CNC Machine Grounding

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Wiretech

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Have any of you ever delt with CNC machine tools ?
Most OEM's tell you that you have to drive a ground electrode for each CNC machine, and disconnect your EGC from the machine, to reduce RFI. and EMI,
or they they will not honor their equiptment warranty !!!
They even make you sign and date a check list when they come and set up a newly purchased machine, and one of the items on the check list is whether or not you have installed the ground electrode and "disconnected your building ground".
To me this in violation of several parts of Art. 250, maybe 647.6 others??
I didn't find anything in Art. 670 about it.
How would this setup clear a fault?, unless the conduit, (EMT, RMC, LFMC) served as the return path ?
When I install one of these machines I run the EGC to equip. ground buss in the machine, if the setup service tech. insist, I will then install a grounding electrode and disconnect the EGC.
When he is finished, I put it back the way I had it. Somtimes I even pull the ground rod, and reuse it the next time we buy another new machine.
A lot of extra work for nothing ain't it.:mad: Should have stuck with wiring houses :rolleyes:
I have had to do this on about 6 of the last 10 machines we've bought.
We're up to about 30 CNC machines know. I hope the OEM's learn somthing soon. You would think, after being employed here 17 years, I would find something else to grip about!! Health insurance premiums maybe??:wink:
Comments: Thanks, Greg
 
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It is a violation.

250.110 Equipment Fastened in Place or Connected by Permanent Wiring Methods (Fixed).

Exposed non?current-carrying metal parts of fixed equipment likely to become energized shall be grounded under any of the following conditions:

(1)Where within 2.5 m (8 ft) vertically or 1.5 m (5 ft) horizontally of ground or grounded metal objects and subject to contact by persons

(2)Where located in a wet or damp location and not isolated

(3)Where in electrical contact with metal

(4)Where in a hazardous (classified) location as covered by Articles 500 through 517

(5)Where supplied by a metal-clad, metal-sheathed, metal-raceway, or other wiring method that provides an equipment ground, except as permitted by 250.86, Exception No. 2, for short sections of metal enclosures

(6)Where equipment operates with any terminal at over 150 volts to ground


Exception No. 1: Metal frames of electrically heated appliances, exempted by special permission, in which case the frames shall be permanently and effectively insulated from ground.

Exception No. 2: Distribution apparatus, such as transformer and capacitor cases, mounted on wooden poles, at a height exceeding 2.5 m (8 ft) above ground or grade level.

Exception No. 3: Listed equipment protected by a system of double insulation, or its equivalent, shall not be required to be grounded. Where such a system is employed, the equipment shall be distinctively marked.
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The grounding that section requires can not be accomplished with a grounding rod.

250.4(B)(4) Path for Fault Current. Electrical equipment, wiring, and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit from any point on the wiring system to the electrical supply source to facilitate the operation of overcurrent devices should a second fault occur on the wiring system. The earth shall not be used as the sole equipment grounding conductor or effective fault-current path.
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No one will get me to energize a piece of a equipment like that without a proper EGC. No matter who asks you to do this it is your license, your liability and your conscience.

I have installed the added grounding electrode on CNC machines as that is allowed and will not hurt anyone.

I suggest you show the inspector, or the AHJ or your plant safety director this 'check list'.

But I am being entirely serious, I would not energize the machine at all without an EGC, just not going to happen.
 
I ran into this once with an ATM machine about 20 years ago. I told the manufacturer that I would need a signed letter from them telling me to make the installation in violation of the rules found in the NEC. I never got the letter so I made the installation per the code.
Don
 
I have seen this many times, it is a violation of the NEC and a life safety issue.


The reason this works is the equipment is NOT GROUNDED, the rod is useless.
There was probably a project they did that had circuiting ground current that resulted in operational issues. They isolated their equipment, drove a stick in the earth and thought they had solved the worlds problems. When in reality they had not fixed the existing problem at this facility and created a possible safety issue.

Hopefully this was not coming from an engineer but a salesman.

Don's fix is the best solution ask that it be signed by the firms engineer of record.
 
Iwire, I didn't mean to make it sound like I had these machines energized, with the EGC disconnected, I realize how dangerous this could be.
What I was trying to say was, I leave the EGC connected while the setup service tech is working on the machine. While he is still there working I install the ground rod, run a grounding conductor into the machines cabnet.
When he finishes his work, I denergize the equiptment, lock and tag it out, land this new grounding counductor, on the equiptments ground buss, and disconnect the EGC.
I leave the EGC inside the cabnet and insulate the end with tape.
The service tech looks in the cabnet, looks at electrode, signs off on his "check list", I sign his check list, he leaves, and I put it back the way its supposed to be. Sorry for any confusion.
Its all just a big waist of time, so they will honor their machine warranty.
 
Wiretech said:
The service tech looks in the cabnet, looks at electrode, signs off on his "check list", I sign his check list, he leaves, and I put it back the way its supposed to be.
Its all just a big waist of time, so they will honor their machine warranty.
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So what happens when the machine needs warranty work done? Do you set the machine up the way they checked it off?
 
don_resqcapt19 said:
I ran into this once with an ATM machine about 20 years ago. I told the manufacturer that I would need a signed letter from them telling me to make the installation in violation of the rules found in the NEC. I never got the letter so I made the installation per the code.
Don
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I'll try that next time. Might save me some work, and send the message to the machine tool manufactuer and service tech. deparment.
 
stickboy1375 said:
So what happens when the machine needs warranty work done? Do you set the machine up the way they checked it off?
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No, they haven't ever checked this, when we needed some kind of warrenty work. Very seldom does this ever happen anyway.
It is usually a few years down the road before something happens to the machine. Usually out of warranty by this time.
 
Why disconnect the ECG after the commision. Leave it connected. If they remove it, let them face any safety issues that may result.
I see no possible reason to drive another rod at the machine. It will not serve any purpose, other than a tripping hazzard. If they want to waste their money, thats fine, but you need to connect the ECG and put in writing that you did, and subsequent removal is their sole responsibility.
 
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