Issues in System Design

Designing a safe and highly productive system requires thinking about safety from the outset, not down the line.

YOU must start thinking safety early. One of the most frequent mistakes system designers make is thinking only of the production process and ignoring the fact that the system will be safeguarded later to prevent injury, before it is put into productive use.

The result is that operators often find the safeguards inhibit their ability to perform their jobs efficiently, resulting in reduced productivity. Many operators therefore unsafely bypass the safeguards in order to keep a sustained production pace or troubleshoot equipment. This has become one the most frequent causes of machine-related accidents in automated production today.

Training personnel is critical to avoiding this. However, the safest systems are those where the required safeguards are considered simultaneously with the production system's design. In these cases, systems are designed that allow personnel to safely and easily perform their jobs with no need to bypass the installed safeguards. As a side benefit, productivity can often be dramatically improved while helping to meet the goal of zero injuries.

A Simple Plan? Hardly
Properly designing a safeguarding system is not a simple task. It first requires a clear understanding the hazards that exist in the system.

This is commonly achieved through a formal Risk Assessment process that identifies and documents all production and non-production tasks and the hazards associated with them. The hazards are then classified based on criteria, such as severity of the potential injury, frequency of access to the hazard, and the possibility of avoidance.

Risk Assessments should be performed during the design phase and prior to commissioning to ensure that no new hazards have arisen in the integration process.

Once the hazards have been identified and classified, up-to-date knowledge of current safeguarding technologies is crucial. There have been many advances in safeguarding technologies that allow for greater flexibility in the design and integration of the safety system. The designer should be well-versed in the productivity-enhancing side of safeguarding, as well. Functionality such as PSDI, or Presence Sensing Device Initiation, which allows the safeguard to perform double duty by safeguarding personnel and simultaneously initiating the machine cycle, has significantly improved productivity for many companies while reducing the ergonomic repetitive stress of a separate cycle-start button.

It is always advisable to select safety components and interfaces that have been third-party-certified to local standards by an accredited agency to ensure their intended functionality.

System designers must then know how to safely apply the safeguards. For instance, designers must understand Safety Distance, or the distance from a hazard that the safeguard must be mounted to ensure that a hazard will cease before personnel can reach it. Safe Distance calculations should be made at the initial commissioning of the system and periodically thereafter to ensure that mechanical wear on the system over time has not caused the hazard?s stopping time to increase, thereby increasing the required safety distance.

Interfacing and Wiring Issues
Properly interfacing safeguards with the machine control and/or e-stop circuitry is another area that requires serious attention. Depending upon the Risk Assessment, safety circuits may be required to be designed such that component failures will not prevent hazardous motion from stopping, and that restart will be prevented until the fault is corrected. This can obviously force an extremely complex circuit design. Prepackaged Safety Relays (DIN Rail mountable pre-wired circuits) are a readily available way to reduce safety circuit design time and wiring errors. Safe PLCs and Safe Bus Networks are also becoming popular ways to safely reduce safety wiring and troubleshooting costs for companies on the forefront of safety technology.

Internal or External Expertise?
So, how should a company handle these complex issues surrounding the safety of its personnel and ensure that its systems are designed properly?

Many companies develop this expertise internally, while others turn to outside safety consultants and manufacturers of safeguarding systems for their expertise, rather than try to keep up with the rapidly changing technologies and industry standards. Either way, the goals of a safe environment and a productive environment are better achieved when considered together.

This article originally appeared in the May 2004 issue of Occupational Health & Safety.

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