Machine safety refers to the measures taken to ensure the safety of operators, workers, and other individuals who may come into contact with or work in the vicinity of machinery. Safety categories and performance levels are two important concepts to evaluate and design safety systems for machines. A risk assessment is a process to identify, evaluate, and prioritize potential hazards and risks associated with a particular activity, process, or system. The goal of a risk assessment is to identify potential hazards and risks and to take steps to prevent or mitigate those risks. The hierarchy of controls can determine the best way to mitigate or eliminate risk. We can use this hierarchy, including elimination, substitution, engineering, and administrative controls, and personal protective equipment (PPE), to properly mitigate risk. Our focus here is on engineering controls and how they relate to categories and performance levels.
The performance level (PL) of machine safety components is a measure of the reliability and effectiveness of safety systems. Defined as EN ISO 13849-1 standard by the International Organization for Standardization (ISO), it is based on the probability of a safety system failing to perform its intended function. Performance levels are designated by the letters “a” through “e” with PLa being the lowest level of safety and PLe being the highest. Assessing the safety function of the machinery and evaluating the likelihood of a dangerous failure occurring determines the performance level.
Four levels of protection
The categories of machine safety components refer to the four levels of protection required to ensure the safe operation of machinery, as defined by the ISO. Figure 1 below shows how the measured risk determines the performance level and category of circuit performance.
- Category 1: The occurrence of a fault can lead to loss of the safety function. Single channel safety circuit.
- Category 2: The occurrence of a fault can lead to loss of the safety function between checks. Single channel safety circuit with monitoring.
- Category 3: When a single fault occurs, the safety function is always performed. Some faults, but not all, can be detected, but the accumulation of those undetected faults can lead to the loss of the safety function. This category can be implemented using control reliable devices in a dual channel redundant safety circuit that includes monitoring.
- Category 4: When a fault occurs, the safety function is always performed. Faults will be detected in time to prevent a loss of the safety function and is implemented using control reliable devices in a dual channel redundant safety circuit that includes monitoring.
Using control reliable devices is crucial in Category 3 and 4 safety circuits. One example of a control reliable device is a safety relay that mechanically interlocks the control contacts to the auxiliary contacts. Being mechanically interlocked means when the relay changes states the auxiliary contact will also changes states. Another example of a control reliable device is a safety PLC. A standard PLC is not rated to control safety functions because it is not control reliable and a malfunction could lead to the loss of a safety function.
The selection of the appropriate category and performance level for devices used to mitigate a risk in a machine is crucial for ensuring the safety of operators and other individuals. While it is important to note that the purpose of this blog is to provide information, it is not enough to qualify individuals to design or test safety systems. In summary, the category of machine safety defines the level of protection required for safe operation, while the performance level measures the reliability and effectiveness of safety systems.
Now let us go automate with a focus on safety!