I was wondering what is the role/purpose of the electric component in the red square. This is the schematic of a 230V single phase induction motor.

enter image description here

What is also not clear to me are the connections on the right side. The first two wires are neutral (blue) and live (brown). What is the third one? Earth? If yes, why is there an tild sign (AC) above it?


This component is a undervoltage protection device. When the voltage is high enough the device will close a make contact so current can flow. When the voltage is to low the contact will open or stay open. This is a over voltage-relay, but imagine it with the smaller than(<) character.

Link to image image description here

  • \$\begingroup\$ Hi, As required by this site rule, when you include something in an answer (e.g. photo, image or text) which isn't your original work, you need to properly reference (cite) it. That image seems to have come from somewhere else, so can you please edit your answer and add a link back to the original web page? Thanks. :-) \$\endgroup\$ – SamGibson Sep 9 at 20:13
  • \$\begingroup\$ @SamGibson Thank you for the information i didn't know this. I appreciate this :) \$\endgroup\$ – Randy Peeters Sep 9 at 20:15
  • \$\begingroup\$ Thanks, but that's not a link to the original web page. It seems that you've added a data block which contains the image data itself, not a link to the original web page. So please try again, the link needs to go to the original web page, which is the source of that image. Alternatively, I have found this page, which contains the correct symbol for an under-voltage relay, not the over-voltage relay image you included. Perhaps it would be easier to use that one (you would need to change some of your text too). \$\endgroup\$ – SamGibson Sep 9 at 20:25

The red square is an undervoltage trip. It prevents the breaker from remaining or being closed when there is not enough voltage.
Commonly found on equipment and machinery that for safety reasons may never start automatically when power returns after a failure.


The black wire provides AC back from the motor as long as the thermal protector is closed indicating normal temperature. The device in the red box probably opens the switch when the thermal protector opens.

  • \$\begingroup\$ does it works like that (let's say motor overheats) : 1) thermal protector switch opens 2) undervoltage trip detects it and open the 3 switches (blue / brown / black). 3) motor shutdown ? \$\endgroup\$ – tigrou Sep 8 at 15:30
  • \$\begingroup\$ Yes, I believe that is what happens. \$\endgroup\$ – Charles Cowie Sep 8 at 15:33

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