Thanks to the development of safety guards and other devices, operating industrial machinery is much safer than it was decades ago. In addition to mechanical safety, your industrial manufacturing safety team should also be following electrical safety standards and best practices. Safety circuit design, a significant part of the third step in the safety lifecycle, is the engineering process of creating electrical circuits designed to safeguard people and equipment through switches, relays, or other technologies that detect hazards and take protective actions to help prevent accidents and injuries.
Before you begin your safety circuit design, you’ll need a completed risk assessment and a Safety Functional Requirements Specification (SFRS). Safety circuit design should be performed by an individual who understands the content covered by safety standard ISO 13849-1, as well as any other standards that apply to the machinery, devices, or safety principles being implemented. The SFRS should outline most of these; however, it may not be an all-inclusive list.
What to Know
Keeping in mind that some answers will come from the risk assessment and SFRS, these are the things you will need to know before beginning the design of a safety circuit:
- Types of devices required to meet the criteria specified in the SFRS
- All devices from the input device to the final output device that will be part of the safety function
- Any special requirements for individual components as outlined in their respective user manuals
- Performance Level required (PLr) by the risk assessment
Two of the most significant elements of safety circuit design are component selection and circuit architecture. You can generally find safety data for the safety components in the product data sheet or user manual. This will come in the form of one or more of the following:
- Performance Level (PL)
- Safety Integrity Level (SIL)
- Probability of Dangerous Failure Per Hour (PFHd)
- Mean Time to Dangerous Failure (MTTFd)
- Number of mechanical operations before 10% of product failure (B10d)
Verification Requirements
Knowledge of the next step in the process, Verification, will be required to select the proper components to meet the Performance Level required (PLr). Performance Levels are designated with letters based on how reliable they are. For example, PLa is the least reliable and PLe is the most reliable for the highest-risk applications.
It’s essential to note that simply selecting components of the same Performance Level (PL) will not always ensure that the safety function meets the specified PL. Additionally, keep in mind that if you need to meet PLe, there are more stringent requirements outlined in ISO 13849-1.
Architecture categories have many requirements, but the basics are as follows:
- Category B – Single channel, Low to Medium MTTFd, and characterized by selection of components
- Category 1 – Single channel, High MTTFd, also characterized by selection of components.
- Category 2 – Single channel, Low to High MTTFd, characterized by structure, and introduces Diagnostic Coverage (DC), which needs to be Low to Medium
- Category 3 – Dual channel, Low to High MTTFd, characterized by structure, and low to Medium DC
- Category 4 – Dual channel, High MTTFd, characterized by structure, and High DC
In practice, Category 4 is less often required because many hazards do not reach a PLr of e, and Category 2 is also less frequently used due to its testing requirements (see ISO 13849-1 for more information).
Points to Remember
Most safety systems will be composed of more than one safety function, so there are additional things to consider before designing a safety system comprised of multiple safety circuits:
- How much safety I/O is required for the entirety of the system?
- Additional I/O such as Resets and Request to Enter
- Where are devices located? Will everything be wired to a central location or multiple places?
- Is there an existing logic system with a safety PLC that can handle the amount of new safety I/O? If not, is it better to replace the existing system or add on safety relay-type logic systems?
In the end, the best way to avoid additional costs and delays in implementing your safety system is to use a competent designer, engineer or company that understands safety and the standards that apply. Partnering with Rockwell Automation for your design and verification means investing in the safety and efficiency of your operations, whether you need us to complete the design or perform the verification of your design. Contact us to embark on a journey towards safer, more productive industrial practices with confidence.
