Intrinsic safety—fieldbus style

Devices and barriers for intrinsically safe areas are designed so the energy released by an electrical fault is insufficient to cause ignition, even in a single- or double-fault condition. The ignition point is a function of available power-voltage and electrical current. How much segment electrical current, voltage, barrier choices, and devices permitted per barrier depends on the type of haza...

01/01/2003



From a software addressing and segment-communication perspective, all 14 devices are on a single segment. However, from an electrical perspective each segment is considered separate with individual segment power provided by the repeating barriers. Also it's worth noting, some fieldbus technologies allow a maximum of four repeater barriers per segment.

Click here to read the expanded version of this article .

Devices and barriers for intrinsically safe areas are designed so the energy released by an electrical fault is insufficient to cause ignition, even in a single- or double-fault condition. The ignition point is a function of available power-voltage and electrical current.

How much segment electrical current, voltage, barrier choices, and devices permitted per barrier depends on the type of hazardous atmosphere in which the devices are located, and which intrinsic safety model is used.

Two models

Fieldbus installations can choose between the entity and the fieldbus intrinsically safe concept (FISCO) models.

The entity model assumes all intrinsically safe apparatus are approved as separate entities including the connecting wire. This conservative approach permits a maximum dc electrical current of 83 mA in the wire and a maximum of 18.4 V.

The entity model is recognized worldwide.

The FISCO model considers the electrical wiring parameters to be distributed along its entire length, thus reducing the energy available at a fault, and permitting more devices on a wire pair. The result is a maximum dc electrical current of 110 mA for Class II C environments and 240 mA for Class II B environments.


By connecting six temperature measurements to a multiplexer and substituting a 4-wire Coriolis meter, the previous design can be simplified and overall power requirements significantly reduced.

The FISCO model is not a worldwide standard, however it is gaining European acceptance and is part of the FOUNDATION Fieldbus physical layer profile specification.

Intrinsic safety barriers are certified on the basis of the model they support. Field devices can be certified for use in one or both models.

Applying multiple barriers

Some fieldbus technologies use multidrop bus architecture, thus permitting multiple barriers on a single segment-each on a separate drop. However, if more than two barriers are placed on a single segment, galvanically isolated barriers are required to prevent signal distortion, and it's best to use a repeater barrier to correct the signal's shape.

For example, a process requires six temperature measurements, a mass flow control loop, a liquid flow control loop, and a liquid level control loop. As shown in the diagram, 'Single-segment, multiple-barriers: one approach,' the design places each process measurement on a separate barrier. While perfectly acceptable, it's not very cost efficient.

Among advantages of fieldbus technology is that digital devices can communicate multiple parameters.

By carefully considering and selecting intrinsically safe devices it's possible to simplify the previous example. (See, 'Single-segment, multiple-barriers: simple approach' diagram.)

Intrinsic safety provides the most practical means of working on field instrumentation while they remain powered, thus eliminating the need for hot permits and fire watchers. However, like any engineered solution, intrinsic safety design and specification requires knowledge of what's allowed and what's not.

-Comments? E-mail dharrold@reedbusiness.com

This article was prepared from readily available online material at Emerson Process Management's free online PlantWeb University , MTL , and R. Stahl .





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