Connecting plants and machines via fibre optic cables is an attractive option, especially in large locations. Using fibre optic cables, distributed sensors and actuators, remote I/O systems, control and monitoring cameras and operating terminals can transfer large quantities of data at high speeds even over long distances. They facilitate fieldbus and Industrial Ethernet networks with cable lengths of 2,000 m or more – with copper cables, the limit was usually 100 m. In complex plants with several frequency inverters and motors, optical networks also provide a key advantage: unlike twisted pair wiring, they do not require elaborate grounding and shielding against electromagnetic interference.
What causes explosion hazards?
Even with fibre optic networks, users must ensure that ignition is prevented if the cables run through areas with explosive atmospheres where automation technology is installed. This is covered by IEC/EN 60079-0:2007 and in particular IEC/EN 60079-28:2006. It is easy to demonstrate why optical radiation from fibre optic cables presents an ignition risk of ignition can be demonstrated easily: sunlight that passes through a magnifying glass onto a highly flammable object will quickly cause the object to ignite. The energy, which is focused onto a small spot, is much stronger at the focal point than in surrounding areas. And optical fibres do, after all, focus light onto a very small spot. In the case of cable damage or an open plug connector, optical radiation can therefore cause an explosion. Other ignition mechanisms notwithstanding, the major light-related hazard is that it may cause an excessive build-up of heat. Particles in the atmosphere as well as the surfaces of devices in hazardous areas can absorb optical radiation. Users must therefore ensure that in the case of cable breaks, a specific irradiance level (i.e. an energy value per surface) is not exceeded and that the light energy released (irrespective of scattering) is not too high in absolute terms. This requires specially built and certified transmitters and receivers, for example. In many applications, typical switches with optical ports are a critical factor: they already operate close to or above the maximum permissible light output, and the light released in the case of failure may well exceed this level. A test certificate is essential for switches and end devices installed in zone 1. Depending on the explosion hazard group and temperature class of the atmosphere at the location of an application, different limits apply with regards to light energy and irradiance. Specifically, the limits for surfaces are 5 or 20 mW/mm². The absolute value independent of scattering is 15, 35 or 150 mW in the case of constant exposure. Pulsed radiation is subject to special rules which are also specified in Chapter 5.2.3 of IEC/EN 60079-28. The industrial standard mentioned above also describes the requirements of fibre optic networks in order to prevent ignition in explosive atmospheres. On the one hand, users can implement protection through locking mechanisms which reliably block the optical radiation in the case of failure. This ignition protection class is called Ex op sh. So-called Protected optical radiation (Ex op pr) is an alternative option. The third standard-compliant solution is inherently safe optical radiation (Ex op is).
Ex op is enables hot swapping
R. Stahl prefers the ignition protection class Ex op is. Inherently safe optical radiation provides the advantages known as a result of electrical intrinsic safety (Ex i), and is therefore especially suited to fibre optic communication networks. Inherently safe optical radiation is based on limiting the energy in a system, specifically within the fibre optic cable, during normal operation and under certain fault conditions, such as transmitter overloading. If critical parameters in an application exclude inherently safe technology, users must fall back on alternatives such as the protected optical radiation class Ex op pr. If this is not the case, users benefit from maximum flexibility in optical networks with Ex op is protection, since cables in such networks can be connected and disconnected during operation in hazardous areas. Thanks to hot swapping, installation, extensions and maintenance works on the communication network in zone 1 can therefore be performed as easily as in safe areas. Since many plants must fulfil functional safety requirements in addition to explosion protection specifications, this is an increasingly important point. While explosion protection requires maintenance intervals of up to three years, functional safety regulations often stipulate considerably shorter time periods between checks. In many cases, safety circuits have to be checked every year, which makes hot swapping indispensable.
Additional solutions for Industrial Ethernet
Suitable components in an inherently safe fibre optic network make a wide range of important performance, safety and convenience features of modern bus technologies accessible. First and foremost, inherently safe fibre optic wiring requires optical isolators. Isolating repeaters, for instance, are available as models for zone 1 and zone 2, and for safe areas, enabling the implementation of redundant ring structures for various bus technologies in addition to point-to-point and line wiring. Isolators integrated into a switch are a possible solution for Ethernet networks. With isolators or switches for fibre optic networks, users can, for example, operate an inherently safe Ethernet-enabled remote I/O system in zone 1. Such a system can then enable users to communicate, e.g. via Modbus TCP or EtherNet/IP (in future also FF HSE and Profinet), in an inherently safe optical Ethernet in hazardous areas. For example the simple yet efficient real-time Ethernet protocol Modbus TCP is ideal since it is supported by nearly all control systems. Modbus TCP lacks standardised configuration, parameterisation and diagnosis mechanisms, but DTMs can be used to make standard software packages such as Pactware, Fieldcare or Fieldmate accessible. A continuous Industrial Ethernet solution not only enables faster and more efficient signal transfer, but also allows users to create an integrated automation solution from the control level to the field and even link it to other IT systems such as SAP. As the fieldbus organisations Profibus International and Fieldbus Foundation are working on integration of remote I/O into their communication architectures, Ethernet with Profinet and FF HSE will become much more important in future automation tasks. And with an inherently safe (ex op is) solution, this can alsobe used for hazardous areas.☐hall 11.1, booth E41
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