Electric tractors, fuses, and the concept of AIR

["Christopher Zach" <czach@computer.org>]

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
>
>
>