Ok, so I'm going to ground my table.

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motoguy
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Ok, so I'm going to ground my table.

Post by motoguy » Fri Jan 29, 2016 1:24 pm

I've been told that grounding my table to an earth rod isn't necessary, but it sounds like it can't hurt. I now understand that it's not a safety/current thing, as much as a RF issue. Since this is known that it CAN be an issue...it seems like an easy way to remove some potential (no pun intended!) variables. My table is in the garage. If I run a 20' cable (maybe less), I can connect the table directly to the grounded copper wire that's used to for the house/panel ground, on the service entrance (outside the house). That will prevent me from needing to sink a ground rod, in sub-freezing (ie, hard ground) conditions.

So, here's my understanding: I'll weld a tab/bracket onto one of the legs of my Bulltear/Starlab table. I'll connect the work lead from the Powermax 85 to this bracket/tab. On this tab I'll also place a ground block (for instance, this: Image).

From the ground block...what do I need to connect?
- Run a wire from the block, through the cable channel, and connect to a spot on the gantry?
- Ground wire from the CandCNC DCHTIV box to the block?
- do I need to run a wire from my water table/slats, since it is welded to the frame on my 6x12 table?
- anything from the chassis of the Mach computer? The computer is located inside the house, though only about 10' from the table. Ethercut, Hub, and HyT Cat 5 cables run through the wall, to the computer. Also running through is the VGA monitor cable, and USB cable running to USB hub in the garage.

Anything else? Also, what gauge wiring is needed?

Here are a couple photos of _Ogre's setup, which I'm modeling mine after:

Image
Image

Photo of the current tab mounted to my water table. I'll run a wire from this to the ground lug (see above), and then I'll mount my magnetic work lead to this lug as well.

Image
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Re: Ok, so I'm going to ground my table.

Post by vmax549 » Fri Jan 29, 2016 2:19 pm

Now here is something to consider. The Ground wire side of a plasma cutter is teh POSITIVE side of teh curcuit. What is going to happen IF you inject all of that current into teh ground field AND your safety ground IF you develope a ground loop ??

Just a thought, (;-) TP

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Re: Ok, so I'm going to ground my table.

Post by acourtjester » Fri Jan 29, 2016 2:27 pm

You don't want to attach the work ground from the plasma unit to the ground for the electronics.
really the only place you need the plasma work ground connected is to the metal you are cutting. If you are using a ohmic sensor that also needs to be connected to the metal you are cutting.
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Re: Ok, so I'm going to ground my table.

Post by motoguy » Fri Jan 29, 2016 2:32 pm

acourtjester wrote:You don't want to attach the work ground from the plasma unit to the ground for the electronics.
really the only place you need the plasma work ground connected is to the metal you are cutting. If you are using a ohmic sensor that also needs to be connected to the metal you are cutting.
If one is using the slats for the plasma connection, isn't that really the same thing? The work lead is usually just connected to the table/frame in that scenario.
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Re: Ok, so I'm going to ground my table.

Post by sphurley » Fri Jan 29, 2016 2:44 pm

CandCNCgroundingpractices.pdf
I follow Tom's sheet, And I ground/bond communications sites daily.
If you need help sleeping Google Motorola R56 certification.

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Re: Ok, so I'm going to ground my table.

Post by tcaudle » Fri Jan 29, 2016 3:13 pm

Establish a LOCAL ground (close to the table) ONE spot. Pull the table structure to that.

The goal is to keep noise from the plasma PWM high current (via the workclamp and electrode wires) away from the control side ground. A ground 20 ft away is not a good ground for noise. Tying the table ground to the entrance ground would result in any noise from the plasma going directly to yor whole AC system via the neutral bonding. There is no such thing as a perfect ground. If you Drive two rods 10ft apart the resistance of the earth causes each to have different characteristics. The closer you get a noisy side ground to the "clean side " ground the better the noise will transfer across . Noise is something hard to predict and quantify. New Blow back arc start plasmas have a fraction of the noise on start than the older HF start but all arc generating equipment (plasma, TIG, MIG, etc) are transmitters. A 20ft length of wire becomes an antenna to certain frequencies even if its grounded on one end.

Either get a new earth ground close to the table (like 6ft or less) or just don't ground it. Tying the plasma side directly over to the entrance ground may ADD problems rather than solve them.

Its better to ground the table and the slats and just clip the workclamp to the material. If you ground the work lead and then tie that to the work you now have two potential conduction paths for the cut current.

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Re: Ok, so I'm going to ground my table.

Post by tnbndr » Sat Jan 30, 2016 8:59 am

If you want to ground, why not just drill a 1/2" hole in the concrete below your table and pound the ground rod down through it. Should be no frost below your garage slab. That's what I planned on doing but never got around to it and I don't seem to have any issues, but I hook my plasma directly to the sheet I am cutting.
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Re: Ok, so I'm going to ground my table.

Post by Capstone » Sat Jan 30, 2016 10:17 am

tnbndr wrote:If you want to ground, why not just drill a 1/2" hole in the concrete below your table and pound the ground rod down through it.
I did do exactly this... in the back corner of my garage, less than 10ft from my table, I drilled through the slab and drove a rod down until just the tip was there to attach a 10ga ground to that and then my table and also my HT PM45. Better to eliminate problems that are only bothersome to some, when it's easy, then to have to sort out where the problem might be later.
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Re: Ok, so I'm going to ground my table.

Post by vmax549 » Sat Jan 30, 2016 10:56 am

The welder has its OWN ground back through the plugin for power it is a safety ground. Adding a second ground there CAN cause problems. AND I doubt it would pass local elec codes . The safety ground is NOT suppose to carry any intentional current only fault currents.

(;-) TP

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Re: Ok, so I'm going to ground my table.

Post by jimcolt » Sat Jan 30, 2016 11:00 am

I agree with Tom (tcaudle). Ground rod should be right next to the machine. Here are my suggestions based on 38 years of plasma cutting installation and troubleshooting:

-We are trying to force stray RF electrical noise to go directly to earth ground. RF travels on the outside of conductors, not on the inside like most electrical energy. Use multi stranded wire (the more strands the better) as opposed to solid copper for conductors to ground components.

-One ground rod. No more than one.

- The "star" grounding point where everything is connected should be firmly bolted directly to the ground rod itself.

- Run the work cable from the plasma directly to the star ground. Shorten the work cable to the correct length (no coiling) and bolt it on the ground rod. (don't use the hand clamp that probably came with the cable from the plasma manufacturer. Notice that the work cable has many strands!

-Run another cable (use the piece of work cable that you cut off so it wasn't coiled) bolted to the star ground on the ground rod directly to the cutting bed. Make sure this cable is firmly bolted to clean steel and use plated star type lock washers.

- All other ground wires from machine components should go directly to the star ground connection on the ground rod. All should be the right length, no coiling. All should be individual cables....no daisy chaining from one component to the next. All should be at least 10 gauge multi-stranded copper wire. Flat braided wire is even better.

- I recommend each of these be grounded individually: the torch carriage and z axis, the gantry, any metal enclosed boxes that house electronics.

-If you have a Powermax plasma...the work ground is internally connected (safely) to internal metallic chassis components....so no additional chassis grounding is required. Other brands...check with the manufacturer for best practices.

Jim Colt Hypertherm

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Re: Ok, so I'm going to ground my table.

Post by _Ogre » Sat Jan 30, 2016 1:20 pm

you already know what i did :mrgreen:
the three #6 ground wires you see on our ground bus go to: ground rod under table, metal table that the pc and plasma cutter are on and the lug welded to the table
off the lug welded to the table we have a separate work clamp, though we rarely attach it to the material
it has worked fine for 2 years :mrgreen:

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Re: Ok, so I'm going to ground my table.

Post by vmax549 » Sat Jan 30, 2016 3:20 pm

That approach would never pass code here as you are NOT allowed to force stray currents into teh safey ground side ONLY fault currents. By bounding ALL of your discharge grounds to teh sytem ground that is exactly what you are doing. AND you are allowing any fault current to flow backwards to your Aux ground rod instead of OUT through the safety ground circuit where it is suppose to go. THAT can create a serious ground loop that can be dangerous. You CAN have a discharge ground system to handle radiant energy but it cannot be bonded to the safety ground system.

We run into teh same type problem with automatic gate controls here . Florida is teh leading lightning strike capital of the US. We know it well. The NEC requires you to Ground the fence to earth but local code prevent you from attaching THAT ground to a safety ground. Which is catch 22 because the Gate contoller case IS bonded to teh safety ground by NEC code. SO we have to bring out the lighten arrestor discharge point to the Earth ground and NOT let it flow through the safety ground circuit.

Just a thought, (;-) TP

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Re: Ok, so I'm going to ground my table.

Post by vmax549 » Sat Jan 30, 2016 3:31 pm

Aslo a lot of DIYers don't know that Common and ground are NOT the same thing when wiring a controller MIXING components such as PC and controller boards and Inouts/Ouputs. Most end up creating a maze of ground loops.

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Re: Ok, so I'm going to ground my table.

Post by motoguy » Mon Feb 01, 2016 7:57 pm

I'm going to sink a ground rod right next to the table, in the garage. Picked up a 1/2" rod and some #6 stranded today. Tom and Jim both recommend grounding the gantry and Z axis. My table is a 6x12 Bulltear/Starlab unit. From the exit point on the cable tray, to a bolt point on the Z axis, is right at 19 feet (snaking through the cable trays). The gantry is made of bolted together, powder coated sheet steel. Short of pulling it apart and grinding through the powdercoat, I don't know that there's actually a conductive path through the gantry. I can bolt to the machined AL legs where the motors are mounted, but higher than that point, and I'm into the powdercoat issue. You can see in this photo how the legs are options, but the gantry itself is all powdercoated/bolt-together sheet:

Image

Tom said 20' ground cable (when going from table to existing service ground) was a bad idea. I assume that 19' from the ground rod/bar (at wire tray exit near CandCNC box) to the Z axis (via wire tray) would be just as pointless? If so, I'm just going to ground the table to the ground rod, and call it good. The PM85 is mounted about 12' from the ground rod, up on a shelf. Sounds like there wouldn't be much point in grounding it, either.

I'm not Image to ground the thing, I'd just like to do whatever I reasonably can to help reduce variables/unknowns as I learn the table.
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Re: Ok, so I'm going to ground my table.

Post by motoguy » Sat Feb 06, 2016 3:47 pm

tnbndr wrote:If you want to ground, why not just drill a 1/2" hole in the concrete below your table and pound the ground rod down through it. Should be no frost below your garage slab. That's what I planned on doing but never got around to it and I don't seem to have any issues, but I hook my plasma directly to the sheet I am cutting.
I did exactly this, earlier today. Sunk an 8', 1/2" ground rod, within 4" of the table leg. I live on the edge of a hill, in a very rocky area (flat at front of house, walk out basement exits to hillside on the rear). I managed to dive the rod until the last 15", when I hit a rock it just would not go through. After hitting it with the hammer, air hammer, and hammer drill...and getting nowhere, I called it done and cut it off about 4" above the concrete.

Using the instructions from here:

https://retrosystems.com/wp-content/upl ... res_07.pdf

I attempted to test my ground. Not good results, according to their methodology. Following their diagram, I'm still showing ~14V between the ground rod, and the neutral on the plug in. The 100W bulb lights up fine. Those of you who sank rods and noticed improvements...did you measure your ground rod after sinking it, or just sink it and go?

Also the conversation here: http://www.plasmaspider.com/viewtopic.php?t=13698

I have my system running behind 2 of these:

http://www.amazon.com/APC-BE750G-Back-U ... 50+battery

1 is powering the mach computer/hub in the basement
1 is powering the Bladerunner controller box on the table

Jim Colt and others have spoken of the UPS being a good way of removing unwanted noise from the control system.
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Re: Ok, so I'm going to ground my table.

Post by sphurley » Sat Feb 06, 2016 5:49 pm

Not sure how your shop is wired but here goes. The power company Bonds the neutral to the ground at the power meter. This is how its done in the three states I hold a license for. We use what is known as a Megger for testing grounds, it is the only real way to test. Not sure why you are getting 14volts on your meter to neutral unless you are a long way from the power meter and the ground wire is small or lose somewhere.
Your goal is to get RF off the table and into the ground. I think you have accomplished that.
Soil conditions vary a lot. We build a site on a mountain Death Valley and the ground system cost more than the solar system did.
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Re: Ok, so I'm going to ground my table.

Post by motoguy » Sun Feb 07, 2016 2:31 pm

sphurley wrote: Your goal is to get RF off the table and into the ground.
Correct. That is the desired outcome of the modifications.
sphurley wrote: I think you have accomplished that.
Great. That is really my concern. As long as I've accomplished that, without creating other issues, I'm satisfied.
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Re: Ok, so I'm going to ground my table.

Post by Dishtech55 » Mon Oct 28, 2019 9:02 pm

I was going to do the same thing..Put an 8 foot ground rod in ground close to my plasma and connect it with and old set of Jumper cables.But got to reading on my plasma cutter...It is Positive Ground system..So I dont think that would work very well.and may burn up my plasma cutter..Its an 80 amp cutter..So I would not want to do that.. I think you should check out your specs on your cutter first..because if it is Pos Ground cutter..that means the ground clamp is the Positive.and the torch is the ground...anyone else know,if Im right or not.thanks
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Re: Ok, so I'm going to ground my table.

Post by acourtjester » Mon Oct 28, 2019 10:42 pm

On the plasma unit the work lead is positive no need to attach it to ground, the plasma unit is grounded through the power input ground lead.
I attach the plasma work lead directly to the metal I am cutting only.
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