[RE-wrenches] DC disconnect placement
William Miller
william at millersolar.com
Sun Dec 20 12:28:49 PST 2009
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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