Product category:
Machinery/plant guarding and protection
News Release from: R Stahl | Subject: Intrinsic safety engineering and fieldbus
Edited by the Manufacturingtalk Editorial
Team on 02 September 2003
Intrinsic safety engineering enters
fieldbus era
With the widespread acceptance of fieldbus, intrinsic safety engineering has to change direction to adapt to digitalisation, de-centralisation, and product standardisation.
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 of flammable gases in hazardous areas
This article was originally published on Manufacturingtalk on 18 Apr 2003 at 8.00am (UK)
Related stories
Isolators designed for intrinsically safe circuits
An innovative and future-leading series of modules for intrinsically safe circuits, an isolator series has two input channels; all standard analogue and digital signals are accommodated.
Crane kits cut costs, improve safety, lighten load
Crane builder has finally solved the problem of transporting long and heavy crane bridges half-way around the world: send them in kit of hoist gear, end carriage and all electrical fittings.
But today, with the widespread acceptance of fieldbus systems, intrinsic safety engineering must take 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 of Stahl's IS-1 Remote I/O System, an IS system concept specifically developed for use with fieldbus systems.
Further reading
Reliable SS plug connectors meet hazards
An updated and optimised version of a mini plug connector system makes operating errors and damage impossible, even when the system is used in dirty or corrosive environments.
Control units operate in hazardous dusts
Control units are designed for use in areas where combustible dusts are a hazard and such areas include mills, malting plants and milk powder manufacturing facilities.
Panel PCs run standard Windows applications
A powerful range of panel PCs with standard hardware interfaces and a standard operating system enable users to run standard Windows applications.
In the past three years, nearly 4,000 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/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 pre-requisite 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 opto-couplers, which require their own power supply.
Since the circuits are usually not connected to earth potential, they have a high immunity to interference.
Whilst 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 (see "Hazardous area nomenclature" at end).
IS-1 is simple 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 cross-connection 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 4, 8 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 - 20 mA 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 data base 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 stand-by).
In this case, all central functions of a field station, power supply and fieldbus connection are present in duplicate.
Special features of IS-1 - 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 start-up, 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.
About Stahl - with an annual turnover of EUR 240 million and over 2,000 employees worldwide, the R.
Stahl Technology Group is a leading supplier of explosion protection and material handling components and installations.
Attachment 1 - are remote I/O systems only 'transitional' solutions? It has been said that Remote I/O-intrinsically safe or otherwise-is only needed during the period of transition where we are currently in, until all sensors and actuators can be driven directly on the fieldbus.
However, the following points should be noted: The connection of simple binary devices such as contacts and solenoid valves or temperature sensors (Pt 100, thermocouple) to Remote I/O is much more cost-effective in comparison with bus capable devices.
Even though it is considered "old" technology, the installed base of HART field devices will continue to grow.
Remote I/O today offers integration of HART communication and thus provides the basis for central control of the field device data base.
Remote I/O will not be replaced by the fieldbus, but Remote I/O will integrate the sensor/actuator fieldbus.
The user is thus able to choose the technology which best suits him.
OPC interfaces, ActiveX-technology etc.
will ensure that Remote I/O will be even better integrated into the structure of automation systems in the future.
Access to process data, device parameters and diagnostic information will become possible from a central engineering station, both for remote I/O and for subordinate fieldbuses and their devices.
Ethernet with TCP/IP will also be increasingly used in industrial automation.
Remote I/O is just the thing to define an interface between field devices and Ethernet.
Finally, one further perspective should not be omitted: the decentralisation of open and closed loop control functions.
Function blocks, such as they are processed today in PLCs, distributed control systems or PCs, can alternatively be integrated into Remote I/O.
Verdict: the future belongs to Remote I/O.
Attachment 2 - hazardous area nomenclature - potentially explosive areas are classified into three zones.
Zone 0 is where an explosive gas mixture is continuously present or present for long periods; Zone 1 is an area in which an explosive mixture is likely to occur in normal operation, and Zone 2 is an area in which an explosive mixture is not likely to occur in normal operation and if it occurs it will exist only for a short time.
There are eight techniques used to prevent electrical equipment from igniting explosive atmospheres: flameproof enclosures (EExd); purged and pressurised enclosures (EExp); encapsulation (EExm); oil immersion (EExo); powder filling (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 increased safety EExe is that intrinsic safety limits the electrical energy so that 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 the Stahl IS-1 system equipment allows disconnection of EExe rated modules without removing the power.
• R Stahl: contact details and other news
• Email this article to a colleague
• Register for the free Manufacturingtalk email newsletter
• Manufacturingtalk Home Page
