welding table ground

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dan7485

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We were asked to investigate an existing school shop building, where there is a plasma cutting table. I did some digging, and found out that the manufacturer requires “three separate 6’ ground rods, 10’ apart, and with 5 ohms resistance or less.” These are required to be “separate from the building ground,” or else the warranty is voided. They go on to state that one of the ground rods has to be connected to the cutting table, (the others are for the plasma cutter case, and the cutter table controller). I was told by the manufacturer that the ground rods are intended solely for harmonic mitigation. I believe this is not a safe condition. According to everything I read from other manufacturers, or ANSI Z49.1, the “workpiece or metal table” has to be grounded. I think that NEC 250.54 requires that in order to be “grounded,” it has to be connected back to the source, via an equipment grounding conductor, regardless of whether there is a “separate ground rod.”

Thanks for any input.
 
According to my knowledge and I can be wrong and if I am someone will correct me. But I believe that the clamp on a welder also knows as a ground clamp is more of a return clamp Wich basically returns the current back into the machine in order for a weld to take place . The egc should be provided on the cord and plug and the outlet

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ANSI Z49.1:2012

ANSI Z49.1:2012

That's what I thought, until I read the following in the ANSI Z49.1 standard:

11.3 Installation of Arc Welding Equipment
11.3.1 Code Requirements. Installation including
grounding, necessary disconnects, fuses, and type of
incoming power lines shall be in accordance with the
requirements of the current NFPA 70, National Electrical
Code®, and all local codes.
11.3.2 The Work. The workpiece 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 an
ungrounded workpiece.
E11.3.2 When the work terminal is grounded, care
should be taken to see that the workpiece is not separately
grounded. Refer to 11.3.2.1. Before welding is attempted
the operator should check to be sure that the worklead is
properly connected. This will eliminate the chance of
welding current being misdirected into the grounding
conductor system of other equipment. Misdirected
welding current can damage conductors which do not
have adequate ampacity. See Article 630.15 of NFPA 70,
National Electrical Code®.
11.3.2.1 Grounding. Grounding shall be done by
locating the work on a grounded metal floor or platen, or
by connection to a grounded building frame or other
satisfactory ground. Care shall be taken to avoid the flow
of welding current through a connection intended only
for safety grounding since the welding current may be of
a higher magnitude than the grounding conductor can
safely carry.
11.3.2.2 Work Lead. Welding current shall be
returned to the welding machine by cable with sufficient
current capacity. However, connection of a cable from
the welding machine to a common conductor or properly
bonded structure on which the work rests, or to which the
work is connected, shall be a permissible alternate
procedure. Single-phase alternating current machines in
groups of three with their inputs connected in delta to a
three-phase supply circuit connected in wye on the
secondary circuits shall be permitted to use a single work
lead from the neutral of the three units to the structure
being welded.
The work lead shall use a single cable of a size suitable
for the current rating of at least one machine.

This appears to me to require the work table to be grounded, separately from whatever work lead(s) may be clamped to the table.

Does anyone out there agree?
 
dan7485 said:
That's what I thought, until I read the following in the ANSI Z49.1 standard:

11.3 Installation of Arc Welding Equipment
11.3.1 Code Requirements. Installation including
grounding, necessary disconnects, fuses, and type of
incoming power lines shall be in accordance with the
requirements of the current NFPA 70, National Electrical
Code®, and all local codes.
11.3.2 The Work. The workpiece 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 an
ungrounded workpiece.
E11.3.2 When the work terminal is grounded, care
should be taken to see that the workpiece is not separately
grounded. Refer to 11.3.2.1. Before welding is attempted
the operator should check to be sure that the worklead is
properly connected. This will eliminate the chance of
welding current being misdirected into the grounding
conductor system of other equipment. Misdirected
welding current can damage conductors which do not
have adequate ampacity. See Article 630.15 of NFPA 70,
National Electrical Code®.
11.3.2.1 Grounding. Grounding shall be done by
locating the work on a grounded metal floor or platen, or
by connection to a grounded building frame or other
satisfactory ground. Care shall be taken to avoid the flow
of welding current through a connection intended only
for safety grounding since the welding current may be of
a higher magnitude than the grounding conductor can
safely carry.
11.3.2.2 Work Lead. Welding current shall be
returned to the welding machine by cable with sufficient
current capacity. However, connection of a cable from
the welding machine to a common conductor or properly
bonded structure on which the work rests, or to which the
work is connected, shall be a permissible alternate
procedure. Single-phase alternating current machines in
groups of three with their inputs connected in delta to a
three-phase supply circuit connected in wye on the
secondary circuits shall be permitted to use a single work
lead from the neutral of the three units to the structure
being welded.
The work lead shall use a single cable of a size suitable
for the current rating of at least one machine.

This appears to me to require the work table to be grounded, separately from whatever work lead(s) may be clamped to the table.

Does anyone out there agree?
Click to expand...
No. The first thing it tells you is to run a proper equipment ground to the table. Then they want you to make sure the "work" lead (that's the one that's not the stinger) is used properly so you don't end up using the building wiring as a return path for welding current.
 
In addition to what Dave said, it is telling you not to clamp the work lead to say any old point on building steel, as that could allow welding current to take many paths to get back to the work lead. This is more critical when working outside a shop area, when in the shop clamping the work lead to a work table and performing the welding operation on that table is an attempt to try to make sure the welding current takes a fairly direct path back to the work lead. If you bring in something to work on that can't be placed on the work table then you should move the work clamp directly to that equipment and not depend on other possible pathways that might exist to carry that current.
 
infinity said:
I think that they're trying to avoid this which is common on construction sites.

View attachment 17774
Click to expand...
I'd move those clamps from the raceways over to the metal stud, when nobody is looking, and keep moving them back there if they keep moving them back to the raceways.

Or slip something insulated in the connection and see how fast they come looking for a problem.
 
table ground

table ground

Thanks everyone for your input. In this case, what I'm really wondering about is whether the grounding conductor for the table (not the work lead), needs to run back to the source, or can just be an auxiliary electrode. My opinion is that it needs to run back to the source at the step-dn transformer.
 
I forgot to hook my ground clamp to my welding table at home one time.
Struck an ark and the return current went through the metal legs of my chopsaw on the table, back through the cord looking for ground, and burnt the cord in half.

:p


JAP>
 
dan7485 said:
Thanks everyone for your input. In this case, what I'm really wondering about is whether the grounding conductor for the table (not the work lead), needs to run back to the source, or can just be an auxiliary electrode. My opinion is that it needs to run back to the source at the step-dn transformer.
Click to expand...

You need an equipment grounding conductor back to the source.
 
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