of the actuator 32 to the position as shown in fig3 b at this time the knob 31a is allowed to protrude in such a direction to break the connection between the power source and power supply circuit 12 nevertheless when the actuator 32 is spaced away from the core 34 by a small distance the knob 31a breaks the circuit therebetween and accordingly the voltage applied across the coil 35 approximates the zero voltage as a consequence the magnetism occurring on the core 34 in the direction expelling the permanent magnet 32a will disappear and the non magnetized core 34 will attract the magnet 32a the switch 31 returns back to the condition as shown in fig3 a at once in other words the application of the breaking signals fails to turn the power supply path off it is desirable for the reason discussed above that the voltage to the coil 35 is of such an amplitude to move the actuator 32 to a sufficient distance where the repulsing force of the spring 33 is stronger than the attracting force between the core 34 and permanent magnet 32a or hence the attracting force does not extend to the core 34 the arrangment of a system illustrated with reference to fig4 and 5 shows one approach to avoiding the above described disadvantage this structure is the same as that of fig1 and 2 with the exception that on and off states of a switch 41 for controlling voltage supply of the power source 11 to the switch driver arrangment 9 are accomplished by a delay circuit 42 10 39 is the switch as illustrated in fig3 the outputs fig5 c at the time t 3 from the timer 5 are introduced into the delay.