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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 upon to prevent serious accidents and injuries due to electrical ignition offlammable gases in hazardous areas.
But today, with the widespread acceptance of fieldbus systems, intrinsic safety engineering hastaken a new direction. Fieldbus systems afford new possibilities in digitalisation, de-centralisation, 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 ofStahl's IS-1 Remote I/O System, an IS system concept specifically developed for use with fieldbus systems. In the past five years, thousandsof IS-1 field stations have been installed; one large chemical manufacturer in Germany has nearly 300 at one site.
Intrinsic safety:state of the art
Intrinsic safety protection systems today generally fall into three categories:
- Safety barriers (Zener diodes);
- Isolator modules with galvanic isolation; and
- Remote I/Owith 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 pre-requisite fortheir 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 opto-couplers,which require their own power supply. Since the circuits are usuallynot connected to earth potential, they have a high immunity to interference.
While the remote I/O approach to intrinsic safety has been around for many years, only recently, due to the acceptance of fieldbus technology in process automation, has this method been adopted ona 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 mergesthe automation system's I/O with the corresponding intrinsically safeinterface, all in one unit. A single fieldbus cable connects the automation system with the IS-1 field terminal, which is located in eitherZone 1 or 2. The IS-1 in turn is directly connected with the sensorsand actuators in Zone 0.
The simplicity of the system
The IS-1 system is very simple. There are only three basic components whichhave to be engineered and installed.
- The BusRail, which mounts on a DIN rail, interconnects the modules electrically. The cross-connection contains data, address and the complete power supply for thesystem. 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 beplugged into any free position, their number is restricted only by thepower supply. Inputs and outputs are intrinsically safe. The I/O modules have 4, 8 or 16 channels and are designed for analogue and digitalinputs 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 - 20 mA signal is processed as with conventional transmitters or actuators. A field station behaves transparently forHART commands, so that the diagnostic or parameterisation data are exchanged directly between a field device and the central HART data basevia the ServiceBus.
A comprehensive redundancy design ensures thateven 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, canbe very rapidly replaced in the event of a fault. The highest availability can be achieved with a redundant CPU and Power Module (hot stand-by). In this case, all central functions of a field station, power supply and fieldbus connection are present in duplicate.
HazardousArea Nomenclature
Potentially explosive areas are classified into three zones.
- Zone 0 is where an explosive gas mixture iscontinuously present or present for long periods;
- Zone 1 is anarea in which an explosive mixture is likely to occur in normal operation, and
- Zone 2 is an area in which an explosive mixture is notlikely to occur in normal operation and if it occurs it will exist only for a short time.
There are eight techniques used to preventelectrical equipment from igniting explosive atmospheres:
- flameproof enclosures (EExd);
- purged and pressurised enclosures (EExp);
- encapsulation (EExm);
- oil immersion (EExo);
- powderfilling (EExq);
- non-sparking (EExn);
- intrinsic safety (EExi); and
- increased safety (EExe).
Only equipment certified for intrinsic safety (EExi) is permitted for use in Zone 0. All the other techniques are for zones 1 and 2, except EExn, which is for zone 2 only.
The difference between intrinsic safety EExi and increasedsafety EExe is that intrinsic safety limits the electrical energy sothat any sparks or heat generated by electrical equipment are so low it won't cause an explosion. EExi equipment can be connected and disconnected under power.
Equipment for increased safety EExe, on the other hand, must be mechanically designed to eliminate the possibility of sparks and hot surfaces. For example the terminals must be constructed so that vibration will not cause them to become unattached. Connection and disconnection of an EExe device is not allowed unless power is removed from the system. Note, however, the special design of theStahl IS-1 system equipment allows disconnection of EExe rated modules without removing the power.
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