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DART—The New Dimension in Intrinsic Safety—Part 3

11 March 2009

The DART safe energy concept allows considerably higher direct power in hazardous areas, because of its safe energy limitation through rapid disconnection. In PART 3 we discuss the power concept and DART in fieldbus.

Fig. 7: An example of the interconnection of a DART-Power-System
Fig. 7: An example of the interconnection of a DART-Power-System

DART—THE POWER CONCEPT

The DART power solution is used as the focal point for the point-to-point supply from the power supply to the load. The resulting simple topology consists of the power supply, cable, and the customary loads at the end of the cable, rendered possible by a simple means of the consideration of the complete system, to provide high intrinsically-safe direct power supplies to the loads.

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Fig. 7: An example of the interconnection of a

DART-Power-System

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The decoupling module enables both the safety of the system and its functional operation to be achieved independently of the characteristics of the respective loads. Fig. 7 shows an example of the interconnection of a DART-High-Power power supply with three loads via a connection cable and a decoupling module.

The decoupling module incorporates soft start and load. Due to the safety-related, easily described behaviour of the system, at the point in time of this publication maximum output data is achievable as follows: Umax = 50 V and Imax = 1.2 A with a cable length of 100 m. Fig. 8 shows the block diagram for a decoupling module.

The soft start and load switch-on components as well as a reservoir capacitor provide for fault-free and straightforward switch-on of the load. The reservoir capacitor takes care of switch-on over-currents and short periods of strong current fluctuations. From a safety perspective the combination of reservoir capacitor, AC and reverse polarity protection provides for a defined DART system.

In order to be able to cover the widest possible range of applications, the possibility of the transfer of data on the power supply line was anticipated in the basic concept. The decoupling elements required for this and the cable terminations to achieve a data transfer or more than 500 kbit/s are already available in the power supply and in the interface circuitry. A 500 kbit/s data transfer via a DART High-Power System has already been successfully tested.

The following applications can be achieved with DART Power in the explosion group Ex ib IIC:

• Industrial PC, operating terminals and displays;

• LED illumination system;

• Sensors with high power requirements, e.g. Coriolis flowmeters;

• Analytical devices;

• Magnetic actuators and high power solenoid valves; and

• Electrical heating systems

Fig. 8: Block circuit diagram of the decoupling module with optional di/dt detection
Fig. 8: Block circuit diagram of the decoupling module with optional di/dt detection

DART FOR FIELDBUS

In the area of process automation two fieldbus systems, Foundation Fieldbus H1 and Profibus PA, have been established as de facto standards.

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Fig. 8: Block circuit diagram of the decoupling module with optional di/dt detection

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A trunk-and-spur-topology is employed utilising a home run cable, also referred to as trunk. Field devices are connected via spur lines to wiring interfaces with short-circuit protection, which can be connected to the trunk at arbitrary points. Fig. 10 shows the principle electric circuit of a fieldbus system.

In comparison with existing intrinsically safe fieldbus solutions, DART enables four times as much power on the trunk line. Power is approximately the same compared to the well-accepted High-Power-Trunk concept, without the disadvantage with increased safety installation methods required for the trunk.

Though highest voltage values would be beneficial for maximum output power, the available power is selected to 24 V. Thus any existing field device conformant with the Entity concept defined in IEC 60079-27 can be connected. Entity enables intrinsic safety to be validated for any topology through a simple comparison of values.

Frequently the plants that are to be automated extend over a wide area, which requires long cable connections. If the cable length is determined as being 1000 m, this results in an available effective power of 8 W. This output power is suitable for up to 24 loads per segment and corresponds with the available power on Fieldbus segments with the generally recognised High-Power Trunk concept.

DECOUPLING THE FIELD DEVICES

As already described, the dynamic behaviour of loads is not defined from a safety standpoint. Decoupling circuits are built into the Segment Protectors.

Irrespective of the actual electric characteristics of the field device, the storage capacitor ensures a defined load behaviour at the cable input terminals.

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Fig. 9: DART Fieldbus system with multiple loads and segment protectors
Fig. 9: DART Fieldbus system with multiple loads and segment protectors

Fig. 9: DART Fieldbus system with multiple loads

and segment protectors

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COMMUNICATION

The communication, in the form of a trapezoidal alternating signal with a peak-to-peak value of 18 mA (+/- 9 mA) is superimposed on the direct current supply signal. The flanks of the signal can be several microseconds short. The system distinguishes these current variations unambiguously and reliably from those that occur in the generation of the spark.

In summary, with DART, very high intrinsically safe power is available for new applications in the process industry, depending on the length of cable employed. The maximum possible power output is strongly dependant on the delay times on the transfer cable. Solutions exist for two application areas: DART Power for maximum power output and DART for the Fieldbus, optimised for Fieldbus applications.

Suitable test methods have been developed for an exact safety evaluation of the energy-limiting behaviour of dynamically operating power supply concepts. Changes to the currently applicable standards have already been investigated. Further steps will follow.

DART enables the use of intrinsic safety in applications with power requirements, which today necessitate other, typically inflexible or expensive types of explosion protection. By means of DART operating processes will become simpler and complexity is reduced. Operating safety will be increased.

The authors: Udo Gerlach, Thomas Uehlken, Ulrich Johannsmeyer Physikalisch Technische Bundesanstalt; and Martin Junker, Andreas Hennecke Pepperl+Fuchs

This is part 3 of the series of articles.

For Part 1, CLICK HERE

For Part 2, CLICK HERE


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