[RE-wrenches] Fuse sizing in battery circuits
Kent Osterberg
kent at coveoregon.com
Tue Apr 9 08:35:55 PDT 2013
Bill,
I have all of those old HP issues in my library and once in a while they
are handy. Thanks for prompting me to take a look at this one again. It
should be an eye-opener for folks that the 5000 AIC rated breakers were
pretty much a failure at opening a shorted set of four golf-cart
batteries at 12 volts. This article makes me wonder how the small DIN
rail mounted CBI breakers and small panel mount breakers used by Midnite
and Outback would do. I fear not very well, and yet they are connected
directly to the battery in thousands of systems.
Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com
On 4/9/2013 7:00 AM, William Dorsett wrote:
>
> This topic comes up often enough that we ought to mention a yellowed
> article in Home Power, (1992 Issue 27, pg 26). Christopher Freitas
> wrote /Overcurrent Protection for Battery-Powered Systems /where he
> describes experiments he did back when he was with Ananda. He put a
> 2000 A Big Switch to initiate a short (4/0 cable) between the
> terminals on a set of four golf cart batteries (2 strings @ 12V). In
> series, he put in a 500A Shunt so he could measure current passing and
> various fuses and breakers. "For comparison, we decided to directly
> short the battery...the meter read 6960 amps peak current (three
> seconds) ...during each test the 4/0 cable lifted off the ground 4
> inches into the air by the forces generated by the extremely high
> current.." They videoed the 250 A ANN buss fuses arc and smoke; the
> 200A Heinemann Series AM breakers (paralleled ones that maybe Roy
> mentioned) went 3 seconds without breaking and the video showed a
> flash and blue smoke.
>
> 175A ITE breaker with 42,000 AIC "simply tripped...but still allowed a
> peak current of 2960 amps
>
> 200A Class T Littlefuse "opened promptly with no external signs of
> stress...1920 amps peak current"
>
> Christopher's recommendations:
>
> "Every AE system must have overcurrent protection able to interrupt
> the maximum current available from the batteries. For most systems,
> the main protection should use current limiting high AIC fuses, such
> as a Class T or Class R. A disconnect switch which allows the fuse to
> be safely changed should be included. A lower cost alternative is to
> mount the fuse in a fuse holder without a disconnect. Although the
> fuse would always be electrically hot, it normally would not be
> changed during the life of the system. The fuse holder should be
> mounted outside the battery enclosure. Fuses should not be bolted
> directly onto the battery terminal, as they are not designed to handle
> the physical stresses that can occur without the protection of a fuse
> holder.
>
> Fuses which have exposed elements, such as ANN fuses, should not be
> used because they are not current limiting and have only 2500 amps
> AIC. They also may be a significant hazard when installed near batteries.
>
> High AIC breakers, like the Heinemann Series CF (25,000 Amps AIC @
> 65VDC) can provide overcurrent protection for individual items. They
> cannot be used to protect lower AIC breakers. This eliminates their
> use as a main disconnect in most systems.
>
> Low AIC breakers, like the Heinemann Series AM (5000A AIC @65 VDC) or
> the Square-D QO (5000A @ 125 VDC) can be used in load distribution
> centers and components, but must be protected by a current limiting
> fuse. Using low-AIC breakers alone will not provide sufficient
> protection with a battery system and may be a significant hazard
> during short circuit situations."
>
> It won't pull up anymore on HP's article search but probably Michael
> has a copy he could post for those interested.//
>
> Bill Dorsett
>
> Manhattan, KS
>
> *From:*re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *John
> Berdner
> *Sent:* Monday, April 08, 2013 7:25 PM
> *To:* Allan at positiveenergysolar.com; RE-wrenches
> *Subject:* Re: [RE-wrenches] Fuse sizing in battery circuits
>
> Allan:
>
> A pretty accurate retention of the Grass Valley / Nevada County
> through 2000.
>
> You only missed the Trace to Xantrex "acquisition" that was the start
> of the divorce J
>
> Class T fuses provide better protection for battery based inverters --
> always been that way.
>
> The move to breakers was a cost reduction issues when Trace introduced
> the DC-250.
>
> I still contend that the way the DC-250 did it was a Code violation
> when used with PV in the box (current from all sources issue) but that
> is history.
>
> For high current applications the Boltswitch Pull outs (the ones we
> used in the Power Centers) came in 100A, 200A and 400 A sizes and 1,
> 2, and 3 pole versions.
>
> I think they are still made today.
>
> Still the best way to do currents above 250 Amps.
>
> You can do a Eaton / GJ1P breaker up to 750 Amps but they are Spin Dee.
>
> As you noted you can use a single pull out to do up to 3 battery strings.
>
> You make a copper "comb" to parallel separate fused inputs to a single
> inverter.
>
> If you have more than one inverter you can have "back to back" fuse
> blocks with an H-H-H shaped bus bar set up.
>
> The biggest one I recall we did was a 12 400 A battery/PV inputs
> going to 8 SW inverters.
>
> You size the copper bus bars for 1000 A / square inch of cross section
> and you are good to go (this is from the UL 508 design guide).
>
> Best Regards,
>
> John Berdner
>
> General Manager, North America
>
> SolarEdge Technologies, Inc.
>
> 3347 Gateway Boulevard, Fremont CA 94538 USA */(*Please note of our
> new address.)/*
> T: 510.498.3200, X 747
>
> M: 530.277.4894
>
> *From:*re-wrenches-bounces at lists.re-wrenches.org
> <mailto:re-wrenches-bounces at lists.re-wrenches.org>
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of
> *Allan Sindelar
> *Sent:* Friday, April 05, 2013 9:00 PM
> *To:* RE-wrenches
> *Subject:* Re: [RE-wrenches] Fuse sizing in battery circuits
>
> William,
> For this I went deep into the catacombs of our hardware file cabinet,
> and quickly found a 1996 spec sheet from an Ananda Power Technologies
> SafeT-Block. For the young'uns, Ananda was a quality company, the
> originator of the listed Powercenter - the breaker and terminal block
> hardware that is part of every batttery-based system nowadays. Ananda
> became APT, then Pulse, then part of Trace (the PC250 and PC500) as
> part of a short-lived marriage followed by a messy divorce, the
> results of which led, as an act of revenge insurrection, to the
> introduction into the U.S. market of the Sunny Boy and AC-coupled
> systems, and the rest is even more wacky history. But I digress, after
> a run-on sentence bad enough that my high school English teacher is
> rolling over in her grave...
>
> I have attached a scan of page two of the spec sheet. In the upper
> left corner is a chart of current vs. time for a Class T fuse. Once I
> understood this chart I never again worried about nuisance tripping,
> as whether described as fast-acting or not, Class T fuses have an
> inherent surge capacity of about 2 1/2 times their rating, and thus
> can handle surges.
>
> If you put them on individual battery strings, the point is to prevent
> overcurrent on one string, which is only likely to occur when either
> 1) a terminal is corroded sufficiently that one string fails to carry
> its share of the load or charge (we have seen this) or 2) one string
> fails when the batteries reach end-of-life. When the Class T fuses
> blows in that situation it has done its job.
> Allan
>
> *Allan Sindelar*
> Allan at positiveenergysolar.com <mailto:Allan at positiveenergysolar.com>
> NABCEP Certified Photovoltaic Installer
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder and Chief Technology Officer
> *Positive Energy, Inc.*
> 3209 Richards Lane (note new address)
> Santa Fe, New Mexico 87507
> *505 424-1112*
> www.positiveenergysolar.com <http://www.positiveenergysolar.com/>
>
>
> 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-bounces at lists.re-wrenches.org
> <mailto:re-wrenches-bounces at lists.re-wrenches.org>
> [mailto:re-wrenches-bounces at 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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