Fuses vs. Breakers (was QO Breakers and Panels)[RE-wrenches]

[Travis Creswell, Ozark Solar]

 Hi Jerry,

I don't have any documentation on the various potential short circuit
amperages of various batteries but I bet someone on this list  does.  I'll
still make an attempt at an answer though.  I'm guessing that a T-105 is
capable of  well over a 1000 amps.

So I would assume that 4 T-105's @ 12 volts could deliver enough current
during a fault to destroy a QO breaker before it tripped which means that
the fault would continue until the battery cable burned in two, probably
doing a lot more damage on the way. Of course, as you parallel batteries the
potential current goes up and I believe there is discussion for the future
revisions of code requiring fuses in-between each paralleled bank.  Also,
the shorter the run and the larger the battery cables the harder time a
breaker would have dealing with a fault.

So to answer your question simply which is the only way I can.  It's
probably always a good idea to have a fuse before the breaker and as close
to the battery as possible regardless of the size of the battery.   But what
makes the best design and safety sense doesn't meet code.  Technically a
fuse is "serviceable" and I believe code doesn't want you to have any
serviceable equipment in the battery box.  I'm pretty gray on that so don't
take my word for it.

Travis Creswell
Ozark Solar

----- Original Message -----
From: "Jerry Caldwell" <solarcowboy at yahoo.com>
To: <RE-wrenches at topica.com>
Sent: Sunday, March 11, 2001 3:30 PM
Subject: Re: Fuses vs. Breakers (was QO Breakers and Panels)[RE-wrenches]


> And what is a good rule of thumb for deciding when the
> battery is large enough to require fusing?  Most of
> the smaller grid tied systems I've installed have not
> had fuses on the battery as have the systems with
> larger battery banks.
>
> Jerry Caldwell


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