[RE-wrenches] 690.47D (revisited)

Brian Crise bcrise at nietc.org
Fri Jun 5 13:45:55 PDT 2009


Kelly,
It is my understanding that all types of grounding electrodes, whether  
supplementary or not, for lightning protection or PV array grounding  
shall be bonded together to form the grounding electrode system.  
(250.106, 690.47(A) which sends you to 250.50,   NFPA 780-2008 4.14).   
If you do not do this and you do get a lightning strike or for that  
matter any high frequency signal injected onto a grounding electrode  
you will get what they call a ground potential rise onto one part of  
the system.  With the other separate grounding electrode not having  
this same signal because of them not being common, you will have a  
potential difference between the two separate electrodes even though  
they are both in the ground.  The ground (earth) is actually a very  
poor conductor when it comes to high levels of high frequency current
Hopefully this helps,
Brian

NABCEP Certified PV Installer

To contact Brian L Crise:

Address:
16021 NE Airport Way
Portland OR, 97230

Office Phone:    (503) 262-9991 x.5054

e-mail:    bcrise at nietc.org



On Jun 5, 2009, at 12:10 PM, Kelly Keilwitz, Whidbey Sun & Wind wrote:

> Mark,
> I don't think that the 690.47D "PV GE" is part of the premises AC & DC
> ground electrode system.
>
> IMO, in order to function to properly as “enhanced protection from  
> lightning
> induced surges” as described in the 690.47(D) section note in the  
> 2008 NEC
> Handbook, the “optional supplementary grounding electrode” should  
> not be
> connected to the premises system AC and DC grounding electrodes, as  
> shown in
> Exhibit 690.6 of the Handbook. If an additional #8 (per 250.166) is  
> bonded
> to the premises AC/DC system GE, it's path will be parallel with, and
> redundant to, the existing EGC. This could set up the potential for an
> inductive ground loop, which might cause more damage to the system  
> during a
> lightning strike than it would prevent.
>
> If the “additional electrode for array grounding” (PV GE) of 690.47D  
> is not
> required, then the PV GEC of that section should not be required.
>
> Of course, these opinions are academic. What I really want to know  
> is what
> the AHJ's think! Any of you have to deal with this, yet?
>
> Thanks,
> -Kelly
>
> Kelly Keilwitz, P.E.
> Whidbey Sun & Wind, LLC
> Renewable Energy Systems
> NABCEP Certified PV Installer
> WA Electrical Administrator #KEILWKM923RB
> 987 Wanamaker Rd.
> Coupeville, WA, 98239
> 360.678.7131
> sunwind at whidbeysunwind.com
> WA Electrical Contractor #WHIDBSW920MS
> WA General Contractor #WHIDBSW946M1
>
>
>
>
> On 6/5/09 10:01 AM, "Mark Frye" <markf at berkeleysolar.com> wrote:
>
>> Kelly,
>>
>> The question is, if the AJH had required you to run the new array  
>> DC GEC
>> directly to an additional DC array GE, would that new DC GE system  
>> need to
>> be bonded with the exisitng AC GE system with a bonding wire other  
>> than the
>> new EGC running from the array to new DC GE?
>>
>> Unfortunately, NEC 2008 is not clear about this with respect to the  
>> new DC
>> array GE.  That leaves us with only the general NEC requirement  
>> that all
>> premise GE's be bonded into a single system.
>>
>> So the total effect of the new 690.47D, assuming that the  
>> "prcaticable"
>> location for the new DC array GE is not within 6 ft of the  
>> exisiting AC GE,
>> is this:
>>
>> A new DC array GEC must run directly from the array to a new DC GE  
>> system
>> and and a new bonding jumper must be run from the new DC GE system  
>> to the
>> exisitng AC GE system.
>>
>> It the new DC GE system can be the same as the existing AC GE by  
>> virtue of
>> it proximity (less than 6 ft) to the practiable location, then the  
>> ne DC
>> array GEC must still be run directly to exist GE system which is  
>> now the
>> AC/DC GE system.
>>
>>
>> Mark Frye
>> Berkeley Solar Electric Systems
>> 303 Redbud Way
>> Nevada City,  CA 95959
>> (530) 401-8024
>> www.berkeleysolar.com
>>
>
>
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