Arcing between subpanel and concrete wall

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Gene B

Member
Location
USA
There's a subpanel mounted directly to a basement concrete wall which shows signs of arcing between the box and the wall. What are the possible causes?

Regardless of the cause, is it a bad idea to mount a metal box directly on a concrete wall because of the possibility of an unintentional earth connection?
 

big john

Senior Member
Location
Portland, ME
Yeah, I can't imagine a circumstance where a low-voltage system would show visible arcing damage to concrete.

You sure this isn't corrosion related?

Mounting directly to concrete is normal and only regulated in wet locations.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
Regardless of the cause, is it a bad idea to mount a metal box directly on a concrete wall because of the possibility of an unintentional earth connection?
The only issues with unintentional earth connections are when they involve the neutral or an ungrounded conductor.

There are no issues with multiple points of connection for the grounding conductor(s) or conductive object connected to grounding conductors.
 

Gene B

Member
Location
USA
Yeah, I can't imagine a circumstance where a low-voltage system would show visible arcing damage to concrete.

You sure this isn't corrosion related?

I'm sure. If it's lightning, wouldn't it be better to have only one earth connection? That way the lightning wouldn't be using the equipment ground as a path.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
I'm sure. If it's lightning, wouldn't it be better to have only one earth connection? That way the lightning wouldn't be using the equipment ground as a path.

Lighting will take any and all paths, including paths of air, to get to it's charge center in the earth. Providing more low impedance (crucial) metal paths to earth will reduce the likelihood of a strike taking an air path and making big sparks.

If the single earth connection isn't sufficient to shunt the bolt of lightning, the result is the bolt using other paths to get back to the charge center.
 

mgookin

Senior Member
Location
Fort Myers, FL
There's a subpanel mounted directly to a basement concrete wall which shows signs of arcing between the box and the wall. What are the possible causes?

Regardless of the cause, is it a bad idea to mount a metal box directly on a concrete wall because of the possibility of an unintentional earth connection?

Can we get some pictures?
We like pictures :)

Panels are mounted on concrete walls all the time.
 

big john

Senior Member
Location
Portland, ME
I'm sure. If it's lightning, wouldn't it be better to have only one earth connection? That way the lightning wouldn't be using the equipment ground as a path.
Can you get photos?

When it comes to lightning not much is an effective insulator, so generally it's far better to ground and bond the heck out of stuff to reduce voltage gradients, than it is to try and stop lightning from causing current flow through isolation.
 

big john

Senior Member
Location
Portland, ME
You sure that didn't exist prior to the installation of that panel?

The exterior of the panel looks pretty unaffected. What does the interior look like?
 

brian john

Senior Member
Location
Leesburg, VA
I'm sure. If it's lightning, wouldn't it be better to have only one earth connection? That way the lightning wouldn't be using the equipment ground as a path.


Lightning can and does anything it wants to, even the best of elaborate grounding systems with TVSS (SPD's) and Lighting protection can sustain damage if the lighting hits in the right location and is of long enough duration and magnitude.

Any chance there is re-enforcing steel exposed (or was exposed) at this location.
 

Travisat419

New member
I'm near Philly, we always mount a back box to a minimum of plywood to keep from corrosion of the metal, I prefer 7/8 metal strut and 1/4 20 spring nuts and the sort if suitable. I deal with wall walls that are 100 years old sometimes it is a mission to get a 2' x 4' cut of plywood to firmly attach. In your pic it looks like a hot conductor came in contact with the top of your panel making a flash. wipe it of, check the inside underneath, clean?
 

Speshulk

Senior Member
Location
NY
Suppose that a panel screw just happens to be close to rebar. The path would be rebar->arc->screw->case->EGC.

We're looking at a picture of an "arc" between a cabinet and a concrete wall that it's mounted to. If two conductive surfaces are touching each other, the path of least resistance between them is going to be through the points of contact, thus, no arc.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
On top of everything else, that panel looks new enough that it must have the code required raised mounting feet that means that spot is 1/4" from the actual concrete. Doesn't that take like 3,000 volts minimum to jump?
 
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