[RE-wrenches] Current limiting fuse at battery bank

[Ray Walters]

I agree installing a fuse at the battery bank is a good idea. However 
actually doing it right is tough.  Putting the  class T fuse and holder 
in a battery enclosure with flooded cells is going to cause more trouble 
than it solves.  I've seen class T fuses oozing green goo out the ends, 
and the fuse holder contacts get corroded to the point that they are 
causing significant volt drop.
If it is installed outside, then you need an enclosure.  Then there is 
the requirement that fuses be serviceable, requiring disconnects on both 
sides.  The Boltswitch pullout system solves that problem, but then you 
have to custom fabricate an enclosure for the Boltswitch disconnect.
End of story, is we aren't usually ever installing that fuse at the 
battery.   Instead we keep the distance from the DC Load center to the 
battery as close as possible.  The number one danger is a wrench falling 
across the battery terminals, and neither the breaker or a class T fuse 
at the main terminal is going to stop that.  We would need fusing at 
each exposed positive terminal of each battery to truly protect against 
short circuits.

R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760

On 10/22/2015 7:26 PM, Tom Ruscitti wrote:
> On battery based systems I always install a Class T fuse as close to 
> the battery bank as possible. 690.71(C) is a little vague in requiring 
> a current limiting fuse only where the available short-circuit current 
> from the battery bank exceeds the interrupting ratings of the inverter 
> breaker. Inverter breakers vary in their interrupting ratings and 
> battery banks are big current sources when things go wrong, so it's 
> just a sensible best practice. My question is about service work on 
> systems we didn't install. We're quoting the second job this year for 
> battery replacement where there is no current limiting fuse installed. 
> It's always a tough question to judge what extra work might be 
> required when you touch another contractor's system, but does anyone 
> have an opinion about recommending or requiring the installation of a 
> fuse at the battery box as part of the battery replacement?
>
> Thanks,
>
> - Tom
>