Our Electronic Timed-delay Unit Trapped Key Interlock Switches are used in applications that require an elapsed time to occur before allowing access to a hazardous area. This interlock switch uses a SI Guardmaster^® safety relay in combination with an EMD Guardmaster safety relay to execute the timing sequence. Turning a non-removable key initiates the timer. When the EMD times out, its output energizes an internal solenoid which then allows the removal of either one or two trapped keys.