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Jeffrey,
Sounds like you need to get involved in the code making process
since you have so many good ideas on how to improve the language.
I like confrontational discussions as long as they lead to a
better understanding and constructive outcomes.
About 30 people worked on this language, so it is definitely not
perfect. However, I don't think it is quite as bad as you make it
out to be. I wanted to jump in since some of your conclusions
were not correct.
This is a circuit requirement, not a disconnecting means
requirement, since it has to do with shock hazard of PV circuits
in and around a building. This is for firefighter safety. 30V is
the international standard for touch safe in a wet location.
240VA is to set a limit on the available power on a circuit.
Contactor combiners, which would be part of a compliant solution,
have 24V control circuits. The other reasoning for 240VA is that
internally, 72-Cell PV modules can be divided into segments of
this power level for the foreseeable future (more on that another
day).
If the conductors stay outside, you have 10' from the array to
place your shutdown device. On large central systems, this would
likely be a contactor combinermost manufacturers sell these. If
the conductors are going immediately into the building, as with
residential and integrated systems, a shutdown device would have
to be within 5' of entering the building. If goes outside for a
while, then inside the building, the total length could be no
more than 10' and no more than 5' inside the buildingthis is not
additive. Remember, all this is for firefighter safety.
As Brian Mehalic and others have pointed out, the language does
not specify where the shutdown initiating device is to be
located. The lack of detail is more for flexibility than it is to
give an AHJ license to make an installer do anything they want.
With grid-tie only systems (no battery backup), it would be most
convenient and cost effective to have a system that initiates the
shutdown on loss of utility. In this way, a firefighter can do
what they normally do, shut down utility power to the building,
and the rapid shutdown would automatically initiate. This does
not necessitate an additional disconnecting means for a load-side
PV connection. The main breaker could be the initiating device.
For a supply-side connection, the NEC already requires that the
PV disconnect switch be located adjacent to the service
disconnecting means (article 230).
The biggest issue with string inverters (central inverters) is
that there is a need to shutdown the capacitor input side of the
inverter since that stays energized for 5 minutes or more. The 10
seconds was to provide a means to rapidly discharge the
capacitors rather than requiring a relay or tripping device.
Doing something other than a relay will require a test laboratory
to evaluate the functionguess what?we don't have a standard yet
to evaluate those products. Sounds like you might want to work on
that committee.
It is more complicated for battery backup systems. Midnite
Solar's birdhouse products are the best I have seen so far to
address this concern. Since dc and ac circuits are not
differentiated, battery backup systems need to have a shutdown
process that works independently of a utility outage for obvious
reasons, and it must shutdown both the dc circuits and the backup
ac circuits. A separate switch, like the birdhouse, would be
necessary that only controls these functions in an emergency
situation.
Is the language not detailedpossibly. This was done to provide
flexibility rather than create problems. Fire departments have
been requiring rooftop disconnects for years in California. These
disconnects are nearly worthless from a shock prevention point of
view since capacitors in the inverter stay charged or there are
multiple disconnecting means feeding each other. We have been
trying to hold the fire community off of rooftop disconnect
requirements so we could work on a solution that actually does
what they want it to do. There is a long discussion on this in
the appendix of my "Understanding the CalFire Guidelines"
document on the SolarABCs website.
The 2014 NEC language was a compromise worked out with the solar
industry (yes string inverter companies as well) in response to
the first version of the proposal which was to require
module-level shutdown. This is not module-level shutdown, it is
PV output circuit shutdown (combiner box shutdown is another way
to look at it). However, the 2017 NEC cycle is this year and
there was a lot of talk about requiring module-level shutdown
this time around.
I hope this helps. I will be writing articles for IAEI journal
and other periodicals on this subject since it was a very
far-reaching and potentially confusing new requirement in the
NEC. Thanks for your interest and let's keep the constructive
dialogue going on the subject. It is time to get involved in the
NEC update process again.
Bill Brooks.
*From:* re-wrenches-boun...@lists.re-wrenches.org
<mailto:re-wrenches-boun...@lists.re-wrenches.org>
[mailto:re-wrenches-boun...@lists.re-wrenches.org
<mailto:re-wrenches-boun...@lists.re-wrenches.org>] *On Behalf Of
*Jeffrey Quackenbush
*Sent:* Thursday, January 16, 2014 1:09 AM
*To:* RE-wrenches
*Subject:* Re: [RE-wrenches] NEC 2014 690.12 Rapid Shutdown
Wrenches,
There is no guidance in the Code text for where the shutdown
should take place. (1) says: "Requirements for controlled
conductors shall apply only to PV system conductors of more than
1.5m (5') in length inside a building, or more than 3m (10') from
a PV array."
So, the provisions /apply if/ the circuit 10' from the array and
5' inside a building, but no mention is made of where the
shutdown actually needs take place in the circuit. In the video
Bill Brooks suggests that the shutdown mechanism should also be
placed within this 10'/5' boundary but that is just an inference
-- nowhere in the text is this actually specified. If that was
the intent of the Code committee, then they've done a poor job
actually expressing it in English.
I'm concerned that some AHJs will interpret this to exclude all
central inverter systems (without the addition of cost-inducing
secondary DC-DC converters like Tigo) because the combiner or
junction box can be many feet from the actual beginning of a home
run under the array. Alternately, permissive AHJs could allow
this function to be fulfilled anywhere, meaning that the
implementation won't meet the intent of the writers.
I'm also concerned, as Isaac mentioned, that there are no
requirements for how the shutdown be initiated, or that it
contains of the accessibility and grouping requirements that are
always included for disconnects. I really think this should be
treated and categorized as a disconnect requirement, not a
circuit requirement, because that is the ultimate function that's
intended.
I'm surprised none of the inverter manufacturers have chosen to
comment here, as this could dramatically impact the sales of
central inverters.
Jeffrey Quackenbush
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