Intrinsic safety engineering in the fieldbus era

The IS-1 remote I/O system embodies an intrinsically safe system concept specifically developed for use with fieldbus systems.

Over the past 30 years, intrinsically safe (IS) explosion protection has developed into a mature and widely accepted technology.

In thousands of process plants around the world, IS methods are relied on to prevent serious accidents and injuries due to electrical ignition of flammable gases in hazardous areas.

But today, with the widespread acceptance of fieldbus systems, intrinsic safety engineering must take a new direction.

Fieldbus systems afford new possibilities in digitalisation, decentralisation and product standardisation, and intrinsic safety technology must adapt to these new levels of simplification and cost reduction.

This has been made possible with the launch of Stahl's IS-1 remote I/O system, an IS system concept specifically developed for use with fieldbus systems.

In the past three years, nearly 4000 IS-1 field stations have been installed; one large chemical manufacturer in Germany has nearly 300 at one site.

Intrinsic safety protection systems today generally fall into three categories: safety barriers (Zener diodes); isolator modules with galvanic isolation; and remote I/O with intrinsically safe inputs and outputs.

Current-limiting Zener diodes are the classic IS solution; they are simple and cost effective, and require no power supply, but an essential prerequisite for their use is high-quality potential equalisation between the barrier and the sensor.

Isolator modules are larger and more expensive than safety barriers.

They solve the potential equalisation problem with galvanic isolation, that is, they separate the intrinsically safe from non-safe equipment by using relays, transformers or optocouplers, which require their own power supply.

As the circuits are usually not connected to earth potential, they have a high immunity to interference.

Although the remote I/O approach to intrinsic safety has been around for many years - Stahl introduced its ICS MUX in 1987 - only recently, due to the acceptance of fieldbus technology in process automaton, has this method been adopted on a wide scale in process plants.

With Stahl's intrinsically safe remote IS-1 I/O system, no additional protective devices need be installed, as with safety barriers and isolator modules.

The IS-1 merges the automation system's I/O with the corresponding intrinsically safe interface, all in one unit.

A single fieldbus cable connects the automation system with the IS-1 field terminal, which is located in either Zone 1 or 2.

The IS-1 in turn is directly connected with the sensors and actuators in Zone 0.

The IS-1 system is very simple.

There are only three basic components which have to be engineered and installed.

The BusRail, which mounts on a DIN rail, interconnects the modules electrically.

The crossconnection contains data, address and the complete power supply for the system.

Several sections can be plugged together for extension.

The combined CPU and power module contains the connections for the various fieldbuses.

A power supply for the I/O modules, including intrinsically safe (EExi) field circuits, is also incorporated.

The module's LED display shows the condition of the station and information about single modules and signals.

The ServiceBus interface permits configuration, parameterisation and complete testing of a field station, if the fieldbus does not support these functions.

The I/O modules can be plugged into any free position, their number is restricted only by the power supply.

Inputs and outputs are intrinsically safe.

The I/O modules have four, eight or 16 channels and are designed for analogue and digital inputs and outputs and for temperature sensors and frequency transducers.

Hart modules round off the range.

The IS-1 system has been designed so that it can be located in any of three different areas: a safe area such as the control room, or in hazardous areas Zones 1 or 2.

The ability to mix the installation types is important for practical applications, and reduces the number of spare parts that need to be maintained in inventory.

The only exception is the CPU and power module, which comes in a special encapsulated version necessary for Zone 1 installation.

The IS-1 modules allow direct connection of Hart field devices.

The standard 4-20mA signal is processed as with conventional transmitters or actuators.

A field station behaves transparently for Hart commands, so that the diagnostic or parameterisation data are exchanged directly between a field device and the central Hart database via the ServiceBus.

A comprehensive redundancy design ensures that even when module faults occur, the effects remain restricted.

The internal IS-1 bus has a redundant design; a defective I/O module cannot, therefore, lead to the failure of a complete field station.

The CPU and power module, which is the "central module" of a field station, can be very rapidly replaced in the event of a fault.

The highest availability can be achieved with a redundant CPU and power module (hot standby).

In this case, all central functions of a field station, power supply and fieldbus connection are present in duplicate.

Even though the IS-1 has been engineered for simplicity, it contains advanced features that are a direct benefit to the user.

The total number of units is significantly reduced.

The high number of channels of the I/O modules reduces cost per signal.

Planning and installation are simplified by the omission of isolator modules, terminals, cables etc.

IS-1 field stations replace the field terminal boxes.

At startup, microprocessors in all modules supply the detailed diagnostic information which is necessary for a rapid and accurate fault localisation.

Rapid replacement of all components is important for maintenance: all modules can be changed during operation in hazardous areas, namely in Zone 2 and Zone 1.

Normally equipment installed in EExe areas (increased safety) must be powered down before replacement, but Stahl has engineered the modules with a combination of EExd, EExe and EExi technologies so that they can be removed and replaced under power, as long as the technician follows a set of procedures.

No additional work is needed, such as the adjustment of functions or bus addresses using DIP or rotary coding switches.

All parameters are automatically reloaded.

The system is designed with maximum flexibility: it can be installed in the control room, Zone 2, or Zone 1.

Intrinsically safe I/O modules in a field station can be combined with non-intrinsically-safe modules.

This signal mixing is initially limited to installations in Zone 2; further development of the IS-1 system will offer these possibilities for Zone 1.

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