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2007-04-26 00:00:00


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IEC 61508 Functional Safety Primer
Ian Loudon

Functional safety

What is functional safety?

We begin with a definition of safety. This is freedom from unacceptable risk of physical injury or of damage to the health of people, either directly or indirectly as a result of damage to property or to the environment.

Functional safety is part of the overall safety that depends on a system or equipment operating correctly in response to its inputs.

For example, an overtemperature protection device, using a thermal sensor in the windings of an electric motor to de-energise the motor before they can overheat, is an instance of functional safety. But providing specialised insulation to withstand high temperatures is not an instance of functional safety (although it is still an instance of safety and could protect against exactly the same hazard).

What is IEC 61508?

IEC 61508 (previously IEC 1508) is a seven-part standard that defines specific direction and guide-lines on the functional safety of electrical/electronic/programmable electronic (E/E/PES) safety-related systems.

Developed by the International Electrotechnical Commission (IEC, Geneva, Switzerland), the stan-dard directs proper management of all components of Safety Related Systems (SRS), from sensors and logic solvers, to the response function applications that will take the process to a safe state when predetermined variables are attained. The standard applies to the entire life cycle of the safety sys-tem, from concept, specification, design, operation and use, to the final decommissioning.

The Seven Parts of IEC 61508

IEC 61508-1

Key to system integrators, part one defines the overall safety lifecycle process. The standard employs qualitative or quantitative techniques to identify the process risk to the safety related system. These techniques may focus on such activities as project management, quality assurance and configuration management. As a consequence of its focus on the industrial process, Part one specifically applies to the performance for the E/E/PES system in the field.

IEC 61508-2

Part two provides objectives for the safety development of the E/E/PES. Software is further defined in part three however, it should be noted that part two maintains jurisdiction.

IEC 61508-3

Part three provides objectives for the safety development of the software residing in the E/E/PES.

IEC 61508-4

Part four provides the definitions, abbreviations and terminology used in the safety process that must be adhered to maintain consistency.

IEC 61508-5

Part five provides the formal approach for determining the Safety Integrity Level (SIL) of the safety system (SIS or SRS).

IEC 61508-6

Part six offers guidelines for applying IEC 61508 parts two and three.

IEC 61508-7

The final portion of IEC 61508 provides the safety techniques and measures relevant to parts two and three. These are industry-wide methods that the standard employs for safety.

More on Safety and how if relates to Omniflex Annunciators and Omniterm B series signal conditioning in the forthcoming issues

Omniwatch - 16 Channel Temperature Scanner

OmniWatch a New generation Temperature Monitoring system with a difference catering for up to sixteen temperatures at once. Temperature inputs may be any 3 wire RTD or Thermocouple and these are pre-programmed options. Cold Junction compensation and Linearisation are all standard features.


Alarms and Alarm Annunciation

Each temperature Input has an Alarm and Trip Setpoint allocated to it. Setpoints are configurable via the front panel keypad. Each setpoint is represented by an LED indicator, which can be configured with an optional annunciator sequence forcing an operator to Silence the audible, and acknowledge the flashing alarm LED. 


Temperature channel Display

Each temperature channel may be viewed in turn on the front panel LCD display with its setpoints by using a scroll button. 


Optional Individual Alarm and Trip Outputs

Each Setpoint should have the option for a relay or open collector output for the Alarm or Trip status providing early warning and shutdown interlock protection to the process or device being monitored.


Group Alarm Function

To save on relay outputs in certain applications  4 Group Alarm functions are used to bring up alarm for the attention of operators or to provide trip protection functionality. 


Optional Re-transmit

The temperature inputs can be re-transmitted to another device as 4-20mA outputs by simply fitting an analogue re-transmit output module if required.


Date and Time Stamp of Alarms

Every Alarm or trip is date and time stamped as it occurs. This is useful where unmanned stations are visited periodically and the recording of Alarm and Trips present an audit trail of what happened. These events can be available to a Network which can the route them to a SCADA Database system for further analysis.


Local History Log

A Local  History Log, which captures the last 64 Trips, and Alarms with its Date and Time stamp. 


Local Area Network Connections and Supervisory Systems

Networking on existing twisted pair cabling in the plant makes for easy retrofitting. Multidropping Up to 126 devices along a single twisted pair cable will provide the capacity a total 2016 Temperature Channels on a single twisted pair cable.  Predictive and Preventive Maintenance systems can then utilize the data for pro-active plant management.

Omni-X Displays does it ! - We offer you ultimate flexibility for annunciator displays

The Ultimate flexibility Omni X Displays made to the row and column configuration that suits your panel. The Omni-X Remote Displays provide from 8 to 144 points of annunciator display in a stand-alone panel-mount package.

Use with remote alarm logic such as the Omni16C RLU series for your safety critical alarms, or connect to your PLC or SCADA computer for an operator friendly display of essential alarm information.


Uses both dual incandescent lamps or ultra-reliable solid state LED back-lighting, these displays provide panel indication where remote alarm handling or display logic is required. Add the optional Integral Control Pushbutton station for easy alarm annunciator operator functions. These displays provide an ideal superior low cost alternative to individual panel lamps.

Last Month:

Last month we talked about several important topics including:

  • IEC 61508 Functional Safety Primer
  • Omniwatch - 16 Channel Temperature Scanner
  • Omni-X Displays does it ! - We offer you ultimate flexibility for annunciator displays

If you missed these or other key discussions,
you can find the back issues on the newsletters page of our website:


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(c) Copyright 2003 OMNIFLEX PTY LTD 

In this Issue
IEC 61508 Functional Safety Primer
Omniwatch - 16 Channel Temperature Scanner
Omni-X Displays does it ! - We offer you ultimate flexibility for annunciator displays
Last Month's Issue

Omniflex and IEC61508

OMNI16 Series Annunciators meet IEC61508 Compliance

Omniterm B Series Signal Conditioning Modules are designed in accordance with IEC61508

Omniterm B Series is ideal for Safety Shutdown Interface applications.

Omniterm B series for Single Loop Integrity protection systems


 Omni16 Annunciators for critical plant alarms - dedicated and safety rated




All-in-one Temperature Scanner-

16 Temperature Channels one unit.


Any RTD or Thermocouple


Viewed in degrees Celsius or Fahrenheit scales.


Full Alarm Annunciation Functions

Individual Indication for Temperature Alarms and Trips (32 Indicators in all)

Alarms are Date and Time stamped for chronological reference.


Group Alarms and Trips facility to save I/O


Optional Individual Independent Alarm and Trip outputs for selective shutdown interlocks.


Configuration via Front Panel or via PC

Alarm History


Log of last 64 alarms


SCADA and OPC compatible with logged events to data base systems.


Click for Datasheet



Typical Omni-X Display


OmniX Displays


Annunciator Displays in the row and column format you want them


Sizes from 8 to 144 Windows


Bright Back-Lit Display.


Incandescent or Solid-State LED Display


Redundant Lamps in every window for fail safety.


Plug-in or Screw Termination for low Cost integration


Window legends easily printed on inkjet/laser


Optional integral pushbuttons and audible.

Click for Datasheet