Two electro-mechanical devices are required per gate because an electro-mechanical device is subject to mechanical fatigue. To acheive a dual redundant circuit 2 limit switches are required incase 1 switch welds or remains in the gate closed position. In the case of a coded magnetic reed switch, in conjunction with a fail-to-safe control relay, they are not suseptiple to mechanical fatigue and are not subject to over voltage as the fail-to-safe relay supplies the coded magnetic reed switches with 30mA which is isolated from the power supply. This is my interpretation of AS4024.1.
