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Comment

01 December 2003

Fieldbus in hazardous areas

By L. Christopher Towle

There is increasing use of fieldbus instrumentation in hazardous areas, and thus there is some activity in the area of international standardization to establish a universally acceptable level of electrical safety.

Let's review the present state of the art and then try to anticipate the general direction of future developments.

The fieldbus we consider is the Manchester encoded, bus-powered system designed in accordance with IEC 61158-2, which is the physical layer standard for fieldbus installations.

This standard embraces the two systems usually referred to as Foundation fieldbus H1 and Profibus, which are the low frequency (31.25 kilobits per second) systems in common use.

The primary objective of the fieldbus intrinsically safe concept (FISCO) is to create an intrinsically safe (IS) fieldbus system that one can configure without a detailed safety analysis and that permits the addition of further devices without having to reconsider that analysis.

The standards situation regarding FISCO systems is now clear with the publication of IEC Technical Specification TS 60079-27. It covers some aspects of apparatus and system design as well as a code of practice material.

The current intention is to publish a standard containing the existing information but extending the contents to include the requirements for nonincendive Zone 2/Division 2 fieldbus systems. The term fieldbus nonincendive concept (FNICO) will describe this technique.

CABLE LENGTH PLAYS INHIBITOR

FISCO comes from research work done in Germany at Physikalisch-Technische Bundesanstalt (PTB) by Ulrich Johannysmeyer.

It is important to recognize that the FISCO standard relies on a large amount of experimental work and some computer modeling, and that the requirements of the standard result from the extent of the experimental model considered.

An interesting result of the experimental work was to demonstrate that within certain limits adding cables to a source of power did not increase the incendivity of the short-circuit spark as expected.

Indeed one set of experimental results shows that 1000 meters of a typical cable increases the current required to cause ignition by 26%. Adding a terminator further reduces the incendivity.

Usually, within IS systems, the distributed cable inductance and capacitance is treated in the same way as the lumped parameters of components, but the experimental work shows that this is a very conservative approach and may eventually lead to the subject being reconsidered.

The permitted specification of cables in the FISCO standard is as follows:

These limits are not restrictive, because a typical fieldbus cable has parameters of 50 ohms per kilometer, 0.8 millihenries per kilometer, and 120 nanofarads per kilometer. The operational constraints determine the useable length of the trunk.

PERMITTED SAFETY DESCRIPTION

Power supply features include individual isolation of the power supply and the fieldbus trunks. This simplifies many of the IS questions and is desirable from an operational and faultfinding viewpoint.

The bidirectional fieldbus signal has to develop across the power supply and transfer to the port, which repeats the signal to the host. This means that the power supply must have low impedance at low frequencies to give a constant voltage as a source of power and high impedance at the signal frequency of 31.25 kilobits per second.

The FISCO standard utilizes recent work on the ignition capability of constant current power supplies to propose acceptable levels of power for supplies with a rectangular characteristic.

This permits greater useable power for the IS trunk, particularly if the less-sensitive gas group IIB (ethylene) satisfies the installation requirements.

The available power is always considerably less than that of the permitted safety description because of the necessary tolerances of the limiting devices. In the particular case of the fieldbus supply one must also allow sufficient margin on the available current to prevent the inrush current of a newly connected field device tripping the over-current protection.

The number of field devices that can connect to a power supply is the total quiescent current required by the field devices, being not greater than the useable output current of the power supply.

The maximum permitted trunk length is then determined from the requirement to have a minimum of 9 volts available at the field device terminals. The trunk length results after applying Ohm's law to the combination of cable resistance (usually 50 ohms per kilometer), current flow, and the minimum power supply voltage.

There are many permutations of the possible combinations, but if an average field device current is about 15 mA, then a typical IIC power supply can feed eight devices at the end of 500 meters of trunk.

The corresponding figure for a IIB supply is 16 devices at the end of 300 meters of trunk.

REDUCING SAFETY INSPECTIONS

A significant advantage of the FISCO concept is the reduction of the safety documentation required. Because there is no necessity to carry out the apparatus compatibility checks or to calculate the permitted cable parameters of the system in the same way as there is for an entity concept system, one can reduce the documentation to a single table of interconnected apparatus.

Preferably this should happen by using an adaptation of one of the available asset management programs, which can also make any relevant certificates readily available. When the field devices have intelligence that enables them to be uniquely identified and checked for operational integrity, there is less need for safety inspections and less frequent reading of indecipherable labels. IT

L. Christopher Towle is the chairman of MTL Instruments Ltd. He has been the secretary of the IEC and CENELEC intrinsic safety committees for the past twelve years.