Table grounding

[millman52]

I am currently building a ox/fuel table. In the near future I am planning on adding plasma to it also.

There will be a 110V solenoid for the pierce ox mounted on the gantry.

(1). Should there be a ground from the breaker box connected to the gantry as well as anywhere else there is 110V power?

(2). Should there be a seperate ground rod near the table & all ground(s) pulled from there? Or a combination of both?

[gar]

071012-1858 EST USA

millman52:

You are required by the National Electrical Code to ground all conductive surfaces with which you can come in contact. There are defined requirements on how to do this. This safety ground must run to the main circuit breaker box where it is terminated to the earth ground system at that point. This safety ground system must not normally carry any load current. This means nuetral and safety ground can not be tied together anywhere except at the main breaker box.

I won't argue for or against an additional ground rod at your table in this application. It is not required by the NEC code, and in general I would argue against it. Your plasma and how it is setup and how it could fail might influence this decision.

You probably need very heavy wire for this safety ground from the table and other equipment back to the breaker box.

.

[millman52]

I am well aware of the need of a safety ground. & in the case of a burning table having mooving parts & on bearings, each individual unit or moving part needs to be grounded seperately. IE torch carrier on gantry, gantry itself & table.

I have read so many semingly conflicting articles on grounding of CNC equipment. Especially when it comes to adding a second earth ground.

I know for a fact you can drive 2 seperate ground rods in this earth around here & get voltage reading between the rods alone.

[gar]

071012-2118 EST USA

millman52:

The voltage difference is a reason in most case I would not use the ground at the machine.

In you location have you ever measued that voltage difference between the main breaker box and a rod at the machine where the rod at the machine is connected to nothing except the meter? Have you made both DC and AC measurements? What are some typical values? The DC measurement is fairly meaningfull. The AC has various problems. It needs a series capacitor to strip any DC component. Also may need a low pass filter rolling off at maybe 100 KC.

The floor you stand on should be at the same potential as the table and machines, or be an insulator. Cement is somewhat conductive so its top surface to some extent will take on the potential of the ground below it. So maybe it is better that you define that potential rather than the earth below. This is a complex problem.

Hopefully your plasma machine is isolated from the AC power source. This means you should be able to isolate the plasma current from the AC line and safety ground. If the part being cut, and thus a plasma lead are connected to the table and it in turn is connected to the safety ground there will be plasma electrical noise in the safety ground wire. I can not estimate the magnitude. But I do not think you want to electrically float from ground the part being cut.

If all your CNC equipment is located at the cutting table area, then you should be able to shield the equipment from noise problems. You do not want to run directly connected RS232 cables to somewhere else. Ethernet may not be a problem since most are isolated by a transformer.

.