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-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Eric Thomas Sent: Sunday, November 14, 2010 7:43 PM To: re-wrenches@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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