Hi Phil;
I'm not sure what 690.71 adds over the long standing requirements of
110.9 and 240.86. We've always complied with the interrupt rating
requirements. Old Ananda power centers had QOU breakers that only had
an interrupt rating of (I believe) 5000 amps DC, so a class T fuse was
added to the circuit as well. The power center is always installed
'adjacent to the batteries', so we've always met that requirement as well.
Nothing in the NEC specifies an actual distance to the batteries the
OCPD needs to be, and it doesn't specify that each series string have
separate OCPDs.
I'm glad to hear that Outback has a solution; Midnite has also had that
type of protection available in their battery enclosures for some time
as well. None of this deals with the truly high fault currents
available from industrial fork lift batteries though. When using AGMs,
you can just put a class T fuse on the battery terminal.
The only battery I've ever seen that actually protected against a short
circuit across its own terminals was in my kid's Barbie Jeep. It had a
fuse integrated right into the top of the sealed battery. Maybe as the
industry progresses towards other battery chemistries, we will see more
integrated OCPDs that are only found on kid's toys today.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 4/6/2013 8:29 AM, Phil Undercuffler wrote:
Ray,
690 does offer the protection, it's just that most maufacturers
haven't made it easy.
690.71 Installation
(C) Current Limiting. A listed, current-limiting, overcurrent device
shall be installed in each circuit adjacent to the batteries where the
available short-circuit current from a battery or battery bank exceeds
the interrupting or withstand ratings of other equipment in that
circuit. The installation of current-limiting fuses shall comply with
690.16.
The telcom industry has been doing this for years. Their norm is to
have each string of batteries on a shelf, and terminate at a breaker.
Each string then is combined at a bus bar or plate, to help ensure
equal current on each string. The loads and charging sources all come
to that same bus bar. It has some advantages, as individual strings
can be monitored and serviced without taking the entire system down.
FWIW, OutBack makes a battery rack for AGM batteries with series
string overcurrent and disconnects on every string. It's also to my
knowledge the first UL1741 Listed device available -- the other stuff
I've seen is either not listed, or is trading on old 508a standards.
AGMs might not be for every installation, but for jobs where they are
appropriate this could be a good, code-compliant solution.
Phil Undercuffler
OutBack Power
On Friday, April 5, 2013, Ray Walters wrote:
I just finished a rewire and we kept the Ananda power center,
Allan :-)
Back when we used class T fuses more they were always over sized
relative to Heinemann breakers.
400 amp class T fuse = 250 amp breaker= 4/0 cable
200 amp class T fuse = 175 amp breaker= 2/0 cable
I still think class T fuses are superior in some ways to breakers:
they have better interrupt capacity and trip faster in a short
circuit condition.
However, If you spend too much time thinking about and looking at
the trip curves, you'll break your brain and special order some
weird fuses and breakers. (anybody need a 350 amp class T fuse?)
Just follow the inverter manufacturer's recommendations and all
will be well.
On the other hand, William has brought up a topic I have harped on
for years: Having the OCPD in the cabinet doesn't protect the
majority of the circuit. Class T fuses at the battery terminals
do, but they're not rated for the corrosive environment. I have
thrown a few class T fuses away that had acid eating away at the ends.
I don't have the solution, but I will continue to point out that
this is a real problem. Dropping a wrench across the battery
terminals can lead to a spectacular failure that not only can
cause a fire, but might even cause a battery explosion, yet NEC
offers no protection.
We use insulated wrenches from experience, and hope for the best.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 4/5/2013 8:29 PM, William Miller wrote:
Friends:
Good topic. Some questions:
1. Most manufacturer's present an installation guide that shows
one OCPD in the battery circuit and that is in the BOS cabinet.
This means the battery leads are unprotected. Do we need an OPCD
at the battery terminals?
2. Class T fuses are generally recommended for this application.
The data shows them as "fast acting." Is this a problem? Will
they act too fast and open during normal surge loads?
Thanks in advance!
William Miller
Troy,
Overcurrent device size is matched to the conductor size. The
inverse time constant nature of an overcurrent device can
typically handle the surge currents as long as conductor sizing
has truly been done correctly for the conductor. Circuit
breakers are preferred to fuses because they can be reset.
There has been volumes written on this issue. The constant
current at lowest battery voltage should be used, plus the ac
ripple content on the battery circuit. This is usually a much
larger conductor than your average designer will plan for. The
best thing is to look at Midnight, Outback, and Schneider and
see what size overcurrent devices they require for their
products. That will give you a good clue as to how to size the
conductor and overcurrent device.
Bill.
*From:* re-wrenches-boun...@lists.re-wrenches.org
<javascript:_e({}, 'cvml',
're-wrenches-boun...@lists.re-wrenches.org');>
[mailto:re-wrenches-boun...@lists.re-wrenches.org
<javascript:_e({}, 'cvml',
're-wrenches-boun...@lists.re-wrenches.org');>] *On Behalf Of
*Troy Harvey
*Sent:* Friday, April 05, 2013 3:38 PM
*To:* RE-wrenches
*Subject:* [RE-wrenches] Fuse sizing in battery circuits
I've got a question about battery string fusing. Typically we
size the wire from the batteries to the inverter based on
continuous rating procedures (max power/efficiency)*125%.
However a 6kW inverter, can peak at 12kW for 5-10 seconds,
doubling the source current. That is no big deal for the wire,
because it is a short time frame... little heat will be
generated. However, in fusing the sub-strings, you need to
account for that peak surge current so you don't blow fuses all
the time. But if you put a 500-1000 amp fuse on a 4/0 wire,
above the max surge draw of the inverter, the wire will be
under-protected for its ampacity rating. Any thoughts on the
catch-22?
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