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DART—The New Dimension in Intrinsic Safety—Part 1

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 1 (of three parts) we discuss the basic concepts, and detecting the spark.

Figure 1 - Block diagram
Figure 1 - Block diagram

The intrinsic safety class of ignition protection is based on the principle that sparks occurring in an electrical circuit are always limited in terms of their energy, so that they cannot cause an ignition to take place in an existing potentially explosive atmosphere.

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Fig. 1: Block diagram: Power supply, cable and load, with Ex-Zones

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Intrinsic safety offers recognised advantages, such as its worldwide acceptance and the simple connection and installation technology. In addition, it is possible to carry out maintenance work on circuits and devices during actual operation and without a hot-working certificate.

Currently intrinsic safety (Ex i) is achieved by limiting the available power, usually to less than 2 W. It is mainly used in the area of control and instrumentation in the power supply to actuators and sensors with low connected load.

A significantly higher direct power with the simultaneous safeguarding of intrinsic safety offers the user a new and essentially wider scope of application. These aims are achieved through DART technology (DART: Dynamic Arc Recognition and Termination).

DART is a means of instantaneous tripping, which dynamically detects an undesired condition or a fault in the electrical system precisely as it occurs and instigates an immediate transition to a safe condition before any safety-critical parameters are exceeded. DART is based on the detection of fault conditions and their characteristic rate of rise of current.

Through the use of DART, systems can be operated at drastically increased direct power output compared to current intrinsic safety solutions. This opens the door to the use of intrinsic safety in many applications relevant to the process industry.

DART BASICS

In normal operation the DART power supply feeds the full nominal power, which can be greater by a factor of between 4 and 25 (8 to 50 W) compared to standards-related permissible values. DART detects the very instant of the onset of a fault incident, due for example to the opening of the circuit. DART senses the resulting change in current and immediately switches off the power supply. In this way, the energy from the electrical system is effectively limited in just a few microseconds and thus a spark capable of causing an ignition is prevented.

Figure 2 -  variation with time of spark current
Figure 2 - variation with time of spark current

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Fig. 2: Variation with time of the spark current, voltage and power of a linear limited break spark

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This procedure is possible due to a very characteristic and therefore easily detectable change in current di/dt during the onset of a fault condition. The reaction of the power supply takes place very quickly—in approximately 1.4 µs. On such a fast reacting system, an additional factor to be considered is the propagation time on the cable. The energy released is determined by the power converted at the point of the fault integrated over the time up to the effective disconnection. The following physical parameters are principally responsible for this:

• The power—determined by the supply voltage and the load current ;

• The time—comprising the signal propagation delay in the cable and the reaction time of the power supply;

• The energy stored in the connection cable; and

• The load behaviour.

The energy liberated in the spark is determined by the power available, integrated over time. The relationships are explained below.

DETECTING THE SPARK

The determination of the intrinsically safe ignition limit values is made with the spark test apparatus specified in the standard IEC 60079-11, in which these values are subjected to a specified ignition probability. It is important to distinguish ‘make’ sparks and ‘break’ sparks. Only break sparks are considered in this context.

A typical example of the behaviour of the electrical parameters of a break spark is shown in Fig. 2. A break spark commences with the voltage UF = 0 V and usually ends on reaching the open circuit voltage at UF = U0, in which the steady increase of the spark voltage is directly associated with a reduction in the spark current IF in a linear circuit. The period of time in between depends on the circuit and is referred to as the spark duration tF. Typical spark duration tF: is between 5 µs and 2 ms.

At the start of a break spark the spark voltage UF jumps in less than a microsecond from 0 V to UF • 10 V. The voltage change is directly linked with a characteristic and easily evaluated current jump di/dt (see curve IF). Directly after this jump in current the spark current and spark voltage remain relatively constant for approximately 1 to 5 µs. During this period there is definitively no possibility of ignition due to the extremely low available spark energy WF and it is referred to as the ‘initial phase.’

There then follows a longer period of time, which as a maximum, persists up to the end of the spark duration tF. This range is the ‘critical phase’ during which an ignition can occur. During this period the spark draws the necessary ignition energy from the system, i.e. from the source, the cable and the consumer loads.

Figure 3 - time history of the spark current
Figure 3 - time history of the spark current

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Fig. 3: Time history of the spark current, voltage and power of a break spark with DART interruption.

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From the knowledge of these variations with time it can be seen that the rapid detection of sparks in combination with a means for the rapid disconnection of the source can be employed to reliably prevent the ignition of an explosive mixture. The task is principally to evaluate the current jump di/dt, while giving due consideration to the characteristic safety values.

Fig. 3 shows the time history of a spark interrupted by a DART power supply. The current jump is clearly evident, which triggers the transition of the circuit into the safe condition. It is clear that with DART a fault condition is not only already detected and evaluated within the ‘initial phase,’ but that it also leads to the disconnection of the power supply. The switch-off time available during this process depends on the system. A frequently used value, based on the physics of the spark is, 5 µs.

Due to the very short rise times of current and voltage during the onset of a spark, the connecting cable between the power supply and the load acts as a wave guide even when the cable lengths are very short. The information that a spark is in existence propagates as a travelling wave or surge on the connecting cable. Thus the power supply receives the information delayed—by up to one cable propagation delay period. The reaction of the power supply in turn becomes effective at the position of the spark only after one cable propagation delay period.

This delay is an important safety parameter. In a typical cable used for instrumentation electric waves travel at approximately half the speed of light or 160,000 km/s. Available power is approximately inversely proportional to the cable length. Further influencing factors to be considered are, for example, the stored energy in the connection cable and in the load.

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

This is part 1 of the series of articles.

For Part 2, CLICK HERE

For Part 3, CLICK HERE


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