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DART—The New Dimension in Intrinsic Safety—Part 2

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 2 we discuss the components of DART, the safe and the normal working ranges, and the loads.

Figure 4 - Block Diagram of Power Supply
Figure 4 - Block Diagram of Power Supply

DART COMPONENTS

A DART power system is comprised of three components—the power supply, the connecting cable/s and one or more loads. A system basically consists of only one source, which can however be provided in a redundant form for reasons of availability. The loads are connected to the power supply via a connecting cable with a fixed, defined surge impedance.

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Fig. 4: Block Diagram of Power Supply

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The output voltage is galvanically isolated from the station supply and limited by multiple redundant circuits. The DART specific behaviour is achieved through the functions represented in the block diagram in Fig. 4.

Co-ordination of functions integrated in the DART power supply leads to the output characteristics, in which the output voltage Uout is represented against the output current Iout described below. In addition to the safe permitted highest values Ulim and Ilim the characteristic is divided into the two operating ranges A and B:

SAFE RANGE A (SEE FIG. 5)

This range, which is called the start-up and fold-back range, represents the characteristic curve of a linear voltage source with safe values. After switching on the source switch S1 is open (Point 1). A very low current of a few mA, the so-called ‘trickle current’ (Point 2) is made available at the output terminals across the resistance RStart. When the load resistance due to the combination of cable and load is sufficiently large (RLast > RL1) it means that no fault is present. The output voltage reaches or exceeds a fixed threshold value Uthr (Point 3) and the source switches after a necessary safety period of approximately 3 ms to Range B, the operating range. However, this is only possible if the current variation di/dt due to the load lies below the prescribed detection threshold during the switch-on phase.

NORMAL—WORKING RANGE B (SEE FIG. 6)

Range B represents an almost ideal voltage source with an internal resistance Ri ≈ 0 Ω. In this operating range the source can provide the optimum power to the load, by which means the maximum power conversion is possible at Point 4 with RLast = RL2. Any variations in the load condition—including that due to faults—are associated with an immediate current variation di/dt. If at this point the prescribed maximum value of the current variation is exceeded in actual value, the source switches off and the operating point returns immediately from Range B to the safe Fold-Back Range A. This likewise takes place if the maximum permissible load current Ilim is exceeded. (see Point 4).

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Fig. 5: Output characteristic of a DART source with a representation of the transition from the safe range A to the optimum operating range B (schematic representation) ****************************************

Figure 5 - Output characteristic of a DART source
Figure 5 - Output characteristic of a DART source

In summary, the dynamic control behaviour of a DART source can be characterised as follows: By contrast with customary electronic current limitation there are the following differences in the case of DART made from a safety viewpoint: a transition into the optimum operating range in the ms range and rapid turn-off to the safe Fold-Back Range in the µs range in the event of faults.

THE LOADS

The following prerequisites have been taken into account in the DART concept with regard to the loads:

• The spectrum of loads that can be used should be as comprehensive as possible;

• It should be as simple as possible to integrate the loads into the system;

• It should be possible to operate already existing components / loads (including the customary field devices) with this technology in the same manner as is possible with previously customary technologies—e.g. FISCO (protection of stocks);

• In order to keep the safety considerations straightforward, only a line topology is envisaged; and

• The loads must not have a negative influence either on the functional or the safety capability of the DART source or other loads (including the cable).

The following particularly applies to the loads: They must not restrict or absorb the propagation of information on the formation of sparks. In this context the load behaviour must be accepted as not being exactly defined.

A decoupling module ensures a well-defined electrical behaviour both from a functional as well as a safety perspective. It permits operation of practically any load with DART. A decoupling module is integrated into the explosion-proof housing of the load and connected in series with it. It essentially fulfils the following tasks:

• Soft start-up of the load with limited current rise di/dt;

• Well-defined electrical behaviour; and

• Optional disconnection in the case of faults through di/dt detection.

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Fig. 6: Behaviour of the DART source in the event of a fault (schematic representation)

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Figure 6 - Behaviour of the DART source in the event of a fault
Figure 6 - Behaviour of the DART source in the event of a fault

TESTING DART

All the safety limit values for spark ignition given in the basic standard on ‘Intrinsic safety’ IEC/EN 60079-11 are based on the spark test apparatus defined there. This apparatus generates both ‘break’ sparks and ‘make’ sparks under prescribed constraints. During the time that passes up to the ignition of the explosive mixture, statistically evaluated predictions can be made on the ignition capability of different circuits. The ignition limit values obtained by this method can be found in the direct current reference curves and in the tables in IEC/EN 60079-11. In addition to this evaluation, the standard now permits the execution of various tests with the spark test apparatus in accordance with Appendix B of IEC 60079-11. A software test is also possible with the ISPARK program.

With none of the listed evaluation methods is it possible to carry out an objective safety assessment of dynamic, intrinsically-safe power sources—like DART—because the achievable ignition limit values with this new concept are way above the values in the standards. The intrinsic safety of these sources can only be ascertained by means of their dynamic principle of operation, i.e. their immediate reaction to fault conditions.

The necessary demonstration of proof demands the introduction of new types of test methods. These must target and reproduce the most critical cases that can be encountered in practice. In order to assess the ignition behaviour of dynamically operating sources these have to be loaded by means of hardware before the occurrence of the fault (spark) with precisely defined scenarios for the especially critical conditions, i.e. a defined spark history must be created. The definition of a ‘worst-case’ scenario is already available. However, due to the complexity of the relationships further investigations are necessary.

In the 6th edition of IEC 60079-11, due for publication around 2010, section 10.1.2 will be supplemented. In cases, in which the spark test apparatus cannot be used—such as in the case of dynamically acting sources considered here - alternative test methods will be permissible. The test methods to be used will be incorporated into the standard at a later stage, when further assured knowledge of these is available. Thus the 6th edition will open up the way for the international application of the DART technology.

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

This is part 2 of the series of articles.

For Part 1, CLICK HERE

For Part 3, CLICK HERE


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