Frequently asked questions
How can manual override of an automatic control system be implemented?
FA23069
10 February 2022
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' 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.