Dart fieldbus causes a sensation

The author: Andreas Hennecke Product Marketing Manager Division Process Automation, Pepperl+Fuchs

First introduced some years ago, Dart technology can cope with higher power levels while continuing to provide Ex i (intrinsic safety) explosion protection alongside its other positive features:

To date, these benefits have only been achievable at extremely low effective power levels, resulting in process automation as the most common application for intrinsic safety.

Dart stands for Dynamic Arc Recognition and Termination and is a new concept developed by Pepperl+Fuchs. Inspection and certification testing were developed by the German metrology institute (PTB).

In 2008, PTB and Pepperl+Fuchs established a consortium of fifteen interested companies funded entirely through private capital. PTB examined a further set of test methods and developed ideas for standardising Dart technology. This and all ensuing projects are referred to as Power-I – the ’I’ standing for ’intrinsic safety.’ The results were then compiled into a ’New Work Item Proposal’ for the IEC, which was subsequently accepted – an action that requires a two-thirds majority vote of the IEC committee and a declaration by members from at least five countries of their willingness to participate. Power-I met these criteria easily.

The current standardisation team comprises 28 experts from 14 countries. They will be publishing an IEC Technical Specification (IEC-TS) as the new chapter 39 of IEC 60079. It is also earmarked for inclusion in chapter 11 ’Intrinsic safety.’ The sense of enthusiasm is palpable: the prospect of smashing through the barrier of 1.5 to 5 W of power and going all the way to 50 W is a source of excitement for everyone involved. The IEC-TS grants Dart/Power-I the legal status of a global and binding document for manufacturers and users alike. All work is scheduled for completion by early 2014.

Whereas conventional intrinsic safety restricts the power (namely current, voltage and active power), Dart is based on the discovery that a spark transmits a very characteristic signal into the electric circuit. This can be reliably detected and disabled at the source as well as at every consumer. The operating current and voltage may be significantly higher. Dart technology intervenes only at the exact moment that the electrical energy in a spark is converted into heat.

Many years ago, PTB devised a testing procedure taking into account the special dynamics of the Dart circuit. This procedure was double-checked and confirmed by a second institute and applied to the Dart fieldbus to enable approvals to be granted in line with IEC 60079-11 intrinsic safety. For example, the Dart Power Hub and Dart Segment Protector are part of the company’s FieldConnex range. These devices are components of the fieldbus infrastructure that form the connection between the control system and the field device via the popular Foundation Fieldbus H1 and Profibus PA protocols – but now with an intrinsically safe High-Power Trunk for the very first time.

An important aim while developing Dart fieldbus components was to ensure that the technology could be handled easily in real-life situations. Experts with prior experience of developing Fisco (Fieldbus Intrinsically Safe Concept) were on the teams at both PTB and Pepperl+Fuchs with a focus on user-friendliness. The result is especially plain to see when it comes to planning and operational reliability.

With Dart fieldbus, the segment is planned and the explosion protection validated in virtually a single step. In addition to the planning requirements of the fieldbus standard, only a few general conditions have to be met:

Using the Segment Checker planning software, the planner can check the voltage level – which is extremely important for the fieldbus – with just a few clicks of the mouse. The correct functioning of the segment and explosion protection compatibility can thus be guaranteed well in advance.

Sparks are produced when a plug is disconnected. A Dart circuit must deal with this event in such a way that the availability of the fieldbus segment is completely unaffected. As explained earlier, the power supply is switched off for a mandatory few milliseconds to extinguish the spark before it is automatically switched back on again. This is a vital function, considering that the vast majority of all sparks are caused by the intentional establishing or breaking of connections to field devices.

A power reservoir in the Dart Segment Protector fills the gap when the main power supply is switched off in order to extinguish the arc, allowing the field equipment to continue to function. Undesired restarts of the instrumentation are prevented even after several short and successive breaks, e. g. due to contact bounces.

The fieldbus protocol continues to run by using its own repeat mechanisms to counteract the effect of temporary breaks. These events are not detected by either the field instrumentation or the control system. As a result, Dart may also be used in conjunction with the PROFIsafe and FF-SIF safety protocols at a consistently high level of plant availability.

FieldConnex Dart fieldbus products, the ’Power Hub’ power supply and the ’Segment Protector’ for connecting field instruments have been certified according to the current IEC 60079-11 intrinsic safety standard. An IEC-TS covering Power-I/Dart will establish interoperability and further simplify intrinsic safety operations. Interoperability safeguards markets and technology: the excitement about explosion protection may well lead to excitement about new product developments if the possibilities for new applications are discovered by even more manufacturers.

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