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Selecting the Right Safety Category Level for Interlock Switches
In safety standards, the term "category" is commonly used to specify a particular level of reliability. ISO 13849, which deals with the safety of machinery and its control systems, defines "category" as the classification of the safety-related parts of a control system based on their ability to resist faults and their behavior when faults occur. In essence, a category explains how a safety circuit is wired, how it detects faults, and how it responds to them. There are five categories: B, 1, 2, 3, and 4. These represent a range from least to most reliable in maintaining safety in case of a fault.
In practice, Banner Engineering’s SI-GL42 Series of locking safety interlock switches, like all mechanical interlock switches, can typically meet Category 2, Category 3, or Category 4 depending on the number of switches used per gate, whether the switches are wired individually or connected in series to the safety control system, and other requirements. Selecting the proper category for your safety function designs requires conducting a risk assessment to identify the hazards and risks that will need to be addressed.
Category 2 Safety (One Mechanical Interlock per Gate)
Category 2 uses well-established principles and components. It includes a test function that checks the integrity of the safety function at “suitable intervals” by the safety control system. Since a Category 2 safety design is still single-channel, one failure between the test intervals could lead to the loss of the safety function.
Category 3 Safety (Two Interlocks per Gate)
Category 3 and Category 4 add a second, redundant channel. The redundant channel (along with well-established principles, components, and monitoring explained in Category 2) allows the system to safely bring the machine to a safe state despite a single fault within the safety system. This circuit also prevents a reset of the safety system until the fault has been corrected. Category 3 can tolerate a single fault without losing the safety function. When mechanical interlocks like Banner’s SI-GL42 safety interlock switches are connected in series, two switches per gate (as shown in the diagram) would achieve a Category 3 safety rating since a single mechanical switch failure could go undetected if another gate with properly working switches is cycled open and closed (a concept known as “fault masking”). Category 3 meets OSHA’s requirement for control reliability.
Category 4 Safety (Two Interlocks per Gate)
Category 4 must detect an accumulation of faults, allowing it to maintain its safety function. In the case of mechanical interlocks like Banner’s SI-GL42 safety interlock switches, wiring safety contacts from two safety switches per interlocked guard in a dual-channel connection to a safety module, safety controller, or other safety related parts of the control system can achieve Category 4 safety. Like Category 3, Category 4 meets OSHA’s requirement for control reliability.
