TAKING FIELDBUS INTO HAZARDOUS AREAS

Benefits and limitations are explained for two systems BY philip saward

Any industrial bus system which does not allow sensors and actuators to be placed in a hazardous area will find limited application in many process industry environments.

Hazardous areas exist in many manufacturing industries, ranging from foodstuffs to oil refining, resulting from the presence of potentially-explosive gases and vapours.

The requirement for hazardous area capability for fieldbus systems has led to the development of Physical Layer specifications which exploit the international acceptance and user-benefits of intrinsic safety (IS). Two such systems are products from the Fieldbus Foundation, (FF), which uses the IEC 1158-2Physical Layer, and Echelon’s LonWorks, which uses an IS physical channel developed by MTL, and has been adopted by the LonMark Interoperability Association.

Intrinsic safety

Intrinsic safety allows live working to be carried out in the presence of a flammable atmosphere, and also offers potential savings in terms of the size and weight of enclosures, when compared with the flameproof technique.

Unlike the other methods of protection for electrical apparatus in hazardous areas, the safety of an IS installation considers the compatibility of the separate items and the cables used to connect them.

This compatibility assessment ensures that no item of apparatus is subjected to higher levels of voltage, current or power than was accounted for when it was certified. Total energy storage in unsuppressed capacitance and inductance in devices and interconnecting cable is also taken into account.

The Foundation Fieldbus Physical Layer (FF-816) for 31.25kbps wire media defines eight profiles for fieldbus devices. Of these, four are specified as suitable for connection to an intrinsically safe fieldbus.

{{* Type 111 – standard-power signalling, powered from the bus;* Type 112 – standard-power signalling, separately-powered;* Type 121 – low-power signalling, powered from bus; and* Type 122 – low-power signalling, separately-powered.}}

The common characteristic which makes them suitable for connecting to an IS fieldbus is that they introduce no electrical energy onto the bus during either reception or transmission of signals.

Safety analysis

The intrinsic safety analysis is therefore simplified because there is only one source of energy onto the bus – the fieldbus power supply.

The output of the power supply must be within the IS energy constraints, and this must be sufficient to energise all the devices connected on the bus. This factor will limit the number of devices that may be connected onto an IS fieldbus.

The difference between the bus-powered and separately-powered profiles is that separate powering provides a means of supplying additional power for application circuits. The interface to the bus is otherwise identical. Bus-powered field devices draw their power as a constant current taken from the bus, and communicate by modulating this current using Manchester bi-phase digital encoding.

If an arrangement of intrinsically safe fieldbus devices from different manufacturers is to be assembled, it must be possible to demonstrate that the complete system is safe. The analysis is similar to that for a conventional instrument loop, except that there is now more than one interconnected device, and the cable may comprise a number of parallel spurs instead of a linear run (depending on the bus topology).

The Fieldbus Foundation Physical Layer Profile Specification itself recommends the minimum requirement for input voltage, current and power with which IS devices should be certified to operate. It also quotes the maximum values for outputs.

Capacitance and inductance

In the same way as for a conventional 4-20mA circuit, the allowable capacitance and inductance for the interconnecting cable may be determined by subtracting the lumped values for the field devices (more than one in the fieldbus case) from the corresponding values allowed by the source.

These may be translated into a maximum allowable cable length for the IS bus, where the capacitance and inductance per metre of the cable is known.

All the relevant safety parameters and data are taken from the approval certificates for the individual pieces of apparatus, which must form part of the documentation file for the installed system.

The voltage available on an IS fieldbus depends both on the number of devices attached and on the length of the bus cable. This is due to ohmic losses in the IS interface and the cable.

The IEC Physical Layer standard requires a minimum operating voltage of 9V for field devices which, together with the requirement for a maximum bus length of 1,900m, places a restriction on the current available to bus-powered devices.

LonWorks advantages

Echelon’s LonWorks has established itself as a popular, low-cost bus system in applications which require frequent communication of short messages between a large number of nodes.

Each node runs independently and is capable of originating peer-to-peer communications when stimulated (for example) by a change in input status. Each device connected to the network contains a Neuron chip which incorporates three microprocessors. Two of the processors handle the network communications, but the third is available for applications software, and can be programmed.

For safe area applications, there are a number of different physical channels, such as twisted pair cable, modulated power lines, radio frequency transmission and optical fibre. There is also a variety of communication speeds.

Transceivers and control modules are available to designers for each of the Physical Layers, to reduce development times for LonWorks-compatible nodes. If the number of nodes required in a network exceeds the maximum allowed for a single bus segment, then two or more segments may be connected together using ‘routers’.

A router may also be used to interconnect different physical channels. MTL has developed an IS physical channel to allow LonWorks networks to be extended into hazardous areas, known as IS-78.

IS-78 hazardous networks

Both power and communications are carried on a single cable. Multiple devices may be connected while maintaining full communications between hazardous area devices and with other safe area physical channels, via routers.

IS-78 supports both bus and free topologies. Bus topology requires all the nodes to be connected to a single run of cable having a maximum length of 1,000m, with terminators connected at each end. With free topology, there are no restrictions on the arrangement of the cable, other than a 300m limit on cable length, with two terminators.

The maximum current available on an IS-78 segment is about 85mA, falling to approximately 60mA with 1,000m of cable. This allows three typical bus-powered nodes to be supported, where each draws 25mA from the bus.

The number of nodes may be increased up to eight if separate intrinsically safe power is available for applications circuitry.

To conclude, intrinsic safety design rules have been applied to two different fieldbus systems, allowing field devices to be located in even the most onerous of hazardous areas, while maintaining the ability to make and break connections to the bus – but without gas clearance.

Demonstrating the safety of an assembled system is straightforward because of simple constraints imposed on the certification of system components.

* The Author is with MTL.