Inverter Output Circuit and DC GEC requirements [RE-wrenches]

[Mike Bernhard]

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Bill,
One more thing.  Start manufacturing  inverters that are NOT separately 
derived systems.
Mike Bernhard

----- Original Message ----- 
From: "Bill Brooks" <bill at brooksolar.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, June 19, 2007 11:15 PM
Subject: RE: Inverter Output Circuit and DC GEC requirements [RE-wrenches]


>
> Mike,
>
> Mike,
>
> What the code allows and what is good design can be two very different
> things. That is why I don't like people getting overly focused on the fact
> that the dc is technically a separately derived system according to the
> code. That idea will lead you, and many jurisdictional authorities down 
> the
> wrong road from a design point of view. It is critical that the dc is
> referenced to the exact same grounding source as the ac to prevent the
> transistors in the inverter from being fried in a transient (e.g.
> lightning).
>
> For a standard building 208Vac separately derived ac system fed from a
> 480Vac main panel, grounding to building steel is no big deal, because a 
> big
> dumb transformer is between the two ac systems. That is simply not the 
> case
> with a PV inverter. With PV there are very sensitive power transistors 
> that
> work exactly like surge arrestors when the dc circuit ground goes to one
> voltage while the ac is at another voltage (they are very expensive surge
> arrestors, however). NEC 690.47(C) should not be read as "permitted", but 
> it
> really should be read "must". Electrically it is not dangerous to have two
> separate grounds--unless you are the inverter--then it is deadly.
>
> If a code inspector requires you to derive a separate ground 
> (690.47(C)(1)),
> ALWAYS tie that ground to the ac grounding electrode. The bummer to this
> method is that you must use the larger of the two GECs to run between the
> two grounding systems (you are creating a grounding electrode system). If
> you hold off and tie into the ac grounding electrode (690.47(C)(2)), the
> conductor only has to be sized according to the dc side, which is almost
> always smaller.
>
> Bill.
>
> Bill Brooks, PE
> Brooks Engineering
> 873 Kells Circle
> Vacaville, CA 95688
> office and mobile phone: 707-332-0761
> fax:707-451-7739
> email: billbrooks7 at yahoo.com
> web: www.brooksolar.com
>
> Ecclesiastes 11:7 Light is sweet, and it pleases the eyes to see the sun.
>
>
>> -----Original Message-----
>> From: Mike Bernhard [mailto:spaight12 at sbcglobal.net]
>> Sent: Monday, June 18, 2007 4:51 PM
>> To: RE-wrenches at topica.com
>> Subject: Re: Inverter Output Circuit and DC GEC requirements 
>> [RE-wrenches]
>>
>>
>> Randy,
>> The answer to your first question is no.  Wire, including NM sheathed,
>> must
>> be protected.
>> Question  Don't think of it as a DC GEC.  It is the GEC for the primary
>> side
>> of the isolation transformer as well as whatever else you would like to
>> call
>> it  Separately derived systems in  Art 250.  The GEC can be attached to
>> ANY
>> building grounding electrode including cold water pipe (within 5 ft of
>> where
>> it comes into the building), building steel, Ufer, ground rod, plate, 
>> etc.
>> Once again, Art 250, separately derived systems.
>> Mike Bernhard
>> ----- Original Message -----
>> From: "Randy Hatch" <randy at third-sun.com>
>> To: <RE-wrenches at topica.com>
>> Sent: Monday, June 18, 2007 11:01 AM
>> Subject: Inverter Output Circuit and DC GEC requirements [RE-wrenches]
>>
>>
>> >
>> >
>> >            I was hoping that someone could help me to clear up a few
>> items
>> > that I've been researching.
>> >
>> >
>> >
>> > 1)       Is the inverter output circuit conductor (utility interactive
>> > inverter) required to be in conduit?  NEC reference?
>> >
>> > 2)       Is the DC GEC required to be run from the inverter to:
>> >
>> > a.       The DC GE and then bonded to the AC GE (690.47(1))
>> >
>> > b.       the AC GE (690.47(2))
>> >
>> > What should I do for school and commercial installations where 
>> > providing
>> a
>> > dedicated DC GEC from the inverter/s to the AC grounding electrode
>> system
>> > can be considerably more difficult in terms of pathing, distance, 
>> > etc...
>> > Is
>> > there a work-around?  NEC reference?
>> >
>> >
>> >
>> > Thanks.
>> >
>> >
>> >
>> > Randy Hatch
>> >=
>
>
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