[CptOvo]

I'm in the process of setting up a new 4x4 Plasma CNC (Dynatorch) and have a question on grounding. I have read many other threads on grounding problems and posts on other forums on electrical code but just wanted to see what the consensus was.

I have a sub panel in a garage that's about 30' from the main building and is wired 220v and is grounded back to the main building ( 4 wire - 2 hots, 1 neutral & 1 ground conductor). In the sub panel the neutral and earth are NOT bonded together.

In the installation notes it says to sink a ground spike and wire the plasma table, plasma cutter, control unit and PC to the same ground spike. In the electrical code doing this against good practice. They say to use the provided ground from the main building (in Canada anyways).

I was planning to wire the table ground directly back to the sub panel (6' away), the plasma cutter (Hypertherm 45) is already grounded to the same panel the add a UPS (uninterruptable power supply) for the Control electronics and PC.



The UPS (the case will be grounded to the sub panel directly) should keep any noise away from the electronics and the grounding cable from the table should earth any RF noise generated by the Hypertherm (the Powermax I understand is very good on low noise).

Does anyone see any problems or does anyone have the same configuration?

Thx

Ovo

[Al_The_Man]

I have a sub panel in a garage that's about 30' from the main building and is wired 220v and is grounded back to the main building ( 4 wire - 2 hots, 1 neutral & 1 ground conductor). In the sub panel the neutral and earth are NOT bonded together.

In the installation notes it says to sink a ground spike and wire the plasma table, plasma cutter, control unit and PC to the same ground spike. In the electrical code doing this against good practice. They say to use the provided ground from the main building (in Canada anyways).

I was planning to wire the table ground directly back to the sub panel (6' away), the plasma cutter (Hypertherm 45) is already grounded to the same panel the add a UPS (uninterruptable power supply) for the Control electronics and PC.

The UPS (the case will be grounded to the sub panel directly) should keep any noise away from the electronics and the grounding cable from the table should earth any RF noise generated by the Hypertherm (the Powermax I understand is very good on low noise).

Does anyone see any problems or does anyone have the same configuration?

Thx

Ovo

I believe you mean just use a local ground rod as ground? rather than supplement WITH a ground rod bonded to the service ground at the Plasma table, the latter of which is acceptable in my part of the Country.
Consider what you would have for ground path etc using just the service ground 36' away.
On the end of this grounded supply would be a Plasma Torch together with a PC that has its internal low voltage circuits bonded to the same ground conductor you are using to ground the Plasma table!!
What I have done is bond all grounds, service, table, control, and machine bonding conductors to a central star point near the ground rod.
I have posted this PDF a few times before on bonding and grounding.
http://www.automation.siemens.com/do.../emv_r.pdf?p=1
Al.

[DareBee]

My electrical contractor drilled a hole in the floor, pounded in a long rod, we wired the table, gantry and control pedestal to the rod. IDK what sort of grounding, neutral, bonding thingamajigging was done in the panel.
My Dynatorch had no provisions for the table or pedestal so we welded something on.

[CptOvo]

I spoke with Dynatorch and basically they said fit the ground rod, no option. I don't want any issues with warranty so I will go ahead and fit the rod as directed. I'm sure my plan would work fine but........what ya gonna do

Just for reference this is what they want.



Thx for the responses.

Ovo

[Al_The_Man]

That is the method I generally conform to, but it does not show the service ground bonding to the same point?
Al.

[CptOvo]

That is the method I generally conform to, but it does not show the service ground bonding to the same point?
Al.

And that's my point. I think the ground rod SHOULD be connected to the sub panel ground. Otherwise it's going to get messy with equipment grounded via the sub panel only and equipment grounded via the rod (varying path to ground etc).

I really don't see the issue with using the sub panel ground tbh as long as I use a electrical filter for the controller and PC, something like Belkin PureAV® Home Theater Power Console PF30 | Surge Protectors | Home Theater | Entertainment | Products | Belkin USA Site

Ovo

[CptOvo]

Duplicate post...opps.....Please delete/ignore.

Ovo

[jimcolt]

The electrical ground at your sub panel is a safety ground. It should stay separate from the ground rod on a cnc plasma system. The ground rod is to divert any stray electrical noise directly to earth ground......this will minimize the chance of capacitively or inductively coupling electrical noise into the sensitive, low voltage computer cables and drive signals. The diagram from Dynatorch is essentially correct, however I suggest the work ground from the plasma goes directly to the ground rod as well, with another cable of the same size running directly to the slat bed. All cables running to the ground rod must be heavy gauge multistranded wires.....as high frequency electrical noise is conducted on the outside of conductors.....so multistrand wires are much more efficient at routing transient noise to the ground rod.

I have been working with CNC plasma's for 35 years....this is a standard installation process on all industrial machines. There are some cases where machines will work without doing this, but many more that need the separate earth ground.

Jim Colt Hypertherm

And that's my point. I think the ground rod SHOULD be connected to the sub panel ground. Otherwise it's going to get messy with equipment grounded via the sub panel only and equipment grounded via the rod (varying path to ground etc).

I really don't see the issue with using the sub panel ground tbh as long as I use a electrical filter for the controller and PC, something like Belkin PureAV® Home Theater Power Console PF30 | Surge Protectors | Home Theater | Entertainment | Products | Belkin USA Site

Ovo

[CptOvo]

The electrical ground at your sub panel is a safety ground. It should stay separate from the ground rod on a cnc plasma system. The ground rod is to divert any stray electrical noise directly to earth ground......this will minimize the chance of capacitively or inductively coupling electrical noise into the sensitive, low voltage computer cables and drive signals. The diagram from Dynatorch is essentially correct, however I suggest the work ground from the plasma goes directly to the ground rod as well, with another cable of the same size running directly to the slat bed. All cables running to the ground rod must be heavy gauge multistranded wires.....as high frequency electrical noise is conducted on the outside of conductors.....so multistrand wires are much more efficient at routing transient noise to the ground rod.

I have been working with CNC plasma's for 35 years....this is a standard installation process on all industrial machines. There are some cases where machines will work without doing this, but many more that need the separate earth ground.

Jim Colt Hypertherm

Hi Jim

Thanks for the response. I've read a lot of your posts and have no doubt about your experience and knowledge, you are a legend.

What is unclear to me is this. The tables metallic parts (the control box is bolted to the table btw) must be grounded as they are connected to an electrical source (inside the control box there are 3 wires, 110v hot, neutral and ground). The control box and frame must be connected to the electrical ground for safety, as they are. I take the supplied large ground cable that is connected to the gantry/rails and frame and connect this to the ground rod, as instructed. The frame now is grounded in 2 places essentially bonding the sub panel to the ground rod.



Is it therefore wrong to bond the ground rod and the sub panel ground together via an additional large gauge conductor? They are already bonded via the 14 gauge 110v input cable ground anyways.

Thanks

Ovo

[Al_The_Man]

What is unclear to me is this. The tables metallic parts (the control box is bolted to the table btw) must be grounded as they are connected to an electrical source (inside the control box there are 3 wires, 110v hot, neutral and ground). The control box and frame must be connected to the electrical ground for safety, as they are.

Ovo

It is my experience that it will be virtually impossible to totally isolate the ground rod earth from the service earth ground, even if it was thought desirable, as you point out, many mount mains fed electrical enclosures, PC based controller, servo cabinets and external modules are mounted to the frame of the table, not to mention any conduits or metallic air lines that may be in contact with the machine.
You would need to remove the service ground conductor from any feed to the above and divert it to the ground rod.
No simple task and almost impossible to achieve.
Al.

[jimcolt]

No safety or earth grounding scheme will be perfect. The safety ground is to keep equipment chassis from being live in the event of internal component failures or shorts....it is designed to increase current to ultimately trip a circuit breaker or fuse so a person cannot get shocked by the equipment. The Earth grounding as we are describing here is designed to route stray RF and other forms of electrical noise that are generated by plasma current control circuitry, plasma starting circuitry, similar welding equipment circuitry as well as PWM drive control circuitry back to earth as efficiently as possible...before these stray electrical sources can inductively or capacitively couple to the low voltage computer circuitry.....which is often in the 2 to 15 volt range.

I have probably spent months of combined hours of my life (most of it about 15 to 30 years ago) in the field solving these types of issues, (mostly with early industrial cnc machines equipped with high frequency start plasma systems)...no two were solved in exactly the same manner. With some we had to drive multiple ground rods, others we had to move equipment to different locations on the cnc plasma cutter, and still others required isolated input AC power. Thes older technology cnc machines had early microprocessor based cnc controls....and over time the manufacturers learned how to proprly design them with optically isolated and filtered inputs and outputs that virtually eliminated electrical noise issues....however all industrial cnc machines (even today) are recommended to be installed with a proper earth grounding scheme...including a ground rod separate from the electrical safety ground back at the power panel.

Today's entry level cnc machines almost always are coupled to newer technology plasma systems that do not use a high voltage, high frequency, or capacitive starting circuit. While this starting technology lessens the chance of picking up electrical noise on sensitive drive, electronic and computer cables, the problem however is increased by the simple fact that all of these entry level machines use standard PC's and Laptops...which are designed to operate in the un-noisy , clean office environment. Now the noise that is picked up is often generated by the plasma systems inverter based power supply...which switches on and off to control output current at somewhere between 20,000 and 70,000 times per second (that is how inverter power supplies work), and the pulse width modulated servo and stepper drives on these cnc machines also generate their operating power in a very similar way. At these switching frequencies electrical noise is generated and can transmit and couple through cables, through the frame of the machine, and through the air (RF). The electrical noise levels are nowhere near what they are with a high frequency start plasma...but often are enough to affect the office PC/Laptop. This is why proper installation, isolation and earth grounding is often necessary. Again, it is not perfect, but it works!

Just a reminder as well. Industrial plasma systems all still (and likely always will) use a high voltage, high frequency starting circuit. Putting an industrial plasma....such as a Hypertherm high definition system on any CNC machine that uses an office computer for the cnc control....can and will cause issues that will require earth grounding, isolation of input power, grounding and shielding of all control cables as well as isolation of the plasma system leads from the computer and drive system leads.

Jim Colt Hypertherm

Hi Jim

Thanks for the response. I've read a lot of your posts and have no doubt about your experience and knowledge, you are a legend.

What is unclear to me is this. The tables metallic parts (the control box is bolted to the table btw) must be grounded as they are connected to an electrical source (inside the control box there are 3 wires, 110v hot, neutral and ground). The control box and frame must be connected to the electrical ground for safety, as they are. I take the supplied large ground cable that is connected to the gantry/rails and frame and connect this to the ground rod, as instructed. The frame now is grounded in 2 places essentially bonding the sub panel to the ground rod.



Is it therefore wrong to bond the ground rod and the sub panel ground together via an additional large gauge conductor? They are already bonded via the 14 gauge 110v input cable ground anyways.

Thanks

Ovo

[Al_The_Man]

Ovo, There is a really good definitive book out there that is updated yearly, 'Soares book on Electrical Grounding and Bonding'.
It is published by the International Assocn. of Electrical Inspectors.
There are some cheaper last years versions out there if you decide on a copy.
This book opens up the basic concept of Earth grounding theory that escapes many on the subject of Earth as an electrically neutral body.
Al.

[CptOvo]

Thanks for all the information guy's it's been great and has answered many of my questions and more.

I do have a fundamental understanding of electrical systems as I studied electronic engineering for 3 years at college and then on to aeronautical engineering but it has been a few years since I've used some of those skills .

I have installed a ground rod and wired everything as close to the manufacturers directions as possible with the addition of the UPS power supply for the control box and PC. I have "touch wood" no problems with my new Dynatorch Super B, its great. You can see it running on my YouTube channel, TheWeldingEmporium - YouTube if you like.

I think the only question now is this:- "Is it therefore wrong to bond the ground rod and the sub panel ground together via an additional large gauge conductor? They are already bonded via the 14 gauge 110v input cable ground anyways." To bond or not to bond: that is the question.

Ovo

[jimcolt]

Yes..it is wrong to intentionaly bond the earth ground rod that is designed to snub stray electrical noise to the sub panel safety ground. This creates a ground loop situation...that can make the situation worse.


Jim

[CptOvo]

Yes..it is wrong to intentionaly bond the earth ground rod that is designed to snub stray electrical noise to the sub panel safety ground. This creates a ground loop situation...that can make the situation worse.


Jim

Thank you Jim. Cleared that up.

Ovo

[Al_The_Man]

Actually that can create a ground loop.
Ground differential between two points!
Re-read the Siemens PDF.
Al.

[C.Michael]

When I was setting up my machine, I built a wooden cabinet that had the computer and all the drivers and BOB system in it..16 gauge cables out to the stepper motors..There was a separate ground rod used only to ground the actual plasma cutter table..Hypertherm 1250 grounded by way of the power cord. And the computer using the house current grounded with the 3rd prong..
It just would not work right!!..Then I read here about the ground loop issue..I took off that 3rd prong that was used on the power strip that powered the computer and all the electronics..It worked perfectly then..So all the electronics of my system do not have any kind of grounding to them..They just sit in a wood box cabinet about 15 feet from the table..no issues at all..Mike

[C.Michael]

From what I was understanding of the ground loop issue was that all of the power that was being sent into the earth by way of the plasma arc was actually traveling down the ground rod of the table, across the dirt to the house ground rod, traveling up the ground cable and into the computer!! Causing all those micro circuits to go over load..My main issue was freezing of the computer and shutting down the whole system during cutting..By removing the ground connection to the computer, that interference could not reach the computer..In case you are wondering why the interference couldn't travel up the power cables from the stepper motors..My BOB is optically isolated!! So that couldn't reach the computer..G11 board from cnc4pc..That is a great board..no hickups at all!!

[Al_The_Man]

Did you completely isolate the M.B. from the case? otherwise if the computer case was to touch ground the internal P.S. is still at ground potential even if the ground wire to the P.C. is removed, the M.B. common is normally connected to earth ground via the M.B. mounting.
Also:
http://www.cnczone.com/forums/genera...g_article.html
In all the years of installing P.C. based equipment, many times in very noisy industrial environments, I have actually enhanced the earth grounding of the P.C. rather than remove it.
Al.

[C.Michael]

I didn't do anything to the computers mother board..I just set the computer on top of this cabinet..running all of the wires from it to a lower shelf to hook up the BOB and the THC to it..I guess that a ground could happen if I touch the case and it goes through me to ground..That isn't very likely..I don't have any limit switches either, but that really shouldn't interfere either as the wires to them are optically isolated at the BOB.
It just seems that the idea to mount all of your drivers and stuff inside a steel box that is mounted to the plasma table is asking for interference troubles..separating the micro circuits as far as possible from that big arc of the plasma, it works good for me..I have been cutting heavy and light steel for 4 years now and never had the computer freeze or anything go wrong..Well i did have an event..I ran the table into the end of the line and ruined a couple of screw nuts because I didn't have a limit switch!!