Save Fingers, Save Lives With A No-Voltage Release For The Shop

Imagine the scenario: you’re spending some quality time in the shop with your daughter, teaching her the basics while trying to get some actual work done. You’re ripping some stock on your cheap table saw when your padiwan accidentally hooks the power cord with her foot and pulls out the plug. You have a brief chat about shop safety and ask her to plug it back in. She stoops to pick up the cord and plugs it back in while her hand is on the table! Before you can stop the unfolding tragedy, the saw roars to life, scaring the hell out of everyone but thankfully doing no damage.

If that seems strangely specific it’s because it really happened, and my daughter was scared out of the shop for months by it. I’ll leave it to your imagination what was scared out of me by the event. Had I only known about no-voltage release switches, or NVRs, I might have been able to avoid that near-tragedy. [Gosforth Handyman] has a video explaining NVRs that’s worth watching by anyone who plugs in anything that can spin, cut, slice, dice, and potentially mutilate. NVRs, sometimes also called magnetic contactors, do exactly what the name implies: they switch a supply current on and off, but automatically switch to an open condition if the supply voltage fails.

Big power tools like table saws and mills should have them built in to prevent a dangerous restart condition if the supply drops, but little tools like routers and drills can still do a lot of damage if they power back up while switched on. [Gosforth] built a fail-safe power strip for his shop from a commercial NVR, and I’d say it’s a great idea that’s worth considering. Amazon has a variety of NVRs that don’t cost much, at least compared to the cost of losing a hand.

True, an NVR power strip wouldn’t have helped me with that cheap table saw of yore, but it’s still a good idea to put some NVR circuits in your shop. Trust me, it only takes a second’s inattention to turn a fun day in the shop into a well-deserved dressing down by an angry mother. Or worse.

Thanks to tipline Ironman [Keith Olsen] for this one.

41 thoughts on “Save Fingers, Save Lives With A No-Voltage Release For The Shop

  1. This is actually a small universe unto itself. You can make a contactor with a couple of pushbutton switches and a DPDT continuous duty relay (how we’ve done it for some time) but it’s worth mentioning that other versions can be very valuable as well. Setting it up so that a restart can’t be done before a certain time has elapsed is SOP in a lot of compressor/refrigeration equipment, and there are a lot of variations to prevent a damaging/dangerous/annoying restart under load, without a proper set of access keys, or other unsatisfactory conditions.

    1. If this is a problem, as stated, the solution is a couple of switches and a realy. Hardley hi tech.

      I am glad the OP’s kid did not lose a hand, but the kid did learn a very valuable lesson about where never to put your hands, and perhaps the OP learned something about supervising their children. Not to be a total dick head, but lack of supervision was the crux of the problem, not the lack of some interlock.

        1. This Ralph Nader concept that the world can be made safe has short circuited Darwinism in so many people. I find it hard to believe that the government needed to get involved in my lawnmower because too many people were surprised that there was a spinning blade under it when they reached under it to clean out the chute with it running. One would think that even a warning would not be necessary. What is even scarier is these same people who are saved by this technology can pump their own gas, drive, and buy firearms, and vote.

          1. “Safety mowers” is one of the success stories from the Carter Administration.

            I also found a tie-wrap is sufficient to fix the problem. I have replace enough blades to know there is one spinning underneath.

      1. While I would agree with your nanny-state comments, I’ve found that most modern companies realize that finding someone to blame is not the best way to prevent future accidents.

        1. Definitely. Same in software development. Don’t let the user shoot themselves in the foot. You’ll get less complaints, less bug reports, and more business. Even for advanced users there’s a clear difference between a complicated UI and an unintuitive/crappy UI.

  2. This is (and has been for 50 or more years) the standard in most industrial machines. In some (but not all) classes of machines (presses, saws, milling machines, shears, and so on) the requirement is that they SHALL NOT restart automatically on reapplication of power after a power failure. There are a lot of approaches (digital control, electromechanical controls, magnetic switches, power monitors with cutoff, for example), but, historically, the most common for `dumb’ machines is a magnetic switch (contactor with a keeper circuit on a` holding’ contact).

    I have refit MANY machines that were delivered or originally installed with nothing more than a glorified lightswitch. (15HP drill press with a contactor and two NO circuits unused, and an actual consumer grade light switch, for example. The upgrade took $US 20 for the red/black control buttons and 10 minutes to rewire… Never done because “it will cost too much”. Shop employees were ecstatic, and the followup was a foot switch that could be selected for hold on or emergency stop with a DPDT toggle switch. Similar upgrade for a 10HP bandsaw running a 160″ 3TPI blade, and the 5HP Powermatic metal cutting vertical bandsaw, where, again, the basic control sets were already in place)

    A mechanical dropout requiring manual reset after power fail is another easy to implement feature, when there may be the need to get up close and personal with incomplete work after power loss (such as with an injection press, for example)

  3. I am constantly amazed at how dangerous US/North American mains power supply and machine tools seem to be designed.
    Hand-held routers and drills should have a “dead man” or spring-return switch/trigger that require you to actually hold it in for the machine to even get power in the first place.
    That old-timey table saw with a simple on/off mechanical switch would still roar back into life when you magnetic contactor switch on the powerstrip is reset, it should be built into the machine itself if possible.

    1. That is the requirement in a commercial environment. The rules for homeowner grade machines are quite lax. Many commercial establishments buy homeowner grade machines, or continue to use existing `grandfathered’ machines in an (illusionary) attempt to save money. Reality hits when an insurer refuses to pay on an injury.

      1. A lot of old machines still are very serviceable. Why would one replace a perfectly good, built like a brick shithouse bridgeport mill with some inexpensive replacement just because it is 70 years old? I doubt a lot of the machines built now will be around that long from now, and I will also bet you with a little care and feeding the machines that are that old now will still be around.

        Decades ago I worked in an electronics shop that had a sister machine shop. I used to love going in and watching all the CNC machines cranking parts out. The old toolmakers had manual hardinges and bridgeports with DRO’s, but the mass produced stuff was on the mazaks. Fast forward ~30 years and those old hardinges and bridgeports are still kicking, and commanding good prices but that generation of CNC stuff is often seen at auction for very little. My guess is that (1) it was not made as well to begin with (2) it was beat to hell in production, and (3) the technology is near extinct so getting replacement parts is hard. and (4) they cost more to fix than they are worth.

        1. It’s not exactly a complicated job to swap over the main power switch on an old industrial machine, as there is usually either a largish box that the switch is mounted into or a place where a new box with the magnetic contactor switch could be mounted.
          As long as the machine is tested and tagged after this electrical modification, it should perfectly fine to use in an industrial setting, even by an insurer’s standards. Considering that all your other electrically powered equipment should also be tested and tagged, it’s not a big deal to add one more machine to the list.

        2. CNC machines are worked much harder than their manual counter parts. They are constantly moving 16+ hours a day doing tool paths you would never do manually. Even manual machines that are on steady production get completely clapped out. Also the controllers in the older CNC machines are totally obsolete. Most machinist are not up to the task of upgrading to a modern controller. Not a difficult task for an electronics guy.

          I’ve got a few mills that I’m restoring. None of them ever seen production work so they are in pretty good condition. My big mill came from a college so it seen some abuse.

          Upgrading the electrical to modern standards isn’t that big of a job.

        3. The old CNC systems being retired is mostly a matter of 3 and 4. The tech used is ancient, the manufacturer no longer provides support and the systems aren’t as capable as newer systems. Old systems thus become a boat anchor. Often they’re too large and complicated for the home-shop hobbyist so they’re just relegated to the scrap heap.

          1. It is funny how many big machines I see essentially going for scrap, and how many smaller machines are still hot commodities, mostly because of the maker community. This seems to be the case with many areas. From cooking to farming.

        4. I have a few old machines that all have built in starter switches that act as a nvr, the oldest of which is a le blond lathe first commissioned in 1942, both the metal sop and wood shop also have an incoming contactor that acts as an nvr, this ensures that when power is restored your not near any machines in case a local nvr fails in the closed position (it happened once on a thicknessing machine, too much dust had got into the box but it worked manually)

      2. Possible. But even my cheap discount supermarket (€99,-) circular table saw and the similarly cheap drill press have magnetic latching power buttons to prevent restart in case of power failure. The green button pushes the contactor in until the self holding contact energizes the solenoid. The red button just interrupts the self holding circuit.

    2. Were it not for big machines lacking guards we wouldn’t have the unique sound of black sabbath. The trouble with spring safeties is your hand cramps during use so many people circumvent them making the tool less reliable at best. Good operating practices are cheap, but you gotta follow them. The scenario in the article could have been avoided by switching the machine off before restarting; a good practice for any tool. It should also be done since saw kickback can land you in the hospital. Of course hindsight is 20/20.

      Safety third.

        1. I agree with this wholeheartedly.
          Had a 10 second power interruption while ripping a 4×8 sheet of plywood a few years ago in my basement.
          Once the lights went out, I couldn’t see the power switch. A few knee fumbles to hit the large stop button failed, power came on, heard the saw spinning, panicked a little fearing kickback which made me twitch actually causing a kickback.
          Brilliantly I slapped the board back down to find the blade between my middle & ring finger.
          Close call with a happy ending.

          Thanks Dan for posting. I’d somehow never thought of this.

          1. I didn’t say that it should be entirely in hardware either. Good operating procedure combined with a safety interlock is the correct solution. You shouldn’t rely on a safety system 100%, either.

          2. I’m looking at the circuitry involved in the two big buttons required to operate a stamping press,, requiring that the operator keep both hands away from the machinery in operation, and I would have to disagree with you

            Also if you drive a car you are most certainly delegating your safety to the piece of circuitry that ensures that the air bag does not deploy while you are driving the car.

            If you walk across an intersection when the “WALK” sign lights up, your life and safety depend on the fact that the circuitry in the traffic light does not malfunction.

            In our modern world we all be wiped out in an afternoon if we could not depend on the safety of electrical circuits.

    3. Most handheld tools do have a dead-man switch. But they also have a dead-man switch locking button which locks the dead-man switch in the on position. This of course is purely for comfort, so people don’t have to clench their hands all the damn time they’re using said tool.

      1. True – except some are designed for handedness so the lock button is easily accessible but out of the way for a right-handed person but ends up in the palm of a left handed person.
        A left-handed person picks up the drill and wonders why the bloody thing wouldn’t stop the first time it was used.

        1. Do you actually know any left handed people or do you just say that? I’m a lefty, and I know other lefties as well, and we all use our right hands for things like scissors, can openers, computer mice, hand drills, and such.

  4. A controller that drops out upon low voltage and doesn’t automatically restart when voltage is restored is a LVP. An LVR controller will allow automatic restart when power is restored.

  5. Nice hack, adding a NVR switch to a multi-outlet strip cord, but I’m sure another type could fit IN the strip for field jobs.

    But this doesn’t solve the biggest issue I’ve had: a handheld router left switched ON spinning like crazy when plugged in: does anyone has a solution to refit the original tiny rocker switch with NVR?

  6. This is really interesting. In embedded systems I spend so much time trying to code logic to retry, to reconnect, and to automatically recover from any fault I can think of.

    This is an obvious good idea, but but probably not something I(As an only occasional power tool user with mostly cordless tools) would have thought to bother implementing.

  7. An NVR with a power strip is totally idiotic. If more than one device is powered up at the same time, the protection is defeated. Use a single, dedicated, socket instead.

    1. Idiotic … I think not.

      Whether in a small shop where one person is working … i.e. using a single power tool on one power outlet, or several people are using machines on one circuit … if you have a power droop, or power drop you want ALL tools to stop and not restart. I fail to see how the protection is defeated when power to any/all tools plugged into said NVR power strip is interrupted and everyone has to stop, shut their tools down, and then restart once you’re sure power is going to stay up.

      Maybe I’m missing something here … in such case, could you kindly outline an alternative proposal that:
      (a.) lists the potential safety issues,
      (b) provides a safe means to prevent injuries, and
      (c) provide an actual parts list and schematic so others can gain from your insight?

      One thing missing from an NVR is a means to prevent a reset when ANY device connected to the NVR is still in the “ON” position (in other words, able to start if the NVR is reset).

      Another nice NVR feature would be a Brake Resistor (or similar EMF dumping device) to more rapidly slow down the rotating cutting device(s) … though without the expense and noise of the Wet Hot Dog Protector (SawStop for the for the sarcasm-deprived).

      All the best … and be sure to read, understand, and follow the instructions for your finger-eating power tools. And, remember (tap, tap, tap) always wear safety glasses).

      1. You are missing something. Think about what causes the nvr to operate, and how that might be influenced by having multiple devices plugged into a power strip drawing current concurrently…

          1. @nsayer: When your electrical supply is that weak, that it does not support the spin up of _one_ power tool, you have a problem anyway. Remember: You are not supposed to start all the tools together with the main switch after a power failure. You should switch them all off and start only the one you are sure to use.

  8. I am pretty sure this is a requirement in the european Machinery Directive (safety rulebook for all non-hand powered contraptions). Basically it says that if it is dangerous when the machine starts unexpectedly (like a power failure/restore) then you have to prevent unexpected restart.
    Even the cheapest table saw or drill press at the cheapest hardware store has this very switch also doubling as emergency stop.

    But good idea to add it to a power strip.

    For extra safety I added a footswitch before this switch so the tablesaw stops as soon as you leave it. No more leaving the saw running while getting the next plank from the other workbench. To restart it you have to push on the footswitch and then on the “start” button.

  9. I have a powerplug outside to charge electrical vehicles (bicycle,cargo-bike) and would like an arm-button inside so that
    when a device is connected i push the arm-button to allow current to flow to the charging equipment.

    To prevent a thief from unplugging the charging cable and plugging in his disc grinder the outlet should be disarmed when there is no current going to the outlet. a no-current circuit breaker basically, but i can’t find it online. Does it exist?

    1. I think your simplest solution is an inline switch. But if i understand you correctly, you want:
      1. plug in bike using outdoor socket.
      2. Go inside house push start button, which energises power socket (and bike starts charging)
      3. go outside house and unplug bike, outdoor socket is automatically DE-energised, without you having to go back inside to turn off socket.
      4. repeat.

      so yeah, i think a switch inside is simplest but you’d have to remember to turn it off before you go outside to use your bike.
      Automatically… hmmmm it’s an interesting problem. and i can’t think of anything other than a cheap power timer. Like you might use for you pool pump or what have you. like this
      in step 2, set the time for however long your bike takes to charge then it will shut off. But it won’t stop a thief using your plug during the charging time.

      the other way, would be mechanical:
      not very cool, but solves your issue.

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