Designers of industrial automated machinery and equipment must follow regulations and standards to ensure they are delivering safe electrical designs; however, there have been cases where uncertainty about the standards has led to overly cautious and costly designs. Or in the worst cases, safety design has been compromised by concerns regarding excessive complexity and cost.
Fortunately, updated standards and newly available safety products are making it easier and more cost-effective to incorporate acceptable levels of electrical safety in more applications.
Machinery manufacturers rely on the ISO 13849 Safety of Machinery standard for direction regarding required safety sensing and electrical interlocking methods for machines and equipment. The standard guides designers to perform a Risk Assessment. The Risk Assessment determines the required performance level (PLr) based on worker exposure, potential injury severity, and the possibility of avoiding the hazard. The PLr may range from PLa at the lowest up to PLe at the highest (most hazardous). Other associated ratings include categories (B, 1, 2, 3, and 4), the diagnostic coverage (DC) rating of the system, and the mean time to dangerous failure (MTTFD), as shown in Figure 1.
Unfortunately, in the past, it was all too easy to misinterpret the requirements and follow a more complex and expensive category 3 approach to providing electrical safety. Categories 3 and higher call for full redundancy, necessitating the use of more specialized devices and doubling the required wiring.
In 2015, ISO 13849 was updated with clarifications regarding category 2, allowing a less restrictive solution where “testing occurs immediately upon demand of the safety function.”
Electrical safety circuits typically use inputs from sensors and emergency stop buttons to identify trouble conditions. When trouble occurs, the circuits electrically interlock or disable physical devices like motors and valves, driving equipment to the safest state possible. Because category 2 does not require redundancy, it is possible to use single input connections, safety relay modules, and outputs, simplifying design and implementation.
In the past, safety components like relay modules were more likely to be rated as category 3 or higher. Without the ready availability of other options, designers were forced to implement more complex fully redundant safety schemes that were beyond what was necessary. In some situations, the expense likely led to less-than-ideal configurations.
Once ISO 13849 was revised to provide better clarity about category 2 requirements, industry suppliers could offer safety relay modules ideally suited for this service (Figure 2). Because these new safety relay modules incorporate the right subset of features to meet the exact needs for category 2 electrical safety, they are more economical and help designers to create proper category 2 safety circuits. These devices enable users to reduce the design effort, save control panel space, and minimize field installation labor.
Category 2 electrical safety relay modules make it much more practical for end users to incorporate proper safety measures in far more situations than before, resulting in improved safety for workers worldwide.
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