[RE-wrenches] Head Spinning..(Con) fused.

[Bill Brooks]

Eric,

 

Your concern over shorting the current carrying conductors to a single
conductor would require multiple faults-not that this is impossible-but we
generally are not required to design for it. Squirrels would probably be the
most likely way to cause such an event.

 

Since all four strings are going into separate input stages, if any of the
conductors were to come into electrical contact with each other or the
conduit, it would likely trip the very sensitive ground-fault detectors.
Series fuses are unlikely to have any plausible scenario that would cause
them to operate. 

 

Bill.

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Eric Thomas
Sent: Sunday, November 14, 2010 7:43 PM
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] Head Spinning..(Con) fused.

 

Thanks William and Daryl,

Here is Auroras backfeed stipulation from the installation manual:

"Installation and Operator's Manual Page 98 of 108

(PVI-3.0/3.6/4.2-OUTD-x-US Rev: 2.1)

8.5 Input Source Backfeed Current

PVI-4.2(3.6, 3.0)-OUTD-x-US Grid Tied Inverters are provided with two
separate and
consecutive power stages:

. Booster Stage (DC-DC converter) connected to DC Input Terminals.
. Inverter Stage (DC-AC converter) connected to AC Output Grid Terminals.

The Booster Stage is provided with forward diodes that allows the current
(power)
flow only from DC Input terminals towards Inverter (Output) Stage.
In case of any fault on Inverter Stage, these diodes avoid any back-feed
current
phenomena towards input terminals.

In case of fault of the forward diode, the corresponding booster MOSfet goes
immediately and permanently in short circuit state and it avoids any current
propagation form output to input terminals.

Abnormal Fault tests conducted during UL1741 qualification show also that
these type
of faults produce the opening of internal grid disconnect relays and cause
the external
AC CB protection devices to trip, preventing any power flow from the grid.

In conclusion for PVI-4.2(3.6, 3.0)-OUTD-x-US-y models it is not possible to
have
any single fault responsible of Input source back-feed current flow. For
these models
the Back-feed current into DC Source is negligible."

According to this, the inverter will not backfeed even under most fault
conditions. My biggest concern is the four independent strings shorting
together somehow. I guess technically 4 strings of 8A for a total of 32A
doesn't exceed the ampacity of 10AWG XHHW-2....until you derate it for roof
temps. 

So whats the consensus?

-- 
Eric Thomas
Solar Epiphany LLC
(206) 919-3014
www.solarepiphany.com

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