Intrinsically Safe Fieldbus


#1

here are several Intrinsically Safe Fieldbus technologies in the market. The most commonly used are:

Entity Barrier Concept

This Design takes the concept of normal Field barriers that has been successfully used in the analog signal world(4-20 mA). These barriers use an infallible resistor (wire-wound), Zener diodes and a fuse, and require a good intrinsically safe ground. While this barrier limited energy sufficient for Zone 0/1 all Gas Groups (Class I Div 1 all Gas Groups), it only provides 80 mA for the Fieldbus segment. This optimistically could only power four Fieldbus devices which typically take 15-26 mA each. The Entity barrier concept is safe, but its low power limitations and engineering requirements effectively eliminate many of the benefits of using bus technology.

FISCO

Fieldbus Intrinsically Safe Concept (FISCO), which was first developed by PTB (Physikalisch-Technische Bundesanstalt, the national metrological institute of Germany) as a method to provide higher power to a Fieldbus segment in hazardous areas. The FISCO concept, considers the entire circuit of the Fieldbus segment. The maximum total cable length in a FISCO system is 1 km in Gas Groups A and B (IIC) and 1.9 km in Gas Groups C and D (IIA and IIB), while the maximum allowed spur length (length from the segment junction box/protector) is 60 metres for Gas Groups A through D (IIC, IIB and IIA). Additional constraints are also placed on the power conditioners i.e. load-sharing redundant power conditioners are not allowed in a FISCO power supply.

Certifying devices to a standard before implementation, allows them to be integrated into systems without the engineering requirements necessitated by the Entity approach. This then allows FISCO power supplies to generate more power (and allow more devices per segment) than the Entity barrier solution. The bottleneck of this solution is that it requires each part of the system, including devices, cables and power conditioners to be FISCO compliant and that FISCO design and installation rules are strictly followed. While it does provide more power than the Entity barrier approach, still the system can only support four to five devices per segment as the trunk current is limited to 115 mA.

HPT with Field Barriers

A more recent enhancement for intrinsically safe applications is the High Power Trunk (HPT) with field-based field barriers (FBs), which limits power at the spur, rather than the trunk.This method significantly changes the equation for end users of Fieldbus in hazardous settings. It increases the amount of available power and therefore the number of connected devices on a segment. It also lets end users maximize the length of their trunk cables without the restrictions of FISCO/Entity Barrier Concept.

While the HPT model does provide some significant improvements(500 mA at Fieldbus segment), it is not without its downsides. The field barrier is in essence a field-based isolated power conditioner. So even though the segment can be powered by load-sharing redundant power conditioners at the host, the practical MTBF (Mean Time Before Failure) is still that of a single power conditioner, since most field barriers are not redundant.

Intrinsically Safe-High Power Trunk/High Power Intrinsically Safe Trunk (HPIST)

The High Power IS Trunk (HPIST) technique provides an enhanced level of safety and simplicity in installation, along with the ability to use it for all devices (FISCO and Entity) and hazardous Zones and Divisions. It delivers approximately 350 mA of IS power to segments located in hazardous areas.

This is achieved by utilizing a split-architecture design that puts part of the barrier in an isolator card located in the safe area (power supply rack) & the other part in each of the spurs of field-mounted device couplers/segment. The barrier in the isolator allows 350 mA to be run through the segment up to the Spur/junction Box through trunk cable. Since infallible resistors are used, devices from Zone 0/1 or 2 can be direct connected. Having 350 mA, now allows users to power up to 16 Fieldbus devices (20 mA each) at 500 m while retaining intrinsic safety.