How can I wire an emergency stop that requires a secondary reset before the circuit is closed again? I am working on a CNC, and am brand new to the electrical component of them. The problem I am coming across is if I activate the e-stop which would be connected to the contactor and reset it, the spindle power would resume. What electrical component am I looking for that breaks when power is lost, and requires a reset before continuing? It could also be the actual spindle on/off switch too, if you have an idea with that in mind. Thank you!

  • \$\begingroup\$ Look into safety relays and safety controllers... Or design a circuit that latches and stayes latched till a circuit is opened via a reset button? \$\endgroup\$
    – MadHatter
    May 10, 2020 at 6:28
  • \$\begingroup\$ Useful search term : NVR switch \$\endgroup\$
    – user16324
    May 10, 2020 at 9:06

2 Answers 2


It's the latching circuit, that you require, which is the basis for relay logic.

The schematic shows how it's to be configured for the E-Stop function.

enter image description here

When the mechanical-latch-type E-Stop button is actuated, it de-energises the 'Control' relay and shuts down the machine.

When the E-Stop button is released, the machine remains shut down till the 'Control On', 'Spindle Start' and 'Automatic Start' buttons are actuated with all starting preconditions fulfilled.

This safety feature is called 'Zero Volt' safety. When power fails, the machine shuts down and does not restart on restoration of power until it is commanded to.

The manual E-Stop button is actuated when a danger is foreseen by the operator. Automatic E-Stop can be triggered when the 'NC' contacts of other fault tripping mechanisms are wired in series with the E-stop 'NC' contact.

  • \$\begingroup\$ Zero Volt Safety! That's what I was looking for. I'll work up a demo, but for now my question has been answered. Thank you :) \$\endgroup\$
    – Rulon Rock
    May 10, 2020 at 21:18
  • \$\begingroup\$ Anytime, Rulon Rock! \$\endgroup\$
    – vu2nan
    May 11, 2020 at 3:20
  • \$\begingroup\$ In addition to the above, it's good practice to wire the E-stop switch directly in series with the motor contactor coil. This makes the action of the E-stop switch on the motor as direct as possible I.e. with no other intermediate components to fail.) \$\endgroup\$ May 11, 2020 at 4:58
  • \$\begingroup\$ Hi Li-aung Yip, That would not be necessary since the E-Stop button effectively cuts the control voltage. Failure of the E-Stop button itself could be problem. But that could be taken care of by using more E-Stop buttons in series. \$\endgroup\$
    – vu2nan
    May 11, 2020 at 5:05
  • \$\begingroup\$ Thank you, Rulon. \$\endgroup\$
    – vu2nan
    Jun 22, 2020 at 10:36

Your looking for a latch circuit, sometimes called a start/stop circuit, the E-stops would be what "stop" the circuit, and your reset would be the "start" that enables the latch and then the output from that latch can power your circuit or another existing start/stop system.

enter image description here

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