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Short circuit test

Discussion in 'ELECTRICAL ISSUES - POWER YOUR MACHINES & SHOP' started by martik777, Sep 23, 2011.

  1. martik777

    martik777 Canada Active User Active Member

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    I am getting a slight tingling (like strong static electricity) when I touch the cross slide crank on my 1945 South Bend 9 with original drum switch and single phase 110 motor. I have checked the connections and all looks ok, machine is grounded. When I engage the motor it seems to go away. Could this be caused from the old windings starting to break down?
     
  2. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Sounds like you have a leakage problem, although if the machine is really earth grounded, you shouldn't get a shock. Not standing on a metal grate that might be energized? You do need a leakage test, or a hi-pot test. These things can progress from a nuisance to a real danger with no notice. Get a meter (a real meter, not one of those fancy DVM's) and check for high ohm resistance between the handles you touch and the machine ground. So this with the supply shut off and locked out. Verify with meter before assuming there is no power to the machine.

    By real meter, I mean a VOM like a Simpson 260/270. You should read very low, 2 ohms or less between any exposed metal and the earth ground terminal in the junction box.
     
    Last edited by a moderator: Oct 25, 2011
  3. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    I don't suppose that someone, somewhere, sometime has put the ground through the switch, have they? As in, the frame is not actually grounded until the switch is thrown? If the frame is hot, I can't really visualize a way for the motor being turned on killing that.

    If you can't figure it out, I second calling in an electrician. It's not worth the gamble. Have you grabbed the handle with a sliver in your hand? That is a treat on a leaky machine.
     
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  4. jbltwin1

    jbltwin1 Active User Active Member

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    Try disconnecting the motor from the equation and power up the circuit. Tape the ends OFF. If the "tingle" is still there with the motor out of the circuit, the problem is in the switch or the wiring going to the motor. You have to eliminate one thing at a time and work backwards. If the switch is wired properly, you are switching the "hot" side on and off and the motor SHOULDN'T be hot when off so look closely at the drum switch and the insulators inside IT. Also check the wiring for continuity to ground.
     
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  5. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    [quote author=EdK link=topic=3670.msg26832#msg26832 date=1316796620]
    [quote author=Tony Wells link=topic=3670.msg26817#msg26817 date=1316786049]
    If the frame is hot, I can't really visualize a way for the motor being turned on killing that.
    [/quote]

    Possibly the motor is somehow putting a load on the leakage so the current is still there, just not as detectable. Just a WAG.

    I haven't done that one yet but in the winter my hands get very dry and crack terribly. I'll reach for something and get zapped by ESD right into the crack. Can you say "Ouch!!!!!!!"?

    Ed
    [/quote]

    If one side of the windings is leaking, and happens to be hot, as jb suggests, then it would gain a return path when the switch is turned on.

    Can we see a sketch of the current wiring?
     
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  6. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    True, John. This could indicate a failed ground rod, failed ground bonding to neutral in the distribution panel, or other things. Definitely needs to be addressed, ASAP.

    Floating grounds can act this way.
     
  7. martik777

    martik777 Canada Active User Active Member

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    OK, looks like I found the problem. The extension cord I plug into has 3 outlets and one of them has a loose ground. I tested this by connecting my ohm meter from the lathe to a known good ground, tested all 3 outlets and one intermittently shows no continuity to ground. To check if the hot leg was correct, old plug was not polarized, I measured AC voltage between the lathe and the known good ground. I measured 99 VAC so I reversed them but strangely, I still get 12VAC ???

    Thanks for all the help!
     
  8. Hawkeye

    Hawkeye Canada Active User H-M Supporter-Premium

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    Now we have the rest of the story. As I see it, the wiring is quite old. Two-prong, non polarized plug. Decades ago, they used the neutral as a ground and connected it to the chassis of fridges, radios and, it appears, power tools. Without a polarized plug, you had a 50/50 chance of getting it backwards.

    If that is the original wiring, I would advise replacing it with new cable and a three-prong plug. Do not copy the old wiring, but instead, connect the ground wire to the chassis and leave the neutral to complete the power circuits.

    I'm curious, though. What else were you touching that completed the circuit through you, so that you could feel it? If you didn't have a completed circuit, you'd be like a bird on a wire and feel nothing.

    Mike (Journeyman Electrician)
     
    Last edited: Oct 11, 2011
  9. martik777

    martik777 Canada Active User Active Member

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    [quote author=Hawkeye link=topic=3670.msg26896#msg26896 date=1316838315]
    Now we have the rest of the story. As I see it, the wiring is quite old. Two-prong, non polarized plug. Decades ago, they used the neutral as a ground and connected it to the chassis of fridges, radios and, it appears, power tools. Without a polarized plug, you had a 50/50 chance of getting it backwards.

    If that is the original wiring, I would advise replacing it with new cable and a three-prong plug. Do not copy the old wiring, but instead, connect the ground wire to the chassis and leave the neutral to complete the power circuits.

    I'm curious, though. What else were you touching that completed the circuit through you, so that you could feel it? If you didn't have a completed circuit, you'd be like a bird on a wire and feel nothing.

    Mike (Journeyman Electrician)
    [/quote]

    Actually it was a 3 prong plug but the ground wasn't connected (to the chassis) which I fixed right after I purchased the lathe. Regarding "completing the circuit", I was standing on a piece of carpet on a dry cement floor wearing sneakers.
     
  10. markba633csi

    markba633csi United States Active Member Active Member

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    It's good practice for machine tools to have a separate safety ground apart from the one in the power cord. The safety ground should connect to the closest cold water pipe or other earth ground.
    MS
     
  11. JPMacG

    JPMacG Active User H-M Supporter-Premium

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    I chose to install a GFCI breaker on the line powering my machines. Also, when I rewired my machines I grounded the motor housings, mill/lathe main castings and the metal cabinets they were mounted onto.

    I did radio and TV repair when I was young and I was always getting zapped by the cheap tube radios that were designed with hot chassis. Over the years I have become a bit of a grounding zealot.
     
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  12. talvare

    talvare United States Ted A H-M Supporter-Premium

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    Since this lathe motor is simply single phase 120V, you could provide yourself a good safety factor by just plugging it into a GFCI wall receptacle or put one on that extension cord you're using.

    Ted
     
  13. rwm

    rwm Active User H-M Supporter-Premium

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    I thought large motors were never supposed to be run on a GFI circuit? Wrong?
    R
     
  14. abrace

    abrace United States Active Member Active Member

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    You checked voltage between each leg and a known good ground and got 99VAC on one and 12VAC on the other? That is a problem. One leg should be reading around 120V and the other 0V (the neutral). Either the ground you used to test this wasn't good after all, or there is something going on with your neutral bond in your panel somewhere. That looks like the neutral at that receptacle is 'floating'. Obviously an extension cord with a broken ground is a problem, but you may have more than one issue here...
     
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  15. JPMacG

    JPMacG Active User H-M Supporter-Premium

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    You may be thinking of arc fault circuit interrupters (AFCIs). AFCIs have a reputation for not getting along with motors. As far as I know, ground fault circuit interrupters (GFCIs) are compatible with large motors.

    Combination circuit breakers that include both AFCI and GFCI functions have become popular. But I would not recommend them for our purposes. I installed a circuit breaker that has just the GFCI function. They are actually more costly than the combination breakers - about $50 versus $40 at the big box stores.
     
  16. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    I have two motors which occasionally trip a GFIC circuit on startup. One is a synchronous motor on a bathroom fan and the other is brush type universal motor on a can opener. I am not sure that I would want a GFIC circuit on my machinery. I know that I wouldn't want it on the CNC.
     
  17. talvare

    talvare United States Ted A H-M Supporter-Premium

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    I am not aware of any reason to not run a motor on a GFCI. As a matter of fact, most construction sites have plug-in modules with multiple receptacles which are GFCI protected and that is what the workers plug power tools into. I have been running my drill press, pedestal grinders, stationary belt sander, etc. up to three HP on GFCI receptacles for years. A GFCI simply monitors the current balance between the line and neutral conductors and if there is an imbalance greater than 5mA, it instantaneously opens the circuit. The only thing I would not recommend running on a GFCI receptacle is your refrigerator and freezer or life support equipment simply because some other device on that circuit could cause the GFCI to trip and you could lose your food or your life.

    Ted
     
  18. abrace

    abrace United States Active Member Active Member

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    AFCIs will also trip on ground faults. Instead of the 5ma threshold they are around 15ma. A lot of the issues with AFCI's tripping in the home are really caused by ground faults. It is amazing the amount of household equipment that leaks small currents to ground. Some of the wall switch timers, wall switch occupancy/motion detectors are notorious for this...especially the 'CFL' compatible ones from 5-10 years ago that didn't require a neutral. They actually used the ground as the 'neutral' to operate and as long as they didn't pull more then a certain amount of current UL would pass them.

    The old school versions that worked with incandescent bulbs didn't have that problem. They worked by actually passing a small current through the light when the light was switched off...a very small amount. It wasn't enough to light the bulb, but it passed enough current for the electronics in the motion detector/timer to work. That didn't fly with CFLs and LED bulbs.

    This is why the electrical code basically outlawed switch loops a few cycles ago. You now have to pull neutrals to the wall switch box even if you aren't going to use it. There are some exceptions to this if the back of the wall remains accessible, or if you run conduit and therefore could pull a neutral later, but the general rule is a neutral is required now.
     
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  19. rwm

    rwm Active User H-M Supporter-Premium

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    Very informative discussion. Should I consider putting a 240 vac GFI on my lathe circuit? That voltage scares me more than the 120. (which I get shocked by at least once a year somehow!)
    R
     
  20. abrace

    abrace United States Active Member Active Member

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    As long as your gear doesn't have any issues that cause nuisance trips, a GFCI is never a bad idea.

    I don't know where it sits currently with the code panel, but last I heard they were actually looking at extending the GFCI requirement to all 240V 15A and 20A receptacles as well in wet areas, garages, basements etc. It is very possible that it will be a requirement in the 2017 code anyways. As of the current 2014 code it is only required on 120V 15A and 20A circuits.

    As for danger, A 240V circuit (assuming standard single split phase residential service) isn't any more dangerous than a 120V one. You are never more than 120V with respect to ground. The only way you can get hurt more is if you somehow manage to touch both legs at the same time...but that doesn't usually happen unless you are really trying.
     
  21. rwm

    rwm Active User H-M Supporter-Premium

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    That's a good point about potential to ground. I never thought about that. I could see myself finding both hot ends, but then I don't think a GFCI would help any way.
    I was told that the builders in NC fought hard against code changes. My house built in 2006 only has GFCI to the bathrooms and kitchen.
    Robert
     
  22. Silverbullet

    Silverbullet Active Member Active Member

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    I had a motor at one time that worked really well. Only problem at some point the varnish on the windings had got scratched off making a short. It had push buttons on the bell housing for one off of the motor . Worked fine as long as it wasn't grounded. Had it hooked up to a little air compressor. If you touched a ground you got a tingle. Far as I know it's still in my old shop working with a sign on it don't touch shock hazzard. I always ment to respray the windings but always had more to do then time to do it. Besides lay your sweaty arm on some metal your welding that tingles too.
     
  23. jim18655

    jim18655 United States Active User H-M Supporter-Premium

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    Nothing listed in the code change listing I saw. They did clarify unfinished basement and work, storage areas as far as GFCI requirements.
     
  24. abrace

    abrace United States Active Member Active Member

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    I just took a look at the revisions. Looks like they went half-way. They are requiring it for non dwelling units. That would apply to detached garages, detached shops (assuming it is an outdoor outlet, or the shop has a 'garage door' in it and inspector considers it capable of garaging vehicles, etc). That means that the next cycle they will push it on dwelling units including attached garages, basements etc, the next goaround. :)

    Language from the 2017 NEC is below with the changed section underlined. Since split single phase < 150V to ground on each leg, this would apply to all 250V single phase receptacles 50A and smaller . Will be fantastic for welders!

    Looks like we can thank Schneider Electric for submitting this change. I am sure they only have our safety in mind. It would, of course, have nothing to do with the fact that they own SquareD, the makers of GFCI circuit breakers.

    210.8(B) Other than Dwelling Units. GFCI protection is required for single-phase receptacles rated 50A or less not exceeding 150V to ground and 3-phase receptacles rated 100A or less not exceeding 150V to ground installed in the following locations:
     
    Last edited: Dec 23, 2016
  25. tq60

    tq60 United States Active Member Active Member

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    Do not worry about gfi on your machines but do worry about wiring them correctly and this is stupid simple.

    First check the breaker that supplies the tool.

    Say it is a 30 amp breaker.

    That determines the wire size for all wires (assuming correct size for machine)

    Up to a certian size the safety ground needs to be same size as breaker rating as it has only one job to trip that breaker.

    Next establish your point of entry to the machine and this should be near the motor or control panel but should be a sturdy point.

    Next determine if safety ground is included in power cord or seperate.

    This example is using power cord for safety ground.

    Attach safety ground to the frame or casting of the machine if possible with a bolted on connection to a clean and shiny spot.

    Add additional wires from anything that has power to the frame as well as between any parts that are painted interfaces.

    Example is motor as it should have a seperate braided strap from a motor bolt to a frame bolt as the mount often swivels or maybe cushion mount.

    Last route all power wires neat and clean so they do not pinch or bind as well as insuring all covers and insulators are proper.

    Next last is to have all powee wires in conduit once it gets past point of entry so it is protected from chips causing shorts or you snagging a wire.

    The gfi looks at current flow in both wires to the equipment and trips if one is more than the other indicating current leaking to ground possibly via you.

    If no wire is placed where you can touch it and every conductive item in the machine is bonded together and to ground any internal short would be routed right back to the load center and the breaker would trip.

    You would feel zip.

    Sent from my SAMSUNG-SGH-I337Z using Tapatalk
     
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  26. Catcam

    Catcam Malta Iron Registered Member

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    I agree with the above one leg should be 110/120 volts ac and the other zero if measuring to a good earth point.
    The fact you are measuring 12vac indicates that the neutral leg is floating above ground potential. This may be a high resistance in the neutral line or the connection bus in the meter box linking the neutral to the main earth.

    I suggest first doing the same test directly at the power point and if this gives the same results then test the neutral link to earth connection at the board for high resistance and also the neutral link to a known earth. This should be less than 2 ohms over all (Australia). But only if you are confident/competent in this as it has a danger associated with it, as does everything in life.

    Overall do not use the equipment before it is fixed as it can go at full mains potential at any time
     
  27. rwm

    rwm Active User H-M Supporter-Premium

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    I was trying to find a resource to see how many deaths there are in the USA from in home electrical shocks. It's very hard to find a clear answer. Maybe they want it that way. I am not talking about the idiot who puts a ladder on the high voltage line or a work place accident; just the regular kind of shock you get from a plug or appliance. Some sources also lump in deaths from electrical fires also to inflate the figures. Water seems to be the biggest factor in death versus injury. My suspicion is the number is very low. Looks like Ireland averages about 1 per year.
    Robert
     
  28. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    You mean from things like this? I thought the CDC kept pretty detailed records on that sort of thing.

    electric water.jpeg
     
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  29. Mark in Indiana

    Mark in Indiana United States Active User Active Member

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    Back when I wore a Safety Manager's hat, an OSHA inspector showed me that the way he would check for an improperly grounded piece of equipment was to hold a non-contact voltage detector (aka volt tick) close to the frame of the equipment. If it chirped, there was a faulty ground.
     
  30. jim18655

    jim18655 United States Active User H-M Supporter-Premium

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    That's water cooled - it allows you to draw more amps from the circuit without melting wires.
     
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