The manufacturers' passport to safety
01 August 2008
In today’s marketplace, the manufacturer has to think globally. And that is where functional safety can help. The overall objective of functional safety is to protect workers by reducing incidents and to reduce risk of loss and costs associated with risk.
The cost of risk is the sum of insurance premiums, claims administration fees, workers’ compensation claims, risk management department costs (salary, travel, fringe benefits, and so on), legal fees, and other related costs such as state fees, assessments and consultants. Cost of risk is a measurable direct cost.
Functional safety benefits the user by cutting costs but not reducing safety. It provides them with a high safety/failure ratio so that they can maximise both production and safety. Using functional safety and its applicable standards requires the availability of data such as probability of dangerous failure per hour or mean time to dangerous failure. This adds the time domain because functional safety allows you to calculate the reliability of your safety system. This should not be regarded as an absolute and certain value but more as an indicative and relative quantification that can prevent the use of unsuitable equipment.
Like a travel passport that authorises you to travel internationally, a control system that uses a functional safety concept gives you a ‘passport to safety’. That is because a system using functional safety meets international safety standards, making your system compliant with safety standards applicable in most nations. As a result, you have significantly increased global market opportunities to sell your product to new customers.
Functional safety is the part of the overall safety implementation that depends on the correct functioning of the process or equipment in response to operational safety inputs. It relates to the physical operation of a machine or process. In other words, functional safety equals the confidence in the ability of the safety-related control system to perform its function over a specified time period.
The term functional safety is often associated only with programmable safety systems, but this is a misconception. It covers a range of devices, such as interlocks, light curtains, safety relays, safety PLCs, safety contactors and safety drives that are interconnected to form a safety system.
An example of functional safety is an over-temperature protection device using a thermal sensor in an electric motor’s windings to de-energise the motor before it can overheat. The thermal sensor performs a function, or action, to help provide safety.
Another example, compare fixed hard guarding to electrically interlocked guarding. The hard guarding is not considered functional safety. Although the fixed guard does perform a safety function — keeping people out and materials in — it is not considered functional safety because there is no input to a system. The interlocked door, however, is an example of functional safety. When the guard is opened, the interlock serves as an ‘input’ to a system that assures a safe state is achieved.
Functional safety falls under the umbrella of the risk reduction process. Risk involves four steps: elimination using inherently safe design concepts; safeguarding and protective measures with hard guarding and safety devices; complimentary safety measures including personal protective equipment (PPE); and safe working practice achieved with procedures, training and supervision.
Functional safety addresses the safeguarding portion of the risk reduction process. When you implement integrated safety by designing systems so that safety and environmental considerations are fundamental elements of doing work, you include functional safety measures as part of the safety system.
Performing a hazard analysis and risk assessment is the first step in the safety life cycle and functional safety is part of the safety life cycle because it is involved in steps 2, 3 and 4 (see table 1).
When designing equipment and associated control systems, a hazard analysis will help determine whether functional safety is necessary to ensure adequate protection against each significant hazard. If so, then users can integrate functional safety into the machine design requirements, implementation and validation.
A hazard analysis identifies what has to be done to avoid hazardous events associated with the operation and maintenance of the machinery. In addition, a risk assessment gives the safety integrity required of the safety system for the risk to be acceptable.
Table 1: Five steps of the safety life cycle
Risk assessment or hazard analysis.
Safety system requirements
Safety system implementation
Safety system validation
Maintaining and improving the safety system
Footnote
A white paper entitled Proving the Value of Safety outlines the long-term financial benefits manufacturers can reap by integrating comprehensive machine safety programmes into their workplaces as a form of insurance against potential risks. To read the White Paper, please click the link below.
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