[RE-wrenches] 690.47D (revisited)

Joel Davidson joel.davidson at sbcglobal.net
Fri Jun 5 19:18:02 PDT 2009


I agree with Kelly and others that all grounding electrodes should be bonded together. My experience in lightning country with PV systems with a ground rod at the array and separate ground rod at the BOS and no wire connecting  the ground rods has always resulted in fried equipment. The same systems, after they were repaired and the ground rods were connected together, survived every lightning storm for several years. I have found that the same holds true for any equipment or structure - good earth grounds all connected together create a grounding field and can create a cone of protection. Properly grounded equipment with all grounding electrodes connected together generally survive lightning storms and have been known to survive direct lightning strikes.

Joel Davidson
  ----- Original Message ----- 
  From: Brian Crise 
  To: RE-wrenches 
  Sent: Friday, June 05, 2009 1:45 PM
  Subject: Re: [RE-wrenches] 690.47D (revisited)


  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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