By Dan Hebert, PE, senior technical editor
This article first appeared in Control in July 2007.
Before beginning any discussion of hazardous areas, lets do a short overview of the area classification system. Class 1 is for gases and vapor, and Class 2 is for dusts. Division 1 designates an environment where hazards are likely to exist under normal operating conditions. In a Division 2 environment, they are not.
Designing Division 1 systems is much more demanding than it is for Division 2. The best designs use less expensive components and wiring in Division 2 areas, and more-expensive Division-1 components and wiring only as required.
Perhaps the most significant recent advance is the Fieldbus Intrinsically Safe Concept for Class 1, Division 1 (FISCO). FISCO says that a fieldbus is intrinsically safe if all the components connected to the bus observe defined limits for voltage, current, power, inductance and capacitance, says Moin Shaikh, DCS marketing consultant for Siemens Energy & Automation.
FISCO allows connection of a certain number of fieldbus devices over a prescribed segment length without performing extensive IS calculations. If FISCO-compliant components are used, then the segment meets FISCO requirements, and no entity parameter calculations are needed. Entity parameter calculations are a laborious process employed to confirm that the inherent energy storage in any device, plus its connecting cable, are within allowable parameters in a particular hazardous environment, says Dr. Dave Rizzo, technical writer at Power PR. FISCO removes those calculations to save hundreds of man-hours, especially since no recalculation is needed if additional devices are added to a segment.
FISCO works for a Profibus system because Siemens provides a fieldbus-isolating transformer that converts standard Profibus-DP signals to an intrinsically safe form since the standard Profibus-DP RS485 signal is not safe for use in hazardous areas, states Shaikh. Our fieldbus-isolating transformer has a built-in power supply that limits current to 110 mA. This means about nine devices can be connected per segment.
FISCO for a Foundation fieldbus (FF) is similarly implemented, but both installations use shorter segment lengths than in a non-hazardous area.
The biggest problem with FISCO installations is the limit on the number of devices per segment. Using multivariable transmitters usually solves that problem. Some of our transmitters make up to eight measurements, and using these types of devices increases the number of parameters that can be measured and controlled cost-effectively in a hazardous area, says Marty Zielinski, director of HART and fieldbus technology at Emerson Process Management.
It is much easier and cheaper to implement Division 2 fieldbus systems. Fieldbus lets systems designers use very cost-effective non-incendive devices in Division 2, usually the bulk of the plant, says Mike ONeill, director of MooreHawke. Designers just use explosion-proof devices where required in the few remaining Division 1 areas. Fieldbus systems are implemented most efficiently using non-incendive components in Division 2, allowing higher levels of segment power than with intrinsic safety, while retaining the ability to remove individual devices from a functioning segment.
Implementation of a bus-based system that uses differently rated devices in Division 1 and Division 2 requires considerable care with wiring and interconnection systems. Wiring in Division 1 must be explosion-proof or intrinsically safe, and both protection techniques impose severe limitations on fieldbus installations, says Brad Larson, director of applications and approvals at Turck. Explosion-proof wiring has its rigid and expensive mechanics, and intrinsic safety has its power limitations.
Using a barrier between divisions 1 and 2 allows users to provide maximum power to segments in Division 2 and to connect as many devices as allowed by the particular fieldbus protocol.