RE: Electric tractors, fuses, and the concept of AIR

["Roger Stockton" <rstockton@delta-q.com>]

This discussion neglects the battery's internal resistance and greatly
over-estimates the potential short circuit current.  While you can't go
wrong with a 25kA interrupt rating, it is highly unlikely that you need
even a 10kA interrupt rating.

Uve's battery page
(http://www.geocities.com/CapeCanaveral/Lab/8679/battery.html) lists the
popular T105/125/145 6V floodeds at about 0.004 ohms each.  The internal
resistance alone limits the short circuit current of the battery to
1500A.  A 24V string of such batteries will have at least 2 external
interconnects if the 3rd interconnect is replaced by an appropriate
fuse.  These interconnects will typically add at least another 0.001
ohms to the total series resistance (most of the interconnect resistance
is in the terminals/connections, not the wire).  A dead short of this
pack results in a maximum of 24V/0.017 = ~1412A, assuming the
fuse/circuit breaker adds 0 resistance, which of course it doesn't.

The Square-D QO series of household breakers includes a 70A model that
is UL Listed up to 48VDC and has an interrupt rating of 5kA at 48VDC.
The 70A breaker will carry 200A+ for a few 10s of seconds and offers
very useful protection for packs that are not required to carry large
loads for long periods.  The breaker is *much* cheaper than a comparably
rated DC fuse and will survive more than one "oops".

Cheers,

Roger.

> -----Original Message-----
> From: owner-ev@listproc.sjsu.edu 
> [mailto:owner-ev@listproc.sjsu.edu] On Behalf Of Seth
> Sent: January 15, 2002 4:24 AM
> To: ev@listproc.sjsu.edu
> Subject: Re: Electric tractors, fuses, and the concept of AIR
> 
> 
> Oh, the bussmann FWH fuses are solid, sand filled. So when 
> they blow, you get a really good insulator to fill the gap 
> where the conductor was. (like the type you were referring 
> to) And they are rated for DC as well as AC. I don't know 
> what the AIR rating for them is. 
> 
> http://www.bussmann.com/library/docs/NAmer_Cat.pdf has 
> mechanical and electrical specs.
> 
> I am not a fuse engineer though...
> 
> 
> -Seth 
> 
> Christopher Zach wrote:

[snip]

> > There are a few DC breakers on the market, however most of 
> > them only have an AIR rating of a few thousand amps. Always
> > keep in mind that a shorted 225 amp T105 battery can produce
> > far more than 225 amps when shorted.

[snip]

> > Our circuits are designed with things like #2, 0, 00, 0000 
> > wire which has a resistance of like zero ohms. Thus the max
> > current that can be pulled is something like
> > 
> > 24/.001= 24,000 amps. Whoops.