Kent,
One of the things to take a look at and to make sure that your
inspector is aware of is the last paragraph of Section 90.4. This was
brought up at the last code meeting to allow new products to come on-
line and to implement new technology into the market. Due to the
timeliness of changes happening to the NEC, it might be perceived as
making the product go "primetime" before it is ready. In this case,
we looked at the possible consequences of a delay until 2014 and
thought that the industry with its current growth rate would be
impacted negatively by not having DC AFCI in article 690 to help
prevent a black eye for solar. Sometimes the code does drive product
implementation, but I would like to think that it is proactive rather
than reactive.
Brian Crise
Lead Instructor
NIETC
On Apr 6, 2010, at 10:15 AM, Kent Osterberg wrote:
Ray,
Considering that we design PV wiring to be efficient with voltage
(and power) loss typically less than 2%, the wire size is nearly
irrelevant to arcing issues. Essentially all the energy available
from the PV array can be dissipated in the dc arc. And since the
current is limited by the nature of the IV curve, breakers alone
usually won't clear the fault. The best combiner breakers can do
(if you have enough parallel circuits) is isolate the fault to one
string in the PV array. With one string being 1 or 2 kW in many
systems there is still the potential for a lot of heat.
With the 2011 code just around the corner and no dc arc fault
protection on the horizon, it looks like our industry is again going
to have a code requirement that no one can fulfill.
Kent Osterberg
Blue Mountain Solar, Inc.
R Ray Walters wrote:
I agree that we don't want to create the code first, and try and
develop the product after. On the other hand, if a DC AFI can be
developed that could stop some of the problems I've seen breakers
not help, I'm installing them, and pushing for code requirements.
AC GFIs were gimmicky too at first, but now have gone on to save
countless lives; usually kids, but a few of us wet booted
contractors too.
As far as running things at 100%, I do agree with you, but I also
think that 156% over rating in many cases is too much. If the wire
and breaker are that oversized, it is less likely to trip when you
want it to.
My most recent damage I saw, the cable a few inches back was in no
way damaged, as it only saw array short circuit current, but the
connector that arced burned up a whole circuit board.
More oversizing would have only increased the arc potential, and
reduced the chances of a breaker tripping.
Proper sizing (not too big, not too small) is the way.
R. Walters
r...@solarray.com
Solar Engineer
On Apr 6, 2010, at 9:16 AM, robert ellison wrote:
I have seen info from independent tests that convinces me that
AFCI's probably don't work for AC and i hate to think what they
would do for DC, if anything. Besides drive the costs up.
This was a few years ago and maybe they have gotten it together by
now.
Anyone remember the original ground faults form Trace (?) after
the code change requiring them in 96? Expensive and prone to
catching fire comes to mind, if i remember correctly. Lets not
encourage more if that type of experimentation in the industry.
Just the same i am not a believer of running anything at 100%, it
will always have a higher failure rate than something run at a
lower capacity, be it a generator, lawn mower or a circuit breaker.
Bob
Mon, Apr 5, 2010 at 5:36 PM, William Miller <will...@millersolar.com
> wrote:
Ray:
It is my analysis that combiner breakers (if present) will protect
only wiring upstream of the combiner -- that is, the individual
string circuits. This protection would happen if there is a fault
in one individual string (in the wiring or the modules) that
allows current from other strings, in excess of the breaker
rating, to be supplied through the breaker feeding the faulted
string.
There are two scenarios at play here:
1. Any fault between the combiner and the feeder destination will
not trip any circuit breakers. The breakers are sized such that
the current from each individual string is less than the breaker
rating (by more than 1.56 times) and they will not open.
2. PV GFDI protection at the destination end of a feeder will not
help. PV GFDI circuits will not remove power from a feeder and
they will open the ground-to-grounding conductor bond.
Analyzing this further: Fault conditions are made more likely
given that PV string circuits are no longer protected by conduit.
Faults are then more likely in individual string circuits (those
circuits without conduit protection). This is most problematic at
installations with two or fewer strings, where there is no
combiner, i.e. residential installations. Statistically,
residential installations offer greater exposure to electrical
fires because: occupancy occurs for more hours per year, fire
alarms and sprinklers are often not installed, children are more
often present and standards are more lenient for residential
wiring systems.
These two facts are PVs dirty little secrets. Further innovation
is needed...
William Miller
At 12:22 PM 4/5/2010, you wrote:
I think the 100% rating exception is an interpretation issue. I
consider the assembly to be defined as the breaker mounted in its
listed enclosure.
I agree that the AFIs would add cost, but they might actually
offer some protection too. (possibly one AFI unit could offer
protection for multiple circuits?)
I've never had a PV circuit breaker actually trip, except some
nuisance tripping due to faulty breakers.
PV breakers seem to only offer protection for very limited
situations ie, a short in a PV wire being backfed by enough other
PV circuits to trip the breaker.
It could happen, but I've never actually seen it. Even completely
shattered modules still have enough internal resistance to limit
the short circuit current to
a value below the breaker trip point.
Ray
On Apr 4, 2010, at 11:47 PM, Kent Osterberg wrote:
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