Bonding Your Skin. Good Practice or Not?

[frizbeedog]

Are you familiar with pole buildings? (wood frame, metal skin/siding)
I install electric services to a lot of these.

Usually I end up installing ground rods at these pole buildings and install a #6 GEC and pass it through a lug that I have installed on the metal skin of the building, just to bond the metal skin. But today I got to thinking...... I said to myself, "self, why are you bonding this metal skin?"

Here is the author's comment from Understanding the National Electrical Code, Volume 1, pg.207, regarding 250.104 (C) Bonding Exposed Stuctural Metal. Mike Holt:

Author's Comment: This rule doesn't require the bonding of sheet metal framing members (studs) or the metal skin of a wood frame building, but it would be a good practice.

So I got to thinking about these skins. Usually they are in vertical sections, mabey 4-6 ft wide. They come pre-painted and they overlap. They are joinded together by wood screws or sheet metal screws and the ones I encounter have a type of gasket under the head of the screw. At this point I am not convinced that there is good continuity between the sections of skin. So it seems to me that by doing this, I only end up bonding the one section.

I have been bonding thse skins for as long as I can remember but....given all of the above, is it really a good practice?

Why, or why not?

 

[Pierre C Belarge]

If you are in an area that has local laws pertaining to this, it would be obvious as to why you would do it.

If you are in an area that has a lot of lightning, again I believe it would be obvious (also it may be the area that has a local law).

I am too lazy this morning (I will be in class for 8 hours a day for the next 3 days), so I am not going to open my book, but I believe I saw in the NEC that the metal siding may be required to be bonded to the service/meter enclosure if one or both of those items are mounted to the metal siding.
I am sure someone will let us know if I am correct or off a little bit.

 

[George Stolz]

...I believe I saw in the NEC that the metal siding may be required to be bonded to the service/meter enclosure if one or both of those items are mounted to the metal siding.

News to me...? :-?

 

[benaround]

dog,

I say keep doing what your're doing, code is just minimum. Some day before

the siding is 100% done, do a quick continuity test, I'll bet it rings thru.

 

[resistance]

Compliments of Mike Holt:
[FONT=Arial, Helvetica, sans-serif]A6 The NEC does not specifically require metal siding or metal framing members to be bonded (not grounded) to an effective ground-fault current path [250.4(A)(4)]. However, exposed structural metal that forms a metal building frame must be bonded to one of the following (effective ground-fault path):[/FONT]

• [FONT=Arial, Helvetica, sans-serif]Service equipment enclosure[/FONT]
• [FONT=Arial, Helvetica, sans-serif]Grounded neutral service conductor[/FONT]
• [FONT=Arial, Helvetica, sans-serif]Grounding electrode conductor where sized in accordance with Table 250.66[/FONT]
• [FONT=Arial, Helvetica, sans-serif]One of the electrodes of the grounding electrode system[/FONT]

[FONT=Arial, Helvetica, sans-serif]The bonding jumper for structural metal is sized to the conductors that supply the building or structure, in accordance with Table 250.66. In addition, the bonding jumper must be:[/FONT]

• [FONT=Arial, Helvetica, sans-serif]Copper where within 18 in. of earth [250.64(A)].[/FONT]
• [FONT=Arial, Helvetica, sans-serif]Securely fastened and not exposed to physical damage [250.64(B)].[/FONT]
• [FONT=Arial, Helvetica, sans-serif]Installed without a splice or joint, unless spliced by compression connectors or by the exothermic welding process [250.64(C)].[/FONT]

 

[bphgravity]

Does the building also have a metal roof? If so, you may want to consider a conventional LPS.

A lightning strike to a dry metal roof will leave a burn mark and likely a hole with the area surrounding the contact point "blistering" because of heating effects.

However, most strikes to a metal roofed building occur during rain which means the roof is wet. The current of the stroke terminating on the wet roof will tend to flow radially along surface of the metal. The under side surface, especially at ridges and corrigations, is rapidily heated and at the same time subject to the cooling effect of the water on the upper surface. The difference in this "adiabatic expansion" between the top and bottom surfaces can cause the metal to be ripped along the channels in which the current is concentrated.

This why some lightning strikes to metal structures appear to have "exploded" or "blown-out" as if it has been subject to wind-borne debris or high velocity winds.

This same pattern of damage can be seen in large trees and the earth itself after being struck by lightning.

 

[brian john]

Siding is generally aluminum (hopefully yu are not bonding the vinyl siding). My understanding of aluminum is that it oxidizes VERY FAST, this oxidation is not conductive and bonding is not viable.

 

[ultramegabob]

Siding is generally aluminum (hopefully yu are not bonding the vinyl siding). My understanding of aluminum is that it oxidizes VERY FAST, this oxidation is not conductive and bonding is not viable.

pole buildings around here are skinned in courogated steel.