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Author Topic: grounding your cnc  (Read 7933 times)

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Offline Jeff_Birt

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Re: grounding your cnc
« Reply #11 on: October 21, 2012, 09:35:46 PM »
Terry - That is a good article! Thanks for sharing. The one use for the term 'ground' they kind of skipped of was 'DC Common'. A lot of guys try to use the machine frame as a current return path for sensor, probes, etc. Since the frame is generally connected to the earth ground you wind up with small control currents running through earth ground which causes all sort of problems. 

Rich - Yeah there are fancy meters for testing how good a ground is. When installing plasma cutters, welding robots, etc we would do a quick 'red neck' test on the local ground rod for the machine. Hook a 100W light bulb between the neutral of the power source and the local ground rod. If the bulb was anything other than very dim the local ground rod was not good. After testing the local ground and building ground were tied together.
Happy machining , Jeff Birt
 
Re: grounding your cnc
« Reply #12 on: October 21, 2012, 09:35:57 PM »
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There is a difference between grounds from an electrical  and noise point of view.

Earth Ground is Earth Ground is Earth Ground, Earth Ground does not change into something else when being used for noise abatement. While the primary purpose of the ground wire being added to electrical system was safety having a low impedance path to earth (a path which had no current flow from any circuit) was also a perfect place to bleed off electrical noise.

A copper ground rod is for safety - it will do nothing at all for noise.

That is incorrect. Crack open the power supply on your PC, where do all the filter parts get bonded to? The Earth Ground! Why? It is a low impedance path to ground. In the case of a machine like a Plasma Cutting table, Automated TIG welding machine etc, the local ground rod provides a low impedance path to the earth rather than rely on the small gauge ground wire that runs all the way back through the plant to the electrical service entrance. Any local ground rod should be bonded to the buildings electrical ground to prevent a potential shock hazard between the two.

I'm not meaning to sound like a 'know-it-all' here, I just want to dispel myths about grounding. I used to design, install and service automated welding and cutting systems and have had first hand experience with about every type of noise issue there is. I just want to share that experience with others.

Jeff,

Sorry, but I have to disagree.  For small signal stuff, like we're dealing with here, the earth ground is not going to do much of anything to reduce noise - the impedance through that path is orders of magnitude higher than the direct path through the ground conductor, even all the way back to the main panel.  It is quite common to have significant potential difference between the "local" earth ground, and the power panel earth ground, if the two are separated by any significant distance.  This creates a local shock hazard if the machine is connected only to the panel ground through the ground terminal of the circuit.  This IS a safety hazard if, for example, the user is foolish enough to be standing in a  puddle of coolant on the floor, and there is a short somewhere in the machine making some part of the machine "live".  With the machine grounded to a local ground rod, this hazard is eliminated.

The situation, and hazards, with high-current welding equipment are quite different from what folks are dealing with here, and if someone is having problems with, for example, spurious E-stop or limit triggers, tying the machine ground to a local ground rod is exceedingly unlikely to make any difference at all with those problems.

Regards,
Ray L.
Regards,
Ray L.

Offline Jeff_Birt

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Re: grounding your cnc
« Reply #13 on: October 21, 2012, 09:48:05 PM »
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The situation, and hazards, with high-current welding equipment are quite different from what folks are dealing with here, and if someone is having problems with, for example, spurious E-stop or limit triggers, tying the machine ground to a local ground rod is exceedingly unlikely to make any difference at all with those problems.

If you'll read my first few responses I also said that unless there is some need, i.e. plasma table etc, then a local ground rod is not required.

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It is quite common to have significant potential difference between the "local" earth ground, and the power panel earth ground,

Also please note I said the two ground rods should be tied together. The article Terry posted the link to says the same thing.
Happy machining , Jeff Birt
 
Re: grounding your cnc
« Reply #14 on: October 21, 2012, 09:48:25 PM »
One common error is the improper connection of neutral wires to ground buses or ground wires to neutral buses. This error allows neutral currents to flow on the bonded system, thereby creating voltage transients. Neutral wires are only allowed to be connected to the bonded system at a service entrance or at a step-down transformer