNDATION Fieldbus HSE build the base for manufacturers to develop and offer solutions. The Ethernet Isolator is an important component of fieldbus infrastructure for an open and integrated architecture – with all the benefits regarding planning, installation, operation and asset management for long lasting and reliable process plants.

For more information contact: Pepperl+Fuchs, Twinsburg, OH, 330-486-0002, www.am.pepperl-fuchs.com, pa-info@us.pepperl-fuchs.com
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