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Intrinsic Safety without Power Limits

July 31, 2012

In a potentially explosive or hazardous area, the type of protection known as intrinsic safety offers recognized advantages such as its worldwide acceptance and the simple connection and installation technology. In addition, it is possible to carry out work on circuits and devices for the purpose of re-equipment, plant extension, and maintenance during actual operation and without a hot-work permit. The intrinsic safety method of 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 of any present hazardous materials.

The traditional intrinsic safety type of protection is currently achieved by limiting the available power. This limitation of power – usually to less than 2 W – provides intrinsic safety (Ex i) and is therefore mainly employed in the area of control and instrumentation, in the power supply to actuators and sensors with low power connected loads.

A significantly higher direct power, coupled with the known benefits of intrinsic safety, offers the user a new and essentially wider scope of application. These aims are achieved through Dynamic Arc Recognition and Termination (DART) technology. DART technology dynamically detects an undesired condition or a fault in the electrical system as it occurs and activates an immediate transition to a safe state before any safety-critical parameters are exceeded. DART is an explosion protection method applicable for Zone 1 & 2 hazardous areas.

Through the use of DART, systems can be operated at drastically increased direct power output compared to current intrinsic safety solutions. More available direct power opens the door to the use of the intrinsic safety type of protection in many applications relevant to the process industry. Examples include weighing equipment, lighting systems, valve control systems, and Fieldbus systems such as Foundation Fieldbus H1 and Profibus PA.

Basic Operating Principles
During normal operation, the DART power supply feeds the full nominal power which, depending on the application, can be greater by a factor of between 4 and 25 (8 to 50 W) compared to standards-related permissible values. DART detects at the very instant of the onset of a fault incident, due for example to the opening of the circuit, 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.

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
* The load behavior

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

Detecting the Ignition of a 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 behavior of the electrical parameters of a break spark is shown in Fig. 1.

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: 5 µs < tF < 2 ms.

At the start of a break spark the spark voltage UF jumps within a very short time (t ≤ 1 µs) 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.

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. 2 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 traveling 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 inverse 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.

Summary and Outlook
Due to 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, optimized for Fieldbus applications.

Table 1: Maximum intrinsically safe output values of DART at typical cable length

Output Voltage Uout      Active Power Pout      Cable length
DART Power
50 VDC          app. 50 W    100 m
24 VDC          app. 22 W    100 m
50 VDC          app. 8 W    1000 m
DART for Fieldbus
24 VDC          app. 8 W    1000 m

Suitable test methods have been developed for an exact safety evaluation of the energy-limiting behavior of dynamically operating power supply concepts.

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.

Brian Traczyk is the product manager for Fieldbus, Remote I/O, and Mechanical Protection at Pepperl+Fuchs (Twinsburg, OH). He has served in this position for four years and previously was an application engineer for two years. Traczyk previously worked as the assistant technical manager for a leading metal detector manufacturer. He earned a Bachelor’s Degree in Electronic Engineering Technology from the University of Akron in 2003. For more information, call 330-486-0002 or visit www.pepperl-fuchs.us.

Pepperl+Fuchs Inc.