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

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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;

Automatic control circuit

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;

Manual override

The 'stay put' or latching 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.