Modern GMAWP, especially the higher level units are using an "adaptive feedback" system... One that senses what is happening at the arc.  It obtains feedback thousands of times per second.  The best way for the power supply to get this information is to place a secondary "voltage sensing" lead (the thin wire) as close to the weld as it can be attached to the work or fixturing.

I don't know anything about "Isolating" the *ground* on the welding machine...  Really does that clamp go to the ground?   Hell no it doesn't.     It's nearly impossible to isolate a power supply from the building it's in... Try to strike an arc on a building column when you have your machine's *work lead* attached to a fixture that is anchored to the slab and see what you get.

Unless you have reviewed the actual Miller Operators Manual and it compels you to isolate the power supply, I think you are getting some junk advice.

The RMD is pretty good at doing what it is told to do.  While the adaptive feedback is very nice and fast and will help with slight discontinuities in fit-up, electrical stick-out and angle variance, it does not change the arc current or voltage unless it is told to do so... It's efforts are to keep that arc the same.

As Lawrence mentioned, the "extra ground" is a volt sensing lead that helps the power source compensate for changes in electrical stickout in order to maintain a stedy arc.
Miller Axcees systems, both manual and automatic use this technology. With manual welding, arc sensing lead use is optional, but with automatic robotic use, it's essential. The closer the lead is attached to the weld joint, the better it works. However, our robotic systems weld parts on rotating positioners, which makes attaching the lead directly to the part impractical. We've found that attaching the sensing lead to the grounding strip, just upstream of the positioner to be effective. It needs to be attached to a separate lug from the machine ground though.
While I've not tried it yet, I understand from Miller that using multiple machines on one part is OK, but I recommend appropriate R&D.
As for the work lead, attach it to the part or positioner. Do not ground through the building.
Miller has a very good support network. I recommend asking your local Rep to visit the site and assess the system set-up.

Tim

Clif,

Another possibility coming from an outside view without nearly enough facts,  many people don't know the difference when wiring up a building and especially welding and/or any shop equipment between a 'ground' and a 'neutral'.  There are a good many applications where both are needed. 

You can go around and around about them being the same, and yet, different but the bottom line is they are different and serve different functions when it comes to our equipment and safety as well as operating parameters. 

Because of the electronics and sensors involved with this particular process you may well need both and that is the 'isolating' that is being talked about.  Not grounds/work leads to the work itself but how the equipment is wired.

Just my two tin pennies worth.

He Is In Control, Have a Great Day,  Brent

just wanted to chime in, When you are running any type of high frequency modulated wave form (like you are with modern GMAW-P) the requirements for good clean power are much more critical. The "extra ground" as everyone mentioned is probably a voltage or arc sensing feedback loop but 90% of the ones I have seen are just integrated into the welding torch.

Alternatively we have had issues with AVC and weld data loggers if you don't have a good true clean ground, thinks like high frequency start, flourescent light ballasts or other equipment can make your incoming power lines somewhat noisey and we have had issues where an AVC system wouldn't work properly until we ran a new true earth line directly drilled into the ground, as the existing ground was getting noise from some other piece of equipment.