Can you provide an example of a Category 3 safety circuit?

An example of a Category 3 safety circuit per ISO 13849-1 or EN 954-1 is a dual-channel emergency stop circuit with monitoring. In this design, the emergency stop button has two normally closed contacts, each contact is wired in a separate channel to a safety relay or safety PLC. Both channels are monitored so that a single fault in either channel does not lead to loss of safety function, and the fault is detected. For instance, Channel 1 goes through contact A to input 1 of a safety relay, Channel 2 goes through contact B to input 2. The safety relay checks for cross faults and for discrepancy between channels. If one contact welds or a wire breaks, the relay detects the mismatch and prevents restart until repaired. The output of the safety relay then controls redundant contactors that cut power to hazardous motion. This architecture provides single fault tolerance and fault detection, which is the core requirement of Category 3. Practical implementations use safety relays like Pilz PNOZ or Siemens 3SK and follow wiring guidelines for separation and monitoring.