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Stray voltage?

rmcphearson

Hot Rolled
Joined
Jan 12, 2008
Location
Rochester, NY
We bought a used reel (cylindrical) grinder a few months ago and I just noticed 50VAC between the frame and the earth ground outlet from an unused receptical on the wall. This is a 110 single phase machine with some solid state circuitry. It works fine but it shocked me one time and it shocks the other user every time he touches it.

I ohmed the ground wire of the power cord and there is an open somewhere in the cord so I'm going to replace it. But my question is: Shoudn't I get NO voltage between the frame and earth ground with the ground wire open? I won't be able to replace it for a few days and I'm going to be :scratchchin: until then.

I have heard some solid state circuits emit phantom voltage that goes away when a load is applied, but 50V? Enough to pop someone?

Roland
 
To check whether this is a phantom voltage or not, hook up the ground properly and then use an AC clamp ammeter on the green wire to see whether any current flows when the machine is running.

If all is well there shouldn't be any appreciable current flow. If there is, it suggests some sort of problem in the power supply.
 
A high voltage on the equipment frame with an open ground wire can be caused by many things, including the normal functioning of things like RFI filters, which incorporate capacitors from the hot conductors to chassis ground.

If the ground conductor opens, the capacitor can pass enough 60Hz AC to show up on a high impedance meter, or even give a mild tingle if you get between the chassis and ground.

Fix the open ground, and I'll bet the problem goes away completely.
 
open ground. Check ground connection to the grounding rod back at the service entrance too.

The latter happened in the home I grew up in - a plumber replaced a section of the copper pipe between the pump and well head with a plastic/polymer section. (they had originally run copper all the way between them). He neglected to install a grounding rod or move the grounded conductor to the well side of the insulating section. I was the lucky one to discover this while standing in the shower, when I stepped back onto the metal drain cap (copper all the way to the septic tank) and completed the connection to ground between the now electrically floating pump and earthed plumbing. We measured ~55 VAC between supply pipe and drain while the pump was running... one of several electrical mis-adventures from my youth that might explain my ending up an electrical engineer.
 
Check for leaky insulation in the motor, too. I've found a couple of used motors in the past few years with voltage from the case to ground. May have the potential (no pun intended) to be dangerous under the right set of circumstances, and should be taken out of service until the nature of the problem is addressed.

~TW~
 
TDegenhart said:
Its very likely that when the ground wire is connected, that the GFI will trip
I was thinking the same thing, but when the operators got shocked there should have been enough unbalanced current to pop a GFCI. Well, at least a household one. Maybe industrial ones have a more robust trip threshold.
 
Any motor has a lot of capacitance to the frame from the windings.... but thaey are not precision capacitors, so one is bound to be bigger than the others, and the voltage on the frame won't be zero to neutral/ground.

50V is not surprising at all.

If there were any "real" connection from frame to hot, you would probably not be seeing 50V, but rather nearly full line volts*......

A great idea is to check that voltage with a "Wiggy".... they draw enough current that a "leakage" won't register, but a real short will.

Just fix the ground problem, and if it makes you feel better, you can measure for any ground current.... there likely won't be any findable.....

* if the short were partway through a winding, you'd get whatever voltage to ground existed there.... but checking for balanced currents on teh motor wires, or for ground current eliminates that possibility. Checking with a "wiggy" also checks this.
 
Suggest you attach large green 'jesus' wire between the machine and a good ground.

Either a breaker will trip or something will start to smoulder inside the machine. Either
way it's not really your problem then.
 
"Jesus" wire. I like that. I have the working table/frames of my lathe and Bridgeport grounded via automotive jumper cables that connect to a #6 stranded copper cable on the back wall. The cable then runs to the 200 amp service and hooks to the ground bus. I also grounded my blast cabinet to prevent static electricity shocks.
 
Suggest you attach large green 'jesus' wire between the machine and a good ground.

Either a breaker will trip or something will start to smoulder inside the machine. Either
way it's not really your problem then.

Back in my younger days (when my hands were still soft) every morning on a construction site, while the building was still damp, I'd get a tngle off the power tools.... Sliksaw, drill, any tool. The boss was getting tired of me camplaining, "They can't all be bad", he said, as he picked up a saw to prove his point. As he bent down to grab the saw, he got a length of re-bar between his legs, and really let out a whoop as he touched the saw. :bawling: That spurred him to action. :D

It turned out the excavator's bulldozer had torn the ground wire off the ground rod of the temporary service, leaving the entire ground system on the site floating. One tool with leaky insulation then enlivened the case on every power tool on the job.

Fix the ground, it'll keep your pacemaker for going into overdrive some day when you least expect it.
 
We bought a used reel (cylindrical) grinder a few months ago and I just noticed 50VAC between the frame and the earth ground outlet from an unused receptical on the wall. This is a 110 single phase machine with some solid state circuitry. It works fine but it shocked me one time and it shocks the other user every time he touches it.

I ohmed the ground wire of the power cord and there is an open somewhere in the cord so I'm going to replace it. But my question is: Shoudn't I get NO voltage between the frame and earth ground with the ground wire open? I won't be able to replace it for a few days and I'm going to be :scratchchin: until then.

I have heard some solid state circuits emit phantom voltage that goes away when a load is applied, but 50V? Enough to pop someone?

Roland

As a electronics engineer I can assure that this half of grid voltage on your machine is absolutely normal. Input filters are usually symmetricaly coupled trough the caps to ground and as the ground is open, machine will have about half the grid voltage.
Ground the machine and you'll be fine. You can measure ground current if you have milliampermeter available, it should be about 1-10mA AC. If it is much higher, you have some faults in your machine.
 
"should have."

Yep, it should be fine. But if something inside the machine starts smoking when you
connect up the jesus wire, I'd step away from the machine and tell the boss "hey,
maybe somebody ought to call the fire department, something's not right here."

Alternatively half the shop may go dark when the breaker pops somewhere. Just
stop work and mention that 'maybe somebody ought to look into this.'
 
All right, If you're worried about smoking - do not put ground wire directly, put something between - a lamp bulb is fine or if you are more familiar with electronics, use 10k resistor. If the voltage on machine considerably drops, you can safely connect it directly.

Or, if you use flexible stranded wire (you should !) then connect only one strand, this will act as a fuse when something is wrong inside.

Also, you can measure resistance between machine body and power input lines. Should be in kilo-ohms.
 
A high voltage on the equipment frame with an open ground wire can be caused by many things, including the normal functioning of things like RFI filters, which incorporate capacitors from the hot conductors to chassis ground.

If the ground conductor opens, the capacitor can pass enough 60Hz AC to show up on a high impedance meter, or even give a mild tingle if you get between the chassis and ground.

Fix the open ground, and I'll bet the problem goes away completely.

When I did medical electronics in the 60s, instrumentation was growing rapidly. People were starting to realize that when they filled a cart with equipment from various sources, often each of them would have its own RFI filter that contributed a few milliamperes to ground. If a ground wire failed, the combined current could be high enough to kill a patient, especially one who was already in a weakened state. I moved to other areas, so I don't know what the resolution was, but I am sure a new regulation resulted.

There is an adage that says "You can't change any one thing." Everything has repercussions.

Bill
 
Just
stop work and mention that 'maybe somebody ought to look into this.'

That was my first move in the beginning. Our electrician did not want to touch it because it may have still been under warranty. We contacted the vendor and they took the lazy approach and made me "check the ground before we come out there."

Anyway, I put on the new cord and everything runs fine. I'll try to get our electrician to amp the ground wire next time he's here.

Thanks all. I learned something. Now I'm wondering why the other operator was being shocked every time he touched it (the first time he used it) and I had been using it for 3 months unaware of the problem. I'm guessing I have more rubber in my soles.

Roland
 
I'm guessing I have more rubber in my soles.

Probably the reason. The DRO on my mill started to flicker and pulse, but I didn't get around to checking it until I finished the job. A meter showed considerable voltage between ground and the machine, which I figured would be cured by tightening the connections on the ground wire. When I did that the arc welded the wire to the service box and tripped the breaker. The motor had developed broken insulation on one of the connections, and that leg was going straight to the machine. All that time I had been working on a hot machine and never guessed. Good ol' Redwings. :D
 
All that time I had been working on a hot machine and never guessed. Good ol' Redwings. :D

Yes, Redwings here also. The other guy is the type to buy chinese boots. I'm not sure if that's a plus or minus. Us Redwing guys will never know if our equipment is screwed up.
 
When you say "Redwings", do you mean the white-sole type?

I have to special order those here.... none of that carbon-loaded rubber for me... we have too much 480VAC and 750 VDC in the lab.
 








 
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