William,
I'm not sure what the struggle is about here. If you have a combiner box, you should always have a disconnect nearby to service the fuses. The 2011 NEC will require such a disconnect. It is a no-brainer on a ground mounted system. A roof-mounted residential system of 4 strings or less should bring the strings off the roof, assuming less than a 100-foot run. That way you would not need a rooftop disconnect-no backhoe problems here unless the backhoe really gets reckless ;-). With the 2011 NEC requirement, integrated disconnects with combiners will become common. Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of William Miller Sent: Sunday, December 20, 2009 12:29 PM To: RE-wrenches Subject: Re: [RE-wrenches] DC disconnect placement Dick: We are gong to install HU361 disconnects at the PV. Touch-safe fuse holders are one way to apply disconnecting means rated at 600 VDC to a Sola Deck device. Fusing at an array and GFDI at an inverter do not de-energize PV feeders. Combiner fuses, if required, protect only adjacent arrays. They will not open if a feeder is violated. In my humble opinion, this is one of the serious, unrecognized hazards of the PV industry. Burial tape is always a good idea, but it is only effective if the backhoe is taking shallow scoops. I don't believe it is required so it does not always get installed. Most of our run is already in the ground without tape. If a PV array were present, I would hope an operator would be more cautious, but without surface marking, it is difficult to be cautious over an unknown distance. If I was digging near an array, I would examine the perimeter of the array looking for a disconnect and/or a telephone number for the installer. Never thought about dumping fill over arcing leads. I'm not sure this is SOP. Each scenario is different, but I like working on de-energized leads and it is standard in other electrical trades to require that it be possible to do so. Without disconnecting means, how is this accomplished? I like avoiding the expense of disconnecting means as much as anyone else, but I do want to leave a legacy of safe installs. Lastly, a 1Kw array at 550 VDC is just as lethal as a 100KW array at 550VDC. This conundrum is new on my radar and I'm still working this out in my own mind. It was brought up by a recent post regarding PV feeders through an adjacent building and also this job with conduit-finding back hoes. I think the issue is warrants discussion. Thanks for participating. I enjoy your view point and the camaraderie of this list. William Miller At 11:34 AM 12/20/2009, you wrote: William, Not sure I understand how buried PV runs protected by touchsafe fuses in combiner boxes at the array and GFCI protection at the inverters constitute a fire hazard. That's not saying I disagree with your disconnect here. I don't. If it was a 1kW system, would you still install one? I would not. Would not a deeper trench with properly located burial tape have been a good call? If the array was installed and visible, maybe the excavation operator would have thought to ask about underground utilities. They should have anyway. Would not common sense imply the operator would drop a bucket of fill on any sign of arcing. Would they know to look for a disconnect at the array in any case? Common sense can be slippery. Dick Solarwind Electric --- You wrote: Friends: I am embarking on a 10 Kw ground mount grid-tie installation. The client prepared for this installation some time ago by stubbing a PVC conduit underground towards the future location of the PV array. Prior to beginning this job, an excavation contractor intercepted the empty conduit with a backhoe. No harm was done because this conduit was empty. Had this conduit been filled with PV feeders, it would have been very necessary to be able to disconnect the PV arrays, both for safety and to prevent a brush fire in our very dry climate. The inverters will be located in a pump house about 130 feet from the PV arrays. I would not normally put disconnects at the array and they are not required. This scenario, however, has caused me to reconsider. The design now calls for a disconnect at each row of modules. I feel that it would not be prudent to delete this important safety item. I wish there were a better selection of compact, low profile disconnects suitable for a roof-top installation. I have put external handle disconnects on roof but they look terrible. I may specify touch-safe fuse holders inside of a Sola Deck for future comp roof jobs. I am not sure what we will do with tile roofs. It seems to me that every source of power should have a means of disconnection as close to the source as possible. This is just common sense. William Miller
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