Selecting the Right Safety Category Level for Interlock Switches

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 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 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.