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Wireless Transmission in Ex D Zone Simple and Safe Wireless Hart Communication? Yes, we Can!

| Author/ Editor: Sabrina Weiland, Dirk Heyden* / Dr. Jörg Kempf

Wireless provision of status, process, and diagnostics data for Ex d field devices — Is it possible to communicate data regarding measurements, diagnostics, and parametrization from flameproof 4…20 mA and Hart field devices in a completely wireless process? Yes it is!

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The WirelessHart adapter “Bullet” features a flameproof Ex d enclosure and is suitable for use in explosion-hazardous areas up to Zone 1.
The WirelessHart adapter “Bullet” features a flameproof Ex d enclosure and is suitable for use in explosion-hazardous areas up to Zone 1.
(Picture: Pepperl+Fuchs; © Sergey Nivens, © fotomek - Fotolia; [M]Tscherwitschke)

When it comes to transferring a signal from an explosion-hazardous area, the particular type of protection a device offers can have significant influence. Previously, if a plant was equipped with Ex d field devices, a wired solution was used for signal transfer, since only intrinsically safe adapters were available for wireless data transfer. If the plant operator wished to establish a wireless connection to the process control system, the flameproof field devices had to be connected via an Ex-d-to-Ex-i configuration.

These setups often turn out to be costly and complex. This complexity comes from the fact that, with Ex d field devices, the circuits are not galvanically isolated, and in many cases the internal capacitance is not known.

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Due to the risk of energy feedback in the event of a fault in the Ex d field device, it is not possible to connect an intrinsically safe adapter for WirelessHart without additional effort.

Various approaches exist for configuring Ex d to Ex i:

  • First approach: In the rare case that the capacitance of the Ex d field device is known and intrinsically safe outputs are available, the circuit can be calculated and designed according to Ex i.
  • Second approach: Another possible approach is to position the WirelessHart adapter outside of the explosion-hazardous area, though this solution results in costs for implementing, routing, and protecting cables using conduits, as well as for installing the adapter.
  • Third approach: A third alternative approach is to make the circuit between the Ex d field device and the Ex i adapter intrinsically safe by using a Zener barrier. In this case, the field device is connected to an Ex d switch box and the Zener barrier integrated in there via a flameproof cable duct and a conduit. There is then another intrinsically safe field connection on the other side of the switch box and Zener barrier, enabling the Ex i adapter to be connected.

All of the alternatives outlined are laborious to some extent and result in increased costs and significant effort. In contrast to these options, wireless connection with a flameproof WirelessHart adapter represents a much more convenient and cost-effective solution.

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