William, 

It is common in our area to install underground, unfused, transformer secondary
laterals. The utilities require 3 feet of cover and marker tape located 12"
below grade. I can't find a requirement for marker tape in the NEC. That's not a
reason not to use it. It's inexpensive & available everywhere. I've seen
excavation contractors (and others) lay it right on top of the conduit. That's a
shallow scoop indeed. When I see that, it's fun to point out that the butt they
save by installing it correctly may be their own.... or another excavator's. For
shallower trenches it's reasonable to put it immediately under the sod. IMHO
it's very rarely reasonable not to use it. 

Are we talking fire hazard or shock hazard here? I thought fire hazard. A 1kW
arc is a very different animal than a 100kW arc. 

Even if there is a disconnect to turn off before the digging starts, caution is
still required. It's a major bummer to have to repair an underground feeder.

If you can describe this scenario for a 1 kW client and get them to go with the
disconnect when for that same money they could buy the better part of another
module, go for it.
In that instance, I think another PV guy may get the job. It's not so hard to
throw some tarps over a small ground mount array.

Dick



--- You wrote:
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-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@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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