That is the electromagnetic contractor (M1 and M2) cannot operate on the supply voltage (5V) alone, but can operate when the output voltages are superimposed on the supply voltage and the voltage is not lower than the set-point voltage (24V) of the electromagnetic coil. The basic concept of the fail-safe relay drive is that malfunctions of driver circuits lead to a change of the timing signal t1 and t2, which should be overlapping operations of M1 and M2 drive, and that will become a protection situation of false timing delay. Relay drive characteristics of M1 and M2 are shown in figure 4.20.