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#29054 - 09/07/03 05:35 PM Welding table, to ground or not to ground?  
INTP  Offline
Member
Joined: Sep 2002
Posts: 14
Dallas, Tx, USA
I have a question about whether or not to ground a metal work table that would be used during arc welding. I know that a good work clamp (which is confusingly referred to by many weldors as the ground clamp) should have a solid connection, as close to the work area as possible. But what happens in the exceptional circumstance?

One thing to keep in mind is that arc welding processes can be DCEP (direct current, electrode positive), DCEN (direct current, electrode negative), or AC (alternating current). Which process depends on the type of metal, filler material, and desirec penetration characteristics.

I've been following a couple discussions at the Hobart Welding site where there are some thoughtful responses, but there is no consensus. Some people there think that a separate grounding rod should be put in the ground for just the table. Others advocate connecting it to the house ground. Still others say that he table should NOT be grounded.

FWIW, my metal welding table is NOT grounded. All other outlets in my work area (garage) are GFCI, except my refrigerator. The garage has its own subpanel, which has a grounding electrode going to the main panel. The grounding electrode and the neutral are NOT connected.

This is one of the discussions that prompted me to come here: http://www.hobartwelders.com/mboard...92&threadid=2691&perpage=15&

Thanks,

Barry


Work Gear for Electricians and the Trades

#29055 - 09/08/03 01:16 AM Re: Welding table, to ground or not to ground?  
Big Jim  Offline
Member
Joined: Jul 2003
Posts: 394
Denver, CO USA
Mine is only one experience but I work an a building apprx. 200,000 sq. ft. and 40 feet tall. Lots of steel structure. All our maintenance welding is done by rolling the welder to the nearest 3phase 480v outlet, plugging it in, and hooking the work clamp to the beam the outlet is mounted on. In over twenty years, I have not heard one report of any problems caused by this setup. I tend to favor EVERYTHING bonded together


#29056 - 09/08/03 04:14 AM Re: Welding table, to ground or not to ground?  
Joe Tedesco  Offline
Member
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA
You should read 630.15 in the 2002 NEC. They added the following fine print note (see 90.3):

Quote
FPN: Connecting welder secondary circuits to grounded objects can create parallel paths and can cause objectionable current over equipment grounding conductors.


The NECH include additional commentary that you should read concerning the safety hazards and why the rule was also revised in the first sentence to read as follows:

Quote
630.15 Grounding of Welder Secondary Circuit.
The secondary circuit conductors of an arc welder, consisting of the electrode conductor and the work conductor, shall not be considered as premises wiring for the purpose of applying Article 250.




[This message has been edited by Joe Tedesco (edited 09-08-2003).]


Joe Tedesco, NEC Consultant

#29057 - 09/08/03 05:55 PM Re: Welding table, to ground or not to ground?  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Good comments here. Let me describe two "gotcha's" I found in the past.

Work {"ground"} clamp/lead attached to one end of a conveyer assembly—with arc welding done on the other end of the conveyor—destroying almost every ball bearing on the conveyor from numerous paths of parallel current flow.

Machine-tool liquidtight flexible metal conduit found with outer vinyl jacket ‘dripping’ off of metal corrugations. Here, work {"ground"} clamp/lead was attached to a steel column about 75 feet distant. Flex became a “non-parallel” return for ~150 amperes of welding current, with the machine {motorized reciprocating saw} becoming a seriously misused ‘welding table.’


#29058 - 09/08/03 06:16 PM Re: Welding table, to ground or not to ground?  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
About 20 years ago I actually tried this & encounterd the objectionable currents that Joe mentiond. It was enough current (I never measured it) to make the #4 bonding conductor I used very hot.

I'll go with Joe, don't do it.


Few things are harder to put up with than the annoyance of a good example.

#29059 - 09/08/03 06:55 PM Re: Welding table, to ground or not to ground?  
MinElectrcn  Offline
Member
Joined: Jun 2002
Posts: 18
pekin,IL,usa
This is a real problem or was I should say in the plant I work in. It has caused so much damage to equipment that the welder(s) doing the job must always connect their neg. clamp within three feet of the weld path. This may seem over cautious to some and I have heard the welders complain as well but I have also seen the damage it can inflict and would not want to be the one paying the bill for us to come repair and most times replace the equipment. I think the worst case I have seen is an Allen Bradly quantum plc cabinet.


#29060 - 09/08/03 09:21 PM Re: Welding table, to ground or not to ground?  
Big Jim  Offline
Member
Joined: Jul 2003
Posts: 394
Denver, CO USA
In my facility, it is almost impossible not to connect the secondary to a grounded object. There is but a small amount of stand-alone equipment. In the example of liquidtite, it would be close to impossible to find a piece here that doesn't terminate on the steel structure on both ends. I'm sure we are unusual in that most of our equipment was built in place as the building was constructed and uses the building structure as an integral part of itself.


#29061 - 09/08/03 10:40 PM Re: Welding table, to ground or not to ground?  
frank  Offline
Member
Joined: May 2001
Posts: 374
windsor ontario canada
Just thought i would add this post Tony put up a few months ago

Tony Moscioni
Moderator posted 06-17-2003 01:44 PM
--------------------------------------------------------------------------------
Millwright electrocuted while using welding machine...
Contact with live parts can cause severe burns to the body or fatal electrical shock. Such was the case with a millwright who suffered a fatal shock when attempting to disconnect the welder-supply cable. Because the work lead wasn’t properly connected, stray welding current caused the failure of a bonding connection between the 600-Volt welder supply switch and its source of power, resulting in voltage being applied to the welder supply cable-bonding conductor, electrocuting the worker.


The work lead, often incorrectly referred to as the ground, in this case wasn’t properly connected as close to the work area or welding location as required. As a result the stray welding current caused damage to the building’s electrical system.


This accident was preventable. Canadian Standards Association, W 117.2-01 Safety in Welding, Cutting and Allied Processes suggested the following procedure and set-up;

a) The work lead shall be connected as close as possible to the work area or the location being welded upon to ensure the welding current returns directly to the source through the work lead.
b) The wire of the work lead cable shall be of equal or greater size than that of the electrode cable.
c) All cables shall be free of damage or exposed conductor and shall not be longer than specified by the welder manufacturer.
d) Electrical grounds connected to a building framework or other locations remote from the work area increase the possibility of output current passing through building systems. This passage of current through building systems can cause extensive damage to equipment and electrical circuits if the work lead isn’t properly installed.


Care must be exercised when attaching the work lead. Failure to do so can result in an electrical shock.

Following safe work practices can prevent such occurrences. For further details and direction, please refer to the CSA document; W 117.2-01, Safety in Welding, Cutting and Allied Processes.

Tony Moscioni
Electrical Inspector
Electrical Safety Authority


#29062 - 11/21/04 04:29 PM Re: Welding table, to ground or not to ground?  
ddahlgren  Offline
Junior Member
Joined: Nov 2004
Posts: 2
Mystic, CT USA
Lincoln told us to do the following and I am not certain it is a good idea any input would be appreciated.

In a new building that was just put up, it is small ony 2000 sq. ft. to do the following, drive copper ground rods 8 ft. deep around the building and then bond these to the rebar in the floor before it is poured. Then ground the work table to one of the rods wherever int might be in the building. This is supposed to keep the high frequecy from the ac tig welder from getting back into the mains and EGC. They had further stated that some small value ,I forget at the moment exactly what it is , capaitors between the supply legs and grond at the junction box / welder disconnect and would further reduce any problems.

What we want to avoid is to have the machine and the work table at two different potentials because of the different ground paths. My thought is to connect the grounded rebar system to the equiptment ground in the panel to avoid this.. Like I said we are up for suggestions on this one. At this moment there are 8 ground rods driven inside the building bonded to the rebar in the floor laid on a 12 X 16 inch grid. Nothing is connected to anything yet as it will be another few months before the building is ready to be put in use. I might add this is in a residential area that is zoned for light manufaturing as well and we don't what every tv on the street rolling when an arc is struck.. So we do have to find a way to kill the rf and not the welder operattor at the same time. FWIW the building has a new 200 amp service, several computer systems including a CNC and that is all up to code.
Dave


#29063 - 11/21/04 07:06 PM Re: Welding table, to ground or not to ground?  
Norstarr  Offline
Member
Joined: Sep 2004
Posts: 86
Wi
My personal opinion is to not ground the work table and require the work lead to be attached as close as possible to the job being welded. More critical is making sure all work cables are kept in good shape and connections are tight. Case in point: Negative cable attached to job laying on work table or it may have been attached to work table itself. Negative cable in poor shape and clamp was used to attach to table but connections were poor. Laying against work table was 4" wide flat stock approximately 20' long which caused it to also lay against the frame of a large shearing machine. High current flowed through the flat stock, through the shear frame to the gounding conductor of the shear. The current was approx. 175 amps and the grounding conductor was a #10. It produced so much heat that it melted the grounding conductor which melted all the conductors adjoining the grounding conductor and took out $2000.00 worth of components and motor for the shear. Plus our labor for a grand total of $4000.00. This could have been prevented had the welder operator kept his equipment in top shape. Electricity is just like me, it likes to travel.
Ron


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