Reinforced concrete- Insulator or conductor

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TVH

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I often see welders on construction sites kneeling or sittng on concrete welding structural steel. One of the basic rules for welders is to insulate yourself from ground paths.
Question: Is reinforced concrete an insulator?
Some say it is and most believe it is safe work positioning for welding without insulating the welder from ground path.
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Electrician - 2017 NEC
In general concrete is an insulator at nominal voltages. Moisture and contaminents may increase its conductivity but its would still be considered an insulator. The lower voltage used for welding would not be enough to pass any reasonable amount of current.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
The NEC considers concrete a grounded surface. See Note 2 to Table 110.26(A)(1).
Condition 2 — Exposed live parts on one side of the working space and grounded parts on the other side of the working space. Concrete, brick, or tile walls shall be considered as grounded.
I am not sure if it is conductive enough to be a shock hazard for welding if it is dry and not on grade. It it is wet or a slab on grade it could be conductive enough to be a hazard.
 

Mule

Senior Member
Location
Oklahoma
A ufer ground is known to have a much lower resistance than a driven ground rod....

Seem like I read somewhere sometime back in a commentary that the intent of GFI's in garages was the unfinished floor and its moisture content....
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Electrician - 2017 NEC
A ufer ground is known to have a much lower resistance than a driven ground rod....

Seem like I read somewhere sometime back in a commentary that the intent of GFI's in garages was the unfinished floor and its moisture content....
You must also consider the surface area and length when comparing the conductivity of the two. A ground rod with the same surface area to ground as the concrete used in the UFER would be need to compare.
 

jdsmith

Senior Member
Location
Ohio
I often see welders on construction sites kneeling or sittng on concrete welding structural steel. One of the basic rules for welders is to insulate yourself from ground paths.
Question: Is reinforced concrete an insulator?
Some say it is and most believe it is safe work positioning for welding without insulating the welder from ground path.
Are welders supposed to insulate themselves from ground paths or from paths that the welding current might take? What is commonly called a ground clamp that welders connect to the steel is technically a work clamp, not a ground clamp. The current coming out of the welding electrode is not looking for ground, it is looking to go back to its source, which is the welding machine. If a person was welding on a steel column base, for example, the work clamp would probably be placed on the base or up the column somewhere. In this case there would be very little current in or out of the ground because the current would flow from the electrode up the steel to the clamp then back to the welding machine. This is just a grounding vs. bonding question in disguise - start here to learn more about the difference and why the term ground is often misused.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
In my opinion, any concrete is to be considered a grounded, conductive surface. I have heard of more than one instance of a welder being shocked or even electrocuted. Concrete conducts.

I have seen plenty of sites where the work clamp is clamped onto the beam closest to the welder generator, and the entire building is welded using that one connection as the return path.

I know that's a case of the steel, and not the concrete, being used as part of the circuit. Nonetheless, as use as Ufer grounds proves, concrete is conductive, even without rebar.
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Electrician - 2017 NEC
Nonetheless, as use as Ufer grounds proves, concrete is conductive, even without rebar.

I agree but I can not agree that it is a good conductor. Also it is the moisture content that varies the conductivity. Without moisture concrete is a good insulator. UFR grounds prove to be good grounds because of the surface area of the concrete that the electrode is in not the concrete itself.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Without moisture concrete is a good insulator.
Are you sure? There's a high mineral content in concrete, and from what I've read, concrete rarely ever dries completely. Concrete can cure even when submerged.

Besides, on just about any construction job where there is structural welding going on, the concrete is relatively fresh.
 

iwire

Moderator
Staff member
Location
Massachusetts
What is commonly called a ground clamp that welders connect to the steel is technically a work clamp, not a ground clamp. The current coming out of the welding electrode is not looking for ground, it is looking to go back to its source, which is the welding machine.
Thank you for bringing that up and I agree the term 'ground' should not be used for the work lead, it just adds confusion.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
The current coming out of the welding electrode is not looking for ground, it is looking to go back to its source, which is the welding machine. If a person was welding on a steel column base, for example, the work clamp would probably be placed on the base or up the column somewhere. In this case there would be very little current in or out of the ground because the current would flow from the electrode up the steel to the clamp then back to the welding machine. This is just a grounding vs. bonding question in disguise - start here to learn more about the difference and why the term ground is often misused.
However the shock issue is between the conductive concrete that is often in contact with the steel that has the work clamp connected to it and the stinger lead and or rod.

If the output voltage of the welder is over 50 volts, the welder must use 1000 volt rated gloves to change the rod:wink:
 
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