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

[Seth <sethea@mediaone.net>]

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:
> 
> No, 25kAmps is the recommended AIR (Amp interrupt rating) for a fuse.
> 
> Bit about the concept of AIR: All fuses blow when the current rating is
> exceeded. The problem with DC circuits is that if you open a shorted
> circuit, an arc tends to form at the point of breakage as the air is
> ionized. In a simple AC type or cheap-o DC fuse, this creates an arc which
> continues to conduct current and (in extreme cases) a neat thing called a
> "plasma ball" (check with Plasma boy for details).
> 
> AC circuits tend not to have this problem mainly because on a 60 cycle
> system the voltage crosses the zero line on a frequent basis, and the arc
> tends to extinguish. On DC circuits however you really need a fuse that can
> extinguish the arc as well as open the circuit. This is sometimes
> accomplished by packing the fuse in sand or sillica, which turns to glass
> when an arc starts to form. The glass then puts out the arc.
> 
> This BTW is why you can *NEVER NEVER NEVER* use an AC rated fuse on a DC
> battery circuit, and why you must *ALWAYS* have a DC fuse with a suitable
> AIR rating in your circuit. AC only rated fuses will open, but the points
> inside the fuse will arc and *NOT* protect your circuit. AC rated fuses will
> light up much like an arc lamp and will also NOT protect your circuit.
> 
> 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. Trace
> makes a 25k AIR breaker, but they cost a good bit.
> 
> Note: Some people at this point might be thinking "OK, why are car fuses so
> wimpy and can I use them?" The answer is yes, they are wimpy, and NO you
> should not use them as crash fuses. Reason why is because one must remember
> that the car wiring system inherently has high resistance (relatively
> speaking). Thus in the event of a short, the max current that will flow is
> something on the order of
> 
> I=E/R or I=12/.2 ohms or 60 amps. The natural resistance in the car's
> cheap-o wires and frame serve as a limit to how much current can flow (note
> the starter is usually wired with thick wires, and is thus protected by a
> fusable link which provides the AIR protection. But I digress)
> 
> 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.
> 
> Thus my question. If the Elec-Trac battery core ever shorts, one can expect
> a melted mess at best; exploded batteries at worst. On my shed, the solution
> was to put a 200amp 25,000 AIR fuse between the two batteries. Thus if the
> panel was ever shorted, the fuse would blow, seperate the batteries, and
> open the circuit. If I drop a tool directly on the battery pile, at most one
> battery will dead-short, and the damage will be limited to one battery
> exploding and a melted tool (BTDT). That's life, but limited by the fact
> that it's only one battery that gets turned into goo.
> 
> Chris
> 
> PS: If you're building a DC shed like I did, get the NEC code, and *read
> it*. Then follow it. All this and more is in there. And although one doesn't
> have to build electric autos to NEC specs, it's not really a bad idea to do
> so IMHO.
> 
> PPSS: This is also why you should always use DC rated switches in DC
> circuits. AC rated switches (even if rated at the amps) usually fail due to
> the arc that forms when you switch things off. Eventually the switch
> internals fuse, and the switch doesn't switch anymore (BTDT).
> 
> ----- Original Message -----
> From: "Seth" <sethea@mediaone.net>
> To: <ev@listproc.sjsu.edu>
> Sent: Monday, January 14, 2002 9:57 PM
> Subject: Re: Going to pick up the ET on Thursday
> 
> > If by 25,000A you mean the short circuit ability of your pile, then I
> > would suggest something like a bussman FWH semiconductor 200A fuse. The
> > voltage rating is more than you need, and maybe you can get a cheaper
> > fuse (but I don't know the bussman designation offhand for a 250ish volt
> > AC/DC fuse) with bolt holes at either end for lugged cable.
> >
> > -Seth
> >
> > Christopher Zach wrote:
> > >
> > > Well, it looks like I am going up Thursday morning to pick up my ET20.
> On
> > > Sunday I went over to a fellow ET person who actually had four tractors
> (3
> > > Wheel Horse and one ET15). Being able to take a physical look (my first)
> at
> > > an ET makes me feel a *lot* more comfortable about what I am getting
> into.
> > >
> > > First: These things are built *solid*. Compared to my 96 Craftsman 16hp,
> > > they are tanks. Much heavier metal, more solid looking overall.
> > >
> > > Second: They're not *really* that big physically. A bit fatter maybe,
> but
> > > about the same size. The short wheelbase should make it a lot more
> > > manuverable than the Craftsman.
> > >
> > > Third: The motor *really* is small. I was expecting something on the
> order
> > > of a OHV ICE motor, not a little thing tucked under the seat. It's
> possible
> > > that I might be able to convert my tractor to a electric motor of that
> size
> > > if I can figure out a way to mount it and fit three 12 volt batteries in
> > > there...
> > >
> > > Fourth: The charger appears to be quite simple. And it doesn't look like
> it
> > > will be too difficult to add an additional quick-lock port for the solar
> > > input. I'd say a 10 amp fuse should be more than enough (with a 100amp
> crash
> > > fuse on the shed side in the event something serious gets shorted).
> > >
> > > Question: Is there any sort of a catastrophic fuse in the battery
> circuit of
> > > the E20? I'm thinking something along the line of a 48 volt 200amp DC
> rated
> > > UL fuse to about 25,000amps? 36 volts is probably enough to produce a
> plasma
> > > ball if there was ever a dead short, this would be bad. If there is a
> cat
> > > fuse, then I can protect the panels with a simple 10amp DC fuse or
> breaker.
> > >
> > > Thanks!
> > > Chris
> >
> >
> >