That tingly feeling

[Drew Marold]

Anyone ever have electrical leakage from their 3520B ? Mine is only a couple months old, and tonight while trying to do some turning, I kept feeling tingly in my fingertips holding the steel of the gouge. At first I thought it was just pins & needles, maybe a bad arm position or something, but it kept happening when I put steel to the tool rest. Then I bumped my elbow against the rest and got a buzz there, so I started to suspect a problem. Got out my multimeter, and with the ground probe in the ground pin of a nearby electrical socket, I measured between 30-50V AC (seemed to vary with speed changes) touching various metal parts with the other probe.

I plan on calling Powermatic in the morning, but thought I'd tap the collective wisdom to see if this had happened to anyone else.

[Tim Rinehart]

Never had it happen. How confident are you of the grounding leg of your connection?

[Ken Fitzgerald]

Drew, I would check the wiring of the AC coming into your lathe. Check to make sure your ground wire coming in is connected properly. Make sure the ground is really "Ground". The body of the lathe should be grounded via the incoming wiring and thus you shouldn't get shocked or measure any AC voltage difference between the lathe body and the ground of another outlet.

I will go check my PM3520B with a meter.

[Ken Fitzgerald]

Drew,

I measured mine though I really didn't think it necessary. I measured nothing from a ground on a 110vac outlet to the body of the lathe with the lathe running at various speeds and then also off.

If you lathe is grounded properly you should not measure any voltage. I would check the incoming AC wiring closely all the way back to the distribution panel. Ground is ground and you shouldn't measure any AC voltage difference between ground and the body of the lathe.

[Drew Marold]

Good call on checking the ground. I traced my circuit & discovered the problem. My 220 line runs from the panel to an outlet for my radial arm saw, then I extended it over to where the lathe is. The line from panel to outlet was 4 conductor (probably because that's what I had at the time), but outlet to lathe is 3 conductor. I connected black->black, white->red & tied the ground wires together. When I looked in the panel, I discovered the 4 conductor had the black & red on the breaker as expected, the neutral wire on the bus bar, but the ground wire was snipped off. Sadly, that was done by an electrician. The breaker was moved in the subpanel when we had an addition done a few years ago and they were putting some new circuits in. It must have happened then, but I never noticed it. Kind of surprising it took this long to get shocked, I've been using the lathe a lot lately and never noticed anything until tonight.
After I tied the (unused) white wire from the 4 conductor to the bundle of ground wires in my junction box, no more leakage current on the lathe.

[Roger Chandler]

Oooooh........that is scary! I am glad you got the tingle and not the big shock! Great that you posted and others replied..........that was a dangerous situation and the information will help others be aware of possible problems and ways to check things out!

[John Coloccia]

Why would there be any energy on the chassis anyway, ground or not? The ground conductor is not meant to be a current carrying conductor. Maybe I'm not thinking about this properly, but it seems like if there's enough energy floating around to make your hands tingle, there's going to be some constant current flowing in that ground conductor, and that's just not normal.

Maybe an electrician can weigh in re: what's going on here.

[Josh Bowman]

Drew, I was not convinced you had corrected your problem. Theoretically, a 220 volt machine IF all insulation is good etc. etc. does not use the ground, EXCEPT if there is a fault. I use a power disconnect on my lathes, therefore the wiring is easy to get to. Soooo I removed my ground and as expected the lathe ran fine. I measured the voltage from a good earth ground and at full speed got about 60 volts! Apparently the VFD uses or induces voltage into the frame of the machine. So sounds like you're in good shape. BTW, when I reattached the ground, the voltage went to zero.

[Duane Meadows]

Here is probably more than you wanted to know on the subject...

http://www.powerlines.com/pq2005paper.pdf


These methods will give meaningful measurements of AC leakage!

[Duane Meadows]

Drew, I was not convinced you had corrected your problem. Theoretically, a 220 volt machine IF all insulation is good etc. etc. does not use the ground, EXCEPT if there is a fault.

However it may well use the neutral, especially with solid state control circuits! This is almost guaranteed to be the case with an electric range!

Oh, there will almost always be some stray voltage measured to ground if the frame isn't grounded. With or without VFD! That is why the proper measurement procedures I posted are necessary for a meaningful leakage measurement. Any readings taken from machine frame to ground with a 20+ megohm input impedance DVM are suspect and meaningless otherwise!

[Ken Fitzgerald]

Josh,

Before I made my recommendation last night I checked the electrical diagram for the PM3520B. It shows 3 leads for incoming power....2 hots and a ground.

Duane,

I agree that the methods to which you refer is the correct methods to accurately measure ground current, leakage current and net current. The only case I have seen where such extremely accurate measurements are of concern is in the field of medical electrical and electronics due to patient safety concerns.

And while you may measure stray voltage, it's been my experience that it is most often measured in the millivolt range and not 30-50 v AC as reported by the OP in his first post. The method he used is a quick and dirty method commonly used to check for bad grounds when someone first experiences a shock. He could have also used the multi-meter in the ohms/resistance function to check for proper grounding between an unrelated ground and the body of the lathe.