I am currently facing the issue that I am currently working on a subsystem for a high power industrial automation system. The panel of this subsystem consists of a lot of components, however here only a 3-position turn switch as well as a safety switch are relevant.

The desired function is the following: whenever the 3-state turn switch contacts to its center contact I would like to turn the safety switch back to its off position. This is so that whenever the operator switches the turn switch from position 1 to position 3 or vice-versa the safety switch is reset into its off position and needs to be manually enabled again.

My question is: How would one go about automatically disengaging a safety switch mechanically in a reliable way? Any help would be greatly appreciated.

  • \$\begingroup\$ what action disengages the safety switch now? \$\endgroup\$
    – jsotola
    Commented Apr 24, 2023 at 16:56
  • \$\begingroup\$ when the 3 position turn switch crosses it's middle contact it should turn off the safety switch \$\endgroup\$
    – user338269
    Commented Apr 24, 2023 at 16:58
  • \$\begingroup\$ What you have described sounds to me like a 1-0-2 selection switch, where left or 1 position connects circuit 1, and the right or 2 position connects circuit 2. If you switch to the middle or 0 position, it's off or nothing is connected. It is used, for example, to switch between two power sources while not making any contact between them. \$\endgroup\$ Commented Apr 24, 2023 at 17:09
  • \$\begingroup\$ Hello and welcome. Can I ask if this is a real equipment installation or theoretical design? \$\endgroup\$
    – jonathanjo
    Commented Apr 24, 2023 at 17:12
  • 1
    \$\begingroup\$ This is easy to do electrically. No idea how to do it mechanically, especially not being able to see what you have available to you. \$\endgroup\$ Commented Apr 24, 2023 at 17:35

2 Answers 2


An electro-mechanical solution is the standard method of achieving this using standard components.


simulate this circuit – Schematic created using CircuitLab

Figure 1. A possible solution.

How it works:

  • A 3-position, centre-off, 2-pole switch is required along with a START (or RESET) button and a relay (SAFETY) capable of switching the load.
  • When the switch is turned to POSN1 power is fed to the open START button.
  • When START is pressed SAFETY relay is energised and power is fed to LAMP1. The SAFETY relay is latched on by its own contact.
  • If the 3-position switch is switched back to centre position then both POSN1 contacts will open and SAFETY relay will drop out disabling the circuit.
  • Switching to POSN2 works in a similar fashion.

Note that very fast switching between POSN1 and POSN2 could mean that the relay doesn't have time to drop out.


simulate this circuit

Figure 2. A safer solution.

Figure 2 ensures that the safety circuits drop out when switching from 1 to 2. START(b) is a second contact of the START button.


There are a lot of mechanical interlock mechanisms which might do what you want.

Here's a sketch of a 3-position rotary switch with a cam. When it is in position 2, the cam releases an e-stop switch of some kind. (Spring-loaded MCB, for example, a conventional e-stop switch, or a microswitch, or many other possibilities.)

enter image description here


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