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Thread: Using a ground as a neutral

  1. #1
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    Using a ground as a neutral

    The VFD on my 120v Jet 1642 lathe burnt up and I would like to replace it with a 240v VFD. (they are smaller, cheaper, and more dependable than 120v) The only problem is that the tach requires 120v. I can put it on a separate 120v circuit, but that seems so inelegant. I would like to run a hot and a ground to it. I am aware it is illegal, but millions of appliances are wired like that and no one ever gets hurt.

    I ran a 120v circuit to it, but it draws less than the 10ma that is the minimum on my meter, so I don't know how much it uses but it can't be much. I then disconnected the neutral and attached that wire from the tach securely to the lathe. The lathe is on plastic pads, so I wired it to my drill press. I measured 121v and the the tach lit up normally. I disconnected the line to the drill press and put it on my leg and took my shoe off and put it on the floor. I felt a very vague tingle, but the tach did not light up; it measured 100v.
    I reconnected the drill press and also connected it to my leg. There was no voltage and no tingle.
    Finally, it occurred t me that the drill press was grounded through it's power cord rather than through the floor. (duh) So I connected the lathe to a chuck on the ground. The tach didn't light up.

    I drew 4 conclusions from this:
    1) The less than 10ma passed by the tach isn't particularly danger, even if worst came to worst.
    2) The concrete floor isn't a good ground; at least not when it is dry.
    3) If I wanted to cover all the bases, I could wire the lathe to the drill press (or something on a separate circuit, if the drill press isn't)
    4) I have to get my lathe fixed because I obviously have too much time on my hands.

    It seems to me that using the ground as a neutral is quite safe. I would be getting less than 10ma, probably much less.
    I could connect the lathe to the drill press to avoid the issue, but that would create a ground loop. Would a ground loop under these circumstances matter?

    Any comments (other than that I am a fool) would be welcome.
    I did wear rubber soled shoes and gloves for all the testing.

  2. #2
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    Replace the tach or double the resistor in the meter. using ground as neutral is safe until it isn't. I forget the exact step but it is a simple undetectable broken ground lead can supply 120 volts to neutral and full amps.
    What shoes, any socks, what is floor made of. Old old physics labs when electricity was just being studied had marble floors. Dry rubber horse stall mats would insulate the floor from electricity, fatigue and heat/cold.
    Bill D

  3. #3
    I think you have a number of profound misconceptions about electricity (e.g. concrete floor is clearly not going to conduct current), but I'll skip over those to answer your question.

    You should buy a 240V to 120V transformer. That would allow you to do this safely. What you are proposing is not safe. Your tests show it may be safe, but don't consider likely failure conditions (e.g. what if something in the tach fails and causes it to draw >10mA?)

  4. #4
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    Being one who has done wiring and worked in the electronics field much of my life there is little advice I will offer people who are not trained in the field.

    1. Don't do it if you don't know it.
    2. Hire a professional.
    3. What can go wrong will go wrong.
    4. That, "I felt a very vague tingle," was just a warning. You may not survive the real thing. That little bit of extra wire to run a proper neutral will cost a lot less than any life lost from bad wiring.

    There are good reasons for neutrals and grounds to be kept separate in home, shop and other wiring.

    jtk
    Last edited by Jim Koepke; 08-12-2022 at 7:32 PM. Reason: typo
    "A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
    - Sir Winston Churchill (1874-1965)

  5. #5
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    1) Find the power supply for the tachometer in the lathes wiring compartment. It might already be a 120v/240v power supply.
    2) Replace the power supply for the tach. with one that is 120v/240v.
    3) Replace the tach. with one that takes a separate power supply and use a 120v / 240v power supply. I know I paid less than $10 for my last electronic tachometer and used an old laptop power supply that was auto switch between 120v / 240v for the tach. You should be able to reuse the existing tach. sensor.
    4) Buy another 110v VFD. They aren't that expensive.
    5) Add a 240v - 120v transformer as suggested above.
    6) Buy a NEMA L14-20P plug and receptacle. You get 2 hots, a neutral and a ground. You will have to add the neutral to your 220v outlet and wire it into the NEMA L14-20R receptacle.
    7) Run a separate 110v power cable and a 220v power cable.
    8) Give up having a tach.

    I would never use a ground as a neutral! I understand the temptation of running a ground as a neutral. I have been tempted to do this before myself but ended up replacing the relays that had 120v coils with relays that had 240v coils in my RPC. The potential consequences just were not worth not doing a proper install.
    Last edited by Michael Schuch; 08-12-2022 at 6:57 PM.

  6. #6
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    Ground and neutral should only be common at the main. Observing this rule is becoming more important.
    Best Regards, Maurice

  7. #7
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    Quote Originally Posted by Maurice Mcmurry View Post
    Ground and neutral should only be common at the main. Observing this rule is becoming more important.
    Further the neutral is relative to the 120V legs coming off the transformer, which is why there are three lines run into most US houses: 120V, 120V 180 degs from the first, and the Neutral which is between the two in the coils of the transformer. The two 120V legs are only 120V _relative_ to the neutral, and NOT the equipment ground. Which is why the Neutral is NOT the same as the ground. If you do NOT run a neutral the 120V legs will vary in the amount of actual power delivered and best case your equipment will stop working or run powerly. Worst case it will fail in a nasty fire.

  8. #8
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    It used to be common to use the ground as a neutral when an electrical item has both 240 and 120 volt parts in it. However, the NEC has required a separate neutral if a 240 volt electrical item has a 120 volt component for decades now. They realized it was not safe to use the ground as a neutral.

  9. #9
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    Add a 240 to 120 control transformer to your installation or change to a 3 wire circuit…..Regards, Rod

  10. #10
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    I read 7 milliamps can stop the heart. It may or may not restart. A GFCI must trip at 5 milliamps or less.
    Bill D.

  11. #11
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    Quote Originally Posted by Michael Schuch View Post
    4) Buy another 110v VFD. They aren't that expensive.
    That sounds like the simplest solution. Amazon has 'em starting at less than $100.

  12. #12
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    I can see a Darwin Award in OP's future. 'shakes head'

  13. #13
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    The only two options I would entertain for this would be to either rewire the circuit properly for four wire 120/240v operation per code or replace the VFD with another 120v VFD.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  14. #14
    Quote Originally Posted by Brian Elfert View Post
    It used to be common to use the ground as a neutral when an electrical item has both 240 and 120 volt parts in it. However, the NEC has required a separate neutral if a 240 volt electrical item has a 120 volt component for decades now. They realized it was not safe to use the ground as a neutral.
    It was only permitted to ground the frame of cooking appliances & clothes dryers to the neutral, up to when the 1996 NEC was adopted, now it is permitted in only in existing installations, it was NEVER permitted to use a grounding conductor as a grounded (neutral) conductor.

  15. #15
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    Quote Originally Posted by Rollie Meyers View Post
    It was only permitted to ground the frame of cooking appliances & clothes dryers to the neutral, up to when the 1996 NEC was adopted, now it is permitted in only in existing installations, it was NEVER permitted to use a grounding conductor as a grounded (neutral) conductor.
    Maybe I got it backwards, but they used to combine the functions of the neutral and ground in one wire. Someone in the past had installed a four wire range receptacle in my kitchen, but it was only wired with three wires. One of the connections on the receptacle had no wire connected. I ran a new four wire cable so the receptacle is connected properly now.

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