NEC ART 630-15 conflicts and ANSI Z49.1 Section 11.3.2 Manufaturer O&M

[Mauricio Solis]

We need some clarification and interpreation on this conflict:
NEC NFPA 70 Article 630-15: "Objectionble current in welding output circuit"

ANSI Z49.1
11.3.2 The Work. The work piece or metal upon which the welder welds shall be grounded independent of the welding leads to a good electrical ground., unless a qualified person assures it is safe to work on ungrounded work piece.

Miller O&M manual Section 8 (OMB-356) recommends to connect the work piece to a building to address AC High Frequency.

Grounding and Arc Welding Safety:
Grounding the workpiece has similar benefit to grounding the welding machine enclosure. When the workpiece is grounded, it is at the same potential as other grounded objects in the area. In the event of insulation failure in the arc welding machine or other equipment, the voltage between the workpiece and ground will be limited.

Question:
If we follow the code and removed the ground wire connection between table and building ground and totally isolate weld table, how do you address difference in potential and AC high frequency that needs to dissipate to ground and can sometimes cause electric shock.

 

[hurk27]

The problem whit all this, is many don't understand that current only wants to return to source, each time you have installed a transformer in a system, it creates a new source, even a welder is a source, and the current it provides will try to return to it.

I have worked in many weld shops as an electrician, and I have seen the results of bonding the work piece to the building independent of the weld leads. the HF AC used in some types of welders only returns to the source that created it, the welder supply.
The problem with bonding the work piece to the building or the building grounding system, is the two paths that the welding current can take, if the welder lead connection to the work piece is lost for any reason (and it happens all the time), then all the current will try to return to the welder power supply via the building through the grounding system or the EGC's of the electrical system (which can have disastrous consequences).

There are two ways to stop this from happening, one is to bond the work lead to the building at the welder power supply, then the bonding of the work piece to the building that is close to the welder power unit will provide a low impedance path back to the welder power supply in the event of a loss of a connection of the work lead.

even with this arrangement, I have seen 600-1200 amp welding blow bolts apart on the building frame, melt stiffener cables that hold the building square.

so we went to just bonding the work lead to the building at the power supply, and not using any other parallel connection, we lowered the connection loss rate of the work lead to the work, by lugging and bolting the work lead to a waste plate that is welded to the work each time, then cut off after the welding is done.