[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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