[RE-wrenches] Fuse sizing in battery circuits

Tue Apr 9 07:00:04 PDT 2013

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] 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
 <mailto:Allan at positiveenergysolar.com> 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] 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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