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    How can manual override of an automatic control system be implemented?

    There are many instances where it is necessary to have automatic control of a motor and also the provision of a manual override.
     

    • A typical automatic control utilising contact S1 (the process monitoring contact e.g. thermostat, float switch, limit switch) to control a contactor (coil KM1) to switch a motor is shown below;


     

    • In order to implement manual override, a three position selector switch; S2 (e.g. Schneider Electric Harmony type ZB4BD3 + ZB4BZ103) is utilised as shown in the circuit diagram below;




    The 'stay put' selector switch has 2 qty N/O (normally open) contacts;
     

    • The N/O contact in position 1 ('Auto' control) is wired in series with contact S1 to enable automatic control.
    • The N/O contact in position 3 ('Manual' control) is wired in parallel with contact S1 overriding the automatic control and therefore maintaining the operation of the contactor.
    • The centre position (2)  provides a manual 'off'  selection as both N/O contacts are open in this position.
    Note: The term 'Hand' control is often used in place of 'Manual' control, such that this arrangement is either referred to as 'Auto - Off - Manual' or 'Auto - Off - Hand'  control.

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