Airbags for Intrinsic Safety

It Is Crucial to Learn About IS, It Is Also Vital to Learn How It Can Be Aided by HPT, DART and Other Coming Tools

By Jim Montague

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Process safety engineers, dandelions, fieldbus networks and small children are sneaky. I mean this in the best possible way, of course, but they're usually trying to get into places where they haven't been before—and sometimes they join forces. So, just as my daughters enjoy blowing seedlings over our newly weeded lawn, many engineers and their accomplices don't stop after implementing intrinsic safety (IS) methods, but instead jointly seek to safely extend IS into new, higher-power and formerly unreachable applications.

Now, fieldbus IS began as a way to bring electricity and communications into potentially flammable or explosive settings, but do it at low enough power levels and temperatures that ignition can't happen. However, there are power, distance and other technical limits to the original "Entity" barriers and later Fieldbus Intrinsically Safe Concept (FISCO). For example, Entity and FISCO initially could only run four or five 15-26 mA devices per segment. As a result, users and developers have been pushing against those restrictions ever since.

To help overcome some of these limits, engineers and suppliers such as Pepperl+Fuchs and others came up with the High-Power Trunk (HPT) concept with field barriers, which limits power at the spur instead of the trunk. Because the trunk isn't IS, it and the field barriers must be installed to comply with Class I, Div 2 or Zone 1.

Likewise, "Bridging the Intrinsically Safe Fieldbus Disconnect," by Moore Industries-International reports spurs can be IS and connect to Entity or FISCO devices located in Div 1 or 2 (Zone 0/1 or 2) areas. "This increases available power and connected devices on a segment, and also lets users maximize trunk cable length without FISCO restrictions," explains Moore's white paper. "However, converting from non-IS to IS at the barrier can be confusing." To help make installation simpler and safer, Moore developed its High-Power IS Trunk (HPIST) method and its Route-Master fieldbus system, which uses a split-architecture design to maintain IS while reportedly delivering 350 mA and powering up to 16 20-mA fieldbus devices at 500 m.  

DART maintains IS in electric circuits by recognizing faults and putting them into a safe state before critical levels are reached.

Beyond limiting power, the other innovation in IS in recent years is P+F's development of Dynamic Arc Recognition and Termination (DART), which maintains IS in circuits up to 50 W by recognizing faults and putting circuits into a safe state before reaching critical levels. This is similar to an airbag that deploys before a car accident—though this airbag prevents the crash from happening. The company's white paper on DART, "Intrinsic Safety Without the Power Limits," reports, "A spark caused by opening or closing an electric circuit has a very characteristic and easily detectable change of current and voltage. This change is detected by DART, and the circuit is switched off in only a few microseconds (μs). Thus, even at higher power levels, sparks never become incendive. At a wave velocity of more than 160,000 km/sec via its power cable, the response of a DART power supply is about 1.4 μs."

Pepperl+Fuchs has been developing and supporting two different versions of DART. The first, DART Power, is designed for point-to-point connections, which enables maximum energy of 50 W on a 100-m cable, far exceeding former 2-W limits. DART Power can be used for valve control, magnetic and Corilois flow measurement, fire and gas detection, optical and acoustic sensors, servomotors, light, analytic devices, scales and balances, and PCs and HMIs. The second, DART Fieldbus, is optimized for maximum cable length, but able to connect up to 20 fieldbus instruments to a segment. It can be used for valves, magnetic and Coriolis flow measurement, and analytics.

So, just as it's crucial to learn about IS, it's  also vital to learn how it can be aided by HPT, DART and other coming tools. Unlike circuits, innovation can't be turned off. Very sneaky.

[Editor's note: Both papers are at: www.controlglobal.com/intrinsic-safety-innovations.html.]

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