High-Power-Trunk Concept for PROFIBUS PA
High-Power-Trunk Concept for PROFIBUS PA
The High-Power-Trunk Concept enables, in device count and cable length, hazardous area fieldbus topologies almost comparable to non-hazardous areas.
Intrinsic safety and fieldbus
Intrinsic safety (I.S.) is the method of choice for instrument connections in hazardous areas. I.S. is the safest method protecting people and processes from harm. The electrical circuit in fieldbus is called a segment. Two concepts are used for implementation of I.S. fieldbus. They are Entity and FISCO (Fieldbus Intrinsically safe Concept) defined in IEC 60079.
FISCO provides slightly elevated power levels compared to the Entity making intrinsic safety applicable to certain applications for small areas such as oil tankers or floating product platforms.
I.S. however often does not satisfy the need of the many installations with regards to cable length and number of devices compared to applications outside of hazardous areas.
The High-Power Trunk Concept provides this power and is an essential component for the general acceptance of PROFIBUS PA in hazardous areas.
The High-Power-Trunk concept (HPTC)
The HPTC is based on the fact that “intrinsic safety“ with its power limitations is really only needed at the device connection where actions such as maintenance or replacement has to be carried out during plant operation. These are actions typically not required on the trunk. The HPTC thus utilizes a trunk, which is protected using increased safety (Ex e) ignition protection: Working on the trunk requires a hot work permit, which is the trade-off for increased power levels. Field barriers or other couplers connect to the trunk and provide the necessary methods of power-limiting explosion protection methods at the spur connections.
Solution for Zone 0…1/ Class I, Div 1…2 (fig. 1)
A Segment Coupler (DP-PA link) acts as power supply and is typically located in the control room at the end of the trunk line. Many power supplies available today are certified for Zone 2/ClassI, Div. 2. It feeds the trunk with a supply current of up to 1000 mA. Ex e type connections and respective cabling carry the high current to the field barriers, which are placed close to the field devices. The field barrier can be installed in Zone 1/Div. 2 and provides galvanically isolated, intrinsically safe outputs for field device connection. Field devices may be located in Zone 1/Class I, Div. 2. Up to 40 mA output current is made available for each device, which is sufficient even for high-performance field devices.
Multiple field barriers can be connected to one segment to increase the number of connection points. Compared to an I.S./FISCO trunk, the device count and reachable cable distance is significantly higher with the High-Power Trunk Concept. This solution is now a well established de-facto standard in the process industry.
Solution for Zone 2 / Class I, Div. 2 (fig. 2)
Many areas of fieldbus installations are classified as Zone 2/Div. 2. Here, a configuration similar to which has been described above provides the solution: The high current supplied by the segment coupler/power supply (without ignition protection) is carried by a “nonsparking” (Ex nA) trunk line. Because explosion protection requirements are less stringent, simple couplers with short-circuit protection are used in place of field barriers. They are Ex nA certified and provide outputs which have short-circuit protection. Intrinsically safe field instruments ‘Ex ic’ Zone 2 / Div. 2 can be connected to the outputs.
Energy limiting ignition protection ‘Ex nL’ will be shortly replaced by Ex ic.
Fig. 1: High power trunk to supply Zone 0...1/ Div. 1...2 installations
Fig. 2: High power trunk to supply Zone 2/ Div. 2 installations
Conclusion
The High-Power Trunk Concept with appropriate energy limiting PROFIBUS couplers provides simplicity and highest flexibility for any installation of PROFIBUS PA segments in explosion hazardous areas:
- Same topology in any hazardous as in non-hazardous areas
- Practically the same reachable cable length and device count
- Simple planning with few rules to ensure explosion protection
The large selection of installation components on the market is a clear indication of the value this solution provides and that it is an accepted and established concept . Best in class process availability is the result of this feature of PROFIBUS PA.
Please note:
Fig. 1 and 2 show all components in the “hardest” zone for which they are approved for.