Grounding screw requirement [RE-wrenches]

Sat Feb 23 10:46:19 PST 2008

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Hi Sky;

I think several on the list would say this is NOT code compliant; but  
here in the lawless outback, we do the same thing. There is an issue  
with the anodized coating on the aluminum, but my testing shows a low  
resistance connection with star washers. We always isolate the copper  
braid from the aluminum with stainless washers too. This method  
passes inspection here, for what ever thats worth. Its not UL, but  
look at other electrical systems that allow mounting to a steel  
backing plate to be the ground connection, or attaching to the steel  
building framing to be the allowable grounding path. (When was the  
last time UL tested an entire building's bolted structure for  
electrical conduction?)
Its interesting that your grounding method might not be allowed, but  
every building in our area is required to have a UFER ground, which  
is a rusty piece of rebar with copper wire clamped directly to it.   
No one here is going to convince me that there will not be galvanic  
corrosion between these dissimilar metals that will cause resistance.  
(we weld, and often the rust is so thick the welder can't pass enough  
current to even work)
So stainless star washers to aluminum rails seems as good or better a  
conductor than copper to rusty rebar IMHO.
On the same idea, we always sink a separate ground rod beside the  
pole, but a 6" steel pipe sunk in 4 ft in the ground in concrete  
should be as good a ground as the allowed rusty rebar or 5/8" ground  
rod.
I've been wanting to buy a clamp-on ground impedance tester, because  
the code allows ( I believe) any ground if you can prove it has an  
impedance of less than 25 ohms.
I think many ground rod and UFER installations might not pass this test.
I don't care so much for dissecting the language of the code, if I  
can field test and confirm good performance.
Talk is cheap, good installation that can be verified scientifically  
is not.

Ray Walters
solarray.com
On Feb 22, 2008, at 2:32 PM, Sky Sims wrote:

>
> Personally I prefer to use bolts with star washers for the grounding
> points. Our rack is secured at four points with bolts and star washers
> to each panel. The rack becomes the ground wire. This eliminates the
> need for ground wire between panels. You don't need to worry about the
> bolts stripping out either.
>
> This method won't work with every type of rack but it should be
> acceptable with most.
>
> Sky Sims
> Ecological Systems
> www.ecologicalsystems.biz
> 220 County Road 522
> Manalapan, NJ 07726
> 732-462-3858   fax)732-462-3962
>
> -----Original Message-----
> From: Brian Wiley [mailto:btw at we-llc.com]
> Sent: Wednesday, February 20, 2008 3:01 PM
> To: RE-wrenches at topica.com
> Subject: RE: FW: Grounding screw requirement [RE-wrenches]
>
>
> Hi Aaron,
>
> I do not think there is any such specification for electrical
> connections.  The 3 thread rule of thumb is for mechanical engineering
> and relates to mechanical strength, not current carrying capacity.
>
> This is why UL requires the manufacturer to provide a specific screw
> with their product.
>
> Tests in my laboratory have established that a 10-32 stainless steel
> screw with 3 threads (as is commonly used on PV modules) is capable of
> carrying enough current to be used with a 10 AWG wire maximum.
>
> Brian Wiley
> Wiley Electronics LLC
>
>
>
> Aaron Wellendorf wrote:
>>
>> Fellow wrenches,
>>
>> I am resending this as it did not seem to post.
>>
>> I am trying to come up with documentation regarding the requirements
> for
>> ground screws, such as those used to mount a mechanical lug inside of
> a
>> disconnect switch or a junction box. I have seen installations with a
>> Tek screw holding ground lugs to electrical boxes. I have always
>> used/required the typical 10-32 ground screw that is self tapping, or
> a
>> properly sized pre-threaded hole to put in the 10-32 screw. I was
> taught
>> that proper thread engagement was 3 threads touching the metal to
> ensure
>> long term contact and continuity. I am searching for the requirement
> for
>> the 3 threads or any other specification relevant. I have searched my
>> electronic copy of the NEC and can't find a reference to this. Can
>> anyone tell me where a written specification can be found?
>>
>> Square D tech support says that to maintain their UL listings, their
>> safety switches require the use of their grounding kit, part  
>> #GTK0610,
>> which has a 12-24 hex head self-tapping screw 0.62" long with a
> washer.
>> The screw must be installed in the location marked with the ground
>> symbol.
>>
>> 	"Aaron,   All of our  safety switches have a UL label inside the
>> switch that lists the UL listed accessories that can be used.  The
>> ground kits for these switches are listed on this label.
>> These UL listed kits are the only accessories that should be used in
>> these switches to retain the UL listing, and warranty.  Using  
>> anything
>> not listed on this label will not be UL listed as an accessory, and
> void
>> any warranty.
>> 	Tim Milbert  Product Support Specialist"
>>
>> Thank you,
>>
>> Aaron Wellendorf
>
>
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R. Walters
Solarray.com
NABCEP # 04170442	

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