Simply intrinsically safe – without power restrictions

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

With Dart, sparks have no chance, no matter where the fault might occur in the fieldbus infrastructure. Dart (Dynamic Arc Recognition and Termination) is a concept based on the principle of detecting a spark as it occurs and extinguishing it before it can ignite a combus-tible atmosphere. Since sparking is only ever a temporary phenomenon, the system returns to normal operation again after no more than a few milliseconds. The instrumentation and the control system can continue to communicate because the separation period is very brief. There is no loss of functional availability. In short, the Dart technology reflects a new approach to power limitation and facilitates much higher direct power.

At first glance, the functional principle of Dart technology is similar to that of a type “a” safety technology. In spite of this, Dart is not related to SIL in any way because it is only a type of explosion protection for intrinsic safety (Ex ia) in Zone 1.

The inventors and developers of Dart attach great importance to interoperability, including between different manufacturers, and especially to straightforward practical applicability. After all, a consortium of 13 German manufacturers is currently working on the creation of an IEC TS (technical specification) under the guidance of the PTB metrology institute in Braunschweig, Germany, to provide a basic definition for manufacturer-independent applicability and interoperability. This consortium is made up of the same people who defined Fisco, the Fieldbus Intrinsically Safe Concept, almost 20 years ago. Fisco has had its own chapter in the IEC standard for explosion protection (IEC 60079) since 2002 and is one of the most important foundation stones for the acceptance and use of fieldbuses in process plants.

Profibus PA and Foundation Fieldbus H1 are two very popular process automation fieldbuses. They facilitate communication between the control technology and field instrumentation, offer an integrated power supply and are particularly suitable for hazardous areas. The FieldConnex Dart Fieldbus series is the first range of products to incorporate Dart technology. These products are certified by the PTB to the international standard IEC 60079-11 and can therefore be used throughout the world.

Dart Fieldbus is part of the popular FieldConnex series specially designed for fieldbus infrastructure products by Pepperl+Fuchs. It uses higher intrinsically safe power exclusively on the trunk. The connection for the DCS is of conventional design. The outputs on the Dart Segment Protectors incorporate spur connections that offer classic intrinsic safety. The intrinsic safety of the trunk and spurs is certified for operation in Zone 1 according to Ex ib IIC. This safety level is adequate because on most fieldbus-compatible field devices with approval for Zone 0, the transmitter head and connection are located in Zone 1.

When used in hazardous areas, Dart technology offers virtually the same cable lengths and number of node connections as applications in safe areas. What’s more, the fieldbus installation is completely intrinsically safe and can be connected to a redundant power supply. For planners and users, a high degree of safety from the project planning stage to operation allows easy, unrestricted operation of fieldbuses in hazardous areas. This safest of fieldbus infrastructures can be used in both new systems and upgrades because intrinsically safe field instrumentation remains compatible, whether in the planning phase or already installed.

Fisco paved the way for simplicity in practical operation. The concept was developed at the PTB in Germany. It was empirically proven that Fisco’s intrinsic safety is guaranteed providing specific limits are maintained for the capacitive and inductive behaviour of power supplies, cables, and instruments as well as a number of other less important constraints. Fisco occupies a permanent chapter of the IEC 60079 standard and is globally accepted.

This degree of simplicity in the use of “intrinsically safe” explosion protection also needs to be applicable to products equipped with Dart technology. In the course of deliberations preceding the certification of the FieldConnex-Dart Fieldbus by Pepperl+Fuchs, this simplicity was defined as a standard. Prior to the development and certification of the Dart Fieldbus Power Supply and Dart Segment Protectors, customer requirements were analysed from the product concept all the way up to final certification.

As a result, only three simple rules must be observed by all stakeholders, from planners and installers to start-up engineers and mainte-nance personnel.

From the segment layout and installation through commissioning to proof of intrinsic safety, all tasks are solved in the simplest way possible. The segment is designed using tools that are suitable for planning Profibus PA or Foundation Fieldbus H1 segments, such as www.segmentchecker.com. Dart supplies sufficient power to operate the vast majority of cable runs in demand today (typically with a trunk length of 400 m). The full spur length is also possible.

The installation is characterised by a common topology that includes cables with no additional requirements. The trunk can be laid much more easily than the High-Power Trunk without having to comply with regulations for increased safety. All connections outside the control technology room are intrinsically safe. Electricians and fitters no longer have to make a distinction.

Current, trusted fieldbus diagnostic tools will continue to be used during commissioning. In particular, the option of stationary physical layer diagnostics is also provided here. It is inserted in the power supply like a module and not only speeds up installation validation but simultaneously enables continuous monitoring and repeat testing after maintenance work. Practical use repeatedly demonstrates that in more than 80 % of all cases, fieldbus faults occur unintentionally, for example through manual intervention during maintenance or upgrading of field devices.

The proof of intrinsic safety and documentation do not require any kind of calculation and are limited solely to the documentation of characteristics and certificates – as already familiar from Fisco. Correct functioning of the segment and simultaneous verification for explosion protection are guaranteed as early as the planning phase.

In all other respects, a Dart Fieldbus is generally operated in the same way as a fieldbus in a safe production area. This is due to its product design and system certificate that allow the procedure described above to be implemented exactly.

An IEC-TS is a binding document for all stakeholders and forms the basis for an IEC stand-ard. An IEC-TS for Dart technology is expected sometime during 2012. In addition to the simple point-to-point connection, operating scenarios involving several nodes similar to the fieldbus may also be incorporated. As a result, it should be possible to implement a wide range of potential Dart technology applications based on a common standard.

The degree of commitment and interoperability created by the IEC-TS, together with its simple applicability in the examples described here, should lead to the widespread acceptance of Dart technology among manufacturers and end users.

Online-Info: www.cpp-net.com/0311457

One product equipped with Dart technology is the modular Protos 3400 X Dart measuring system, which is presently in the development phase. The current version with 20 to 265 V (AC/DC) mains power offers:

This large range of functions needs 10 W of power, which can be produced by Dart with identical functionality. At the same time, the number of required cable glands and terminals decreases, the installation space for the power supply is smaller and there is more room in the area around the terminals. Intrinsic safety reduces installation costs and Protos can be opened at any time for reconfiguration, maintenance or repair.