Short circuit test

martik777

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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?
 
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.
 
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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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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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[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.

Have you grabbed the handle with a sliver in your hand? That is a treat on a leaky machine.

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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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.
 
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!
 
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)
 
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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.
 
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
 
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