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OT: safety switch

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  • OT: safety switch

    Hey gang, what type of switch opens if the power is cut, preventing a saw (or whatever) from unexpectedly coming back on when power is restored?

  • #2
    "Magnetic Switch"; usually a correctly wired contactor.


    • #3
      In UK we would call that type of switch a 'No Volt Release' switch. It would typically be regarded as essential for mains connected power tools.


      • #4
        I believe you are referring to a Safety Relay, mandatory in most European countries and gradually gaining acceptance in N.A.
        Practically all large machinery imported here from Europe have them.
        There is many different options and configurations they can have according to the machine requirements.


        • #5
          Any properly rated relay with at least three sets of contacts can be wired to hold itself on as long as power it present, open on power loss and require intervention to turn back on. One contact set is wired so that when the finger powered on switch is pressed to supply power to the relay coil the contact closes and continues to supply power to the coil when the start button is released. I wired my CNC machine controller that way.
          Free software for calculating bolt circles and similar: Click Here


          • #6
            I wired one years ago as Evan has suggested. The only difference I made was that to turn the machine on, you poked a plastic knob which pushed directly on the relay armature. The knob itself had its own spring, which retracted the knob to its home position, leaving the relay armature free to release without having to push the knob back. To shut off, there was a main switch wired in series with the power.

            To run the machine then, you had to first flip the main switch, then hit the start button.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


            • #7
              I call it an E-stop or the big red button. Some call it the "oh sh!^" button, or their favorite of quite a few other names.

              Any big red "mushroom" button thats built for industrial use will lock itself out once hit, meaning you have to twist or otherwise mechanically "unlock" it to open or close the switch, depending upon if its normally open or normally closed switch. As others suggest, you can also wire a relay or VFD to do this electonically, but for safety reasons many choose not to.
              "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."


              • #8
                Here is a circuit diagram.

                The relay/contactor is normally off. Pushing the Start button applies line Voltage to the relay/contactor coil, energizing it. When the coil is energized, the bottom set of contacts close and then they apply a holding Voltage to the coil. The upper contacts apply line Voltage to the load. When the Stop button is pressed, it breaks the circuit to the coil, releasing it. Likewise, if the line Voltage is removed elsewhere, then the coil is released. In either case, the coil will not be energized again until the Start button is pressed. This gives you both an E-Stop button and a fail-safe when power is interrupted.

                This is actually more reliable than any mechanical locking switch as such mechanisms can fail. If either switch in this circuit fails, the circuit will default to the off condition. Also, if the latching contacts on the relay fail, the circuit will default to the off condition as soon as the Start button is released. In addition, a mechanical lock-out in the switch offers no protection to power cycling at another point in the overall system. This does.

                If three phase power is used, the relay/contactor will need one more set of contacts. Some contactors have one or more lower power contacts for use in control circuits like this.
                Last edited by Paul Alciatore; 11-18-2012, 01:04 AM.
                Paul A.

                Make it fit.
                You can't win and there is a penalty for trying!


                • #9
                  The current regulation is that you can no longer use a momentary break E-stop button, it must be the push maintained - twist release.
                  re NFPA.

                  Last edited by MaxHeadRoom; 11-18-2012, 10:47 AM.


                  • #10
                    A simple dropout relay uses its own contacts to disconnect power from its coil. A momentary contact by-pass switch is needed to power it up. It won't energize on its own.

                    Doh! Just saw Paul's circuit - that's all it takes.


                    • #11
                      The first thing I installed on my sheep shearing machine was an nice big E-stop. You hit it and it breaks the contact. The button must be twisted to restart the machine. My sheep sleep better knowing it is there.
                      How to become a millionaire: Start out with 10 million and take up machining as a hobby!


                      • #12
                        The current regulation is that you can no longer use a momentary break E-stop button, it must be the push maintained - twist release.
                        That is what I have on my CNC machine. In Canada you can buy one for under $10 at Princess Auto. Also, the push to start button is a high force push button and is guarded so it can not be accidentally pushed. The main power switch is also recessed in the panel. The spindle power requires both a mechanical relay to close and a logic signal to a solid state switch in series.
                        Last edited by Evan; 11-19-2012, 01:13 AM.
                        Free software for calculating bolt circles and similar: Click Here


                        • #13
                          An E-Stop is not the same as a No Volt Release. If you are running a machine and then the power drops out, the twist release E-stop does not automatically disconnect. If your machine has a simple mechanical main switch (no relays) and the power then comes back on the machine will restart. This has caused accidents, hence the requirement for No Volt Release and separate E-stop. Paul's circuit is fine, except that the E-stop should be twist release and is normally additional to the ordinary stop button switch.


                          • #14
                            Here is a page with all kinds of information about off the shelf units. Any good electrical supply house should have them if you don't want to mess around wiring your own.

                            Anti-Restart Devices...
                            Arbo & Thor (The Junkyard Dog)


                            • #15
                              I have one on my lathe. It has save many pieces of work. Never had to use it for me! (luck or skill I don't care as long as it hold true) I want one for the Bridgeport but havn't had time. Thanks for the links, I'll order today.

                              Is there a good spot to put in on a BP? I know I don't want it up at the switch. To hight to go for?

                              Thoughts? Right / left on the table on the head. In the future the DRO but for now?

                              I asking because I don't know and what you don't know can get you killed.