By John Rezabek, Contributing Editor
The energy-limiting requirements of intrinsic safety (IS) once led to serious limitations for process buses such as Profibus-PA and Foundation fieldbus, where multiple devices are powered off the same cable pair.
A decade ago, IS requirements limited these buses to fewer than eight devices per segment (network). Since then, this has been improved through new practices such as the fieldbus intrinsically safe concept (FISCO) and fieldbus non-incendive concept (FNICO), which allowed a high enough power budget to increase the number of devices permitted and simplify IS calculations.
However, FISCO and FNICO have very specific requirements. Power supplies must be specifically designed and certified for FISCO or FNICO. Cable resistance, inductance and capacitance must conform to the FISCO/FNICO requirements. Field couplers must be certified for the specific standard, and devices must be certified IS for the zone and temperature limitations of the application. In addition, each segment must be documented, listing all the components above and their manufacturer, model number, method of protection, ambient temperature and temperature classification. The limitations on cable properties and available power (and hence stored energy) limit spur lengths (the distance from a coupler to a device) to about 100 ft, compared with almost 400 ft for non-IS fieldbus segments. The power budget also limits the total devices, depending on their individual current consumption, to about 12 or fewer. FISCO and FNICO also require that only one segment power supply is actively powering the network, so redundancy isn't trivial.
Physical layer suppliers have another solution for these limitations, employing a design called "high power trunk." The trunk itself is not energy-limited, so live working isn't possible without obtaining hot work permits. But the design does accommodate fully redundant, load-sharing, segment power supplies, and as many devices as the host manufacturer can support (typically 16). Field couplers are fitted with energy-limiting circuitry to achieve the IS requirements for each spur, and the required parameters for maximum stored energy are enforced from the field junction box to the device.
All these solutions achieve IS by ensuring any arc or spark from live working (e.g., installing or removing a device, or connecting test equipment leads) doesn't have enough energy to ignite the area's expected hazardous atmosphere.
But what if live working never created an arc in the first place? The engineers at Pepperl+Fuchs asked this question a few years ago, and through the efforts at the consortium the company leads, developed a way to detect a developing arc and remove the power the instant before it forms. This is dynamic arc recognition and termination (DART), which can deliver dramatically increased power for weighing systems, lighting and automated valve networks, as well as Profibus-PA and Foundation fieldbus, with the entire circuit meeting IS requirements from the control house to the field device.
How does it work? P+F's researchers saw that a very recognizable and repeatable phenomenon occurred every time an arc formed. They discovered it was relatively easy to detect a rapid increase in the rate of change of the loop current (di/dt), which preceded spark formation by a few microseconds. They also determined that spark current and voltage remained constant for another 1 to 5 µs thereafter, before the critical phase when a spark could form. The task, then, is to disconnect the power during the quiescent period after detection and before the spark forms, and then quickly restore it before any devices or communications are adversely affected.
Power supplies with this capability have been under development for three years, and tested and certified for use in IS applications by PTB-Braunschweig. By the third quarter of 2011, P+F will begin to deliver DART hardware for new projects. Acknowledging that the marketplace won't tolerate a sole-source solution, P+F will offer to license the technology to other physical-layer suppliers.
In North America, agencies such as UL and FM still are evaluating DART for installations covered by the U.S. National Electrical Code (NEC). DART is certified for Zone 2 (equivalent to Div. 2, hazardous atmospheres normally not present), and Zone 1 (a subset of Div. 1, hazardous atmospheres normally present). The rest of NEC Div. 1 is Zone 0, for which DART is not certified. If users have applications for which DART sounds like a great solution, be sure to have a chat with your electrical code expert.
