[RE-wrenches] Current limiting fuse at battery bank

[Allan Sindelar]

Tom,
Agree with Ray here. But more than that, if you consider a class T fuse good practice, include it. But do no more than offer it when you work on others' systems. The homeowner usually won't see the value in the same way that you do, it won't affect the system 's performance unless there's a catastrophic failure, and you risk being seen as someone they don't know trying to pad a sale with added bells and whistles. 

I haven't checked in recent years, but the original Heinemann GJ breakers in 175 and 250A ratings had 10,000A AIC, which is difficult to achieve with normal cable and terminal resistances. Yes, the class T fuse in a Boltswitch pull-out disconnect offers better AIC protection than a CB, but the industry went the CB route about 15 years ago. 

> On Oct 22, 2015, at 6:46 PM, Ray Walters <ray at solarray.com> wrote:
> 
> 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
> 
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