Hi Drake,

Up to now, like you, we've been using the SMA DATA SHEETS 
(the glitzy ones with the color graphics) to size our wiring and 
breakers. But, those sheets make no mention of a
MAXIMUM OUTPUT FAULT CURRENT. 

MAXIMUM OUTPUT FAULT CURRENT is specified only in the very last pages 
of the installation manuals, around page 104 or so. Because you brought 
this issue to our attention, unless SMA says otherwise, from now on 
we'll use the maximum size breaker SMA recommends, which is also 
spec'd only in the very last pages of the installation manuals.  

When the Sunny Boys drop out because of a loss of grid voltage, 
I believe, the inverters identify that event as a FAULT. 
I would rather they use a different term as that can be confusing 
to us Americans.

When your relay contacts open, there is, of course, an arc. As you know, 
if the sine wave of current is at its peak when that happens, the arc is
greatest.
The current is already jumping the gap. I think it is at this point that 
the inverter dumps extra current, MAXIMUM OUTPUT FAULT CURRENT, 
into the circuit, as it goes off line.

Thanks for bringing this to our attention.

Dick



--- You wrote:
Hi Dick,

Thanks for the suggestion. I will run the concept by the SMA tech 
people.  I did use the SMA spec sheets in sizing the wiring, but 
maybe opening the relays creates fault current at a voltage that will 
jump the gap.

Drake

At 10:50 PM 6/4/2009, you wrote:
>Hi Drake,
>
>I'm sending you some attachments off list as well.
>They are Sunny Boy Specs from the SMA Installation Manuals,
>which are more complete than those on the SMA data sheets.
>
>Note, the Maximum Output Fault Current for the 3000 & 4000 is 30 amps,
>while Maximum Overcurrent Protection is 30 Amps.
>
>The Maximum Output Fault Current for the 7000 is 57.6 amps, the
>Maximum Overcurrent Protection is 50 amps.
>
>This might be your problem.
>Upsizing the conductors and breakers for the 3000 and 7000 might be a fix.
>SMA tech support could probably shed some light.
>
>Dick
>
>
>
>--- You wrote:
>Hi Dick,
>
>There are three inverters, all Sunny Boys, a 3000, 4000 and a
>7000.  The breakers are QO.  The AC conductor sizes are #8 for the
>7000, #10 for the 4000 and #12 for the 3000.  The breakers are set
>for the NEC ratings of the wire (40 A, 30 A and 20 A.).
>
>I sent some information to you off the list with some attachments.
>
>Drake
>--- end of quote ---
--- end of quote ---

