[RE-wrenches] 690.47D (revisited)

Kelly Keilwitz, Whidbey Sun & Wind kelly at whidbeysunwind.com
Fri Jun 5 16:26:13 PDT 2009


Brian,

Thanks for your thoughts. Have you changed your installations to meet
690.47D? If, so, how?

On 6/5/09 1:45 PM, "Brian Crise" <bcrise at nietc.org> wrote:

> 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).

Except that the requirements of 690 supercede any other sections, if there's
a conflict. 690.47(A) is for AC PV systems.

>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.

I'm with you, here, IF the supplementary PV GE is required it makes sense to
bond it to the others (even though the Handbook doesn't show it). We have
been running a bare #6 Cu GEC from remote ground mounts to the GE system
near the inverter for years.

However, it DOESN'T make sense to me to run this redundant (to the EGC) PV
GEC when the the supplementary PV GE is NOT required.


> 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 ³enhanced protection from lightning
>> induced surges² as described in the 690.47(D) section note in the 2008 NEC
>> Handbook, the ³optional supplementary grounding electrode² 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 ³additional electrode for array grounding² (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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Kelly Keilwitz, P.E.
Whidbey Sun & Wind, LLC
Renewable Energy Systems
NABCEP Certified PV Installer
987 Wanamaker Rd, 
Coupeville, WA 98239
PH & FAX 360-678-7131
sunwind at whidbeysunwind.com
 






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