grounding the table on an industrial cnc plasma

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pjwt712

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I'm going to be hooking up a 150a 480v 3ph plasma.
the machine instructions are calling for a ground rod at the machine table, this I understand, but the machine rep is saying that typically it takes 100' of thread together ground rod to get the low impedance ground required. Is this sound correct?
 
I don't understand why a ground rod is needed. I assume you have an equipment grounding conductor so a ground rod is probably never going to give you the low impedance of the grounding conductor.

Soil conditions etc will effect the impedance of the rod so there is no way for us to know.
 
pjwt712 said:
the machine rep is saying that typically it takes 100' of thread together ground rod to get the low impedance ground required. Is this sound correct?
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Based on what? Tested how?? You'll never get lower impedance from a ground rod than from a proper equipment ground conductor.

A ground rod at the machine, while allowed by the NEC (provided 250.54 is followed) and loved by some, is dubious in its worth. To suggest one a 100' long is insane.
 
pjwt712 said:
I'm going to be hooking up a 150a 480v 3ph plasma.
the machine instructions are calling for a ground rod at the machine table, this I understand, but the machine rep is saying that typically it takes 100' of thread together ground rod to get the low impedance ground required. Is this sound correct?
Click to expand...
If you're getting paid good to install this 100' ground rod, install it, take the money, and be happy. Auxiliary ground rods are permitted under Code. Then 110.3(B) comes into play. Whether the rod is necessary or not ends up irrelevant.
 
141210-1421 EST

pjwt712:

You need to ask the machine manufacturer what is the purpose of the ground rod. Then depending upon the answer ask the manufacturer how the 100 ft rod accomplishes that objective.

I suspect it has nothing to do with clearing a power source fault. Rather it probably relates to some noise problem. With a plasma arc there will be a lot of high frequency noise. Does this noise cause problems in the control, or with a communication path like RS232?

RS232 problems are best solved by optical communication means like our I232 optical isolators.

If a ground rod does really solve some problem, then I would try to determine if ten 10 ft rods spaced around the machine table might do a better job. Coupling from the table to whatever should be a wide flat strip of copper. This provides a lower impedance at high frequencies.

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Ground rods at CNC machines can some times help, make no difference, or make the problem worse, per a study by the Electric Power Reliability Institute. What can happen is it creates an additional noise path into the equipment.
 
tom baker said:
Ground rods at CNC machines can some times help, make no difference, or make the problem worse, per a study by the Electric Power Reliability Institute. What can happen is it creates an additional noise path into the equipment.
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Agree. It can also create voltage gradients on the equipment grounding system, especially during nearby lightning strikes.

Electric Power Reliability Institute is typically referred to in my circles by its acronym: EPRI.
 
pjwt712 said:
I'm going to be hooking up a 150a 480v 3ph plasma.
the machine instructions are calling for a ground rod at the machine table, this I understand, but the machine rep is saying that typically it takes 100' of thread together ground rod to get the low impedance ground required. Is this sound correct?
Click to expand...


Incorrect, ground rods accomplish nothing of benefit at the machine.
 
mbrooke said:
Incorrect, ground rods accomplish nothing of benefit at the machine.
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I disagree. the electricians that install them make money doing so, and the CNC techs stop harping over "bad grounds" being the cause of whatever gremlin is affecting the machine, and start looking for the real problem.
 
zbang said:
Neither of which directly benefits the machine itself.
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Note that per the OP, the manufacturer's instructions don't seem to shed any light on the benefits, they just want one there. I'd have to side with petersonra on this ;)
 
petersonra said:
I disagree. the electricians that install them make money doing so, and the CNC techs stop harping over "bad grounds" being the cause of whatever gremlin is affecting the machine, and start looking for the real problem.
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Grounds can cause gremlins. Seriously the whole entire concept of ground having magical properties needs to end. I don't even spec ground rods to most buildings that have metal rebar.
 
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