Ed,
I said, one of the ways to solve this, would be to put a battery on a relay.
Then you don't have 10 feet of
unprotected wire.
Also, there are many ways to protect the system,
but I was talking about a low cost resetable system.
DouglasK
----- Original Message -----
From: Ed Price
To: amphicar-lovers@yahoogroups.com
Sent: Monday, October 28, 2002 2:48 AM
Subject: Re: [amphicar-lovers] Main harness fuse and fires and suggestions
Douglas:
Sorry, but putting your protection device in the dash means that you have a
minimum of about 10 feet of unprotected wire between the battery terminal and
the protection device. Nothing limits the current in this section, except for
the capacity of the battery. We need to protect as close as possible to the
battery.
[I feel that there is little need for fusing on the generator / alternator
output. In an aircraft, the alternator is attached to the engine(s), and runs
whenever the engine runs. The alternator(s) provide primary power for all
on-board avionics, and can source considerable current. Most aircraft don't
carry heavy batteries; it's not cost-effective. The alternator(s) become the
"battery" of a flying aircraft. You may have noticed that most heavy aircraft
need a ground power cart to start their engines, or may use an on-board
auxiliary power unit (APU) as a backup. Anyway, the point is, when you are
flying, you don't want a wiring fault to cause you to stop flying somewhat
earlier than you planned, so you fuse the alternator so that the engine doesn't
need to be shut down. In an Amphi, the engine could be shut down with far less
grievous consequences than if you were about 30,000 feet in the air.]
Let's take our lead from automotive technology. This problem has already been
addressed in automotive design. The possibility of a power bus fault (short
circuit) is rare; a lot of thought goes into the design of an automotive wiring
system. And, a lot has been learned since 1960. <g> However, vehicle collisions
have a way of creating electrical faults. And almost every Amphi has been the
subject of amateur rebuilding, repair and modification.
So, what do the professional car-builders do? They use a little gadget called
the "fusible link." This device is a one-time fuse that is wired in series with
the protected circuit (as close to the current source as possible). The fusible
link (FL) contains a wire, critically sized to melt at a certain current level.
This will open the circuit, just like a fuse does. The wire is insulated with a
very tough plastic jacket that normally protects the wire and contains the
molten metal if the link ever needs to protect. A fusible link looks like a
normal piece of wire, except that it will be a bit thicker and may have some
identification printed on it. (I'm not certain, but fusible links may have to be
red or orange colored.) The fusible link will usually have lug terminals to
match the vehicle cable harness design, but generic ones are available with
simple ring terminals. An FL is cheap, simple, and more reliable than a fuse
(where you need to worry about contact corrosion and shielding the terminals
from accidental contact). The down side is that, once it "protects", it kills
itself, and needs to be replaced. And replacement is much more involved than
simply popping in a new fuse. (However, some believe that protective devices
should NOT be easy to reset. A fault is a symptom of a system problem, and
sticking in a new fuse doesn't fix the real problem. But that discussion will
take us all night!) An FL that has operated, that is, melted open, often looks
pretty bad on the outside. It should still be intact, but it will be mottled and
may have a few burn holes where hot gasses vented.
Now, what should we try to protect? If we simply put an FL in series with the
battery cable, then we have to size it large enough to carry the starter
current. That's a couple of hundred amps (for a few seconds). If we size it for
this load, we will not have enough "sensitivity" to protect against a fifty to
one hundred amp wiring fault current. So, typical design practice simply doesn't
protect the starting circuit. A heavy conductor runs from the battery post, to
the starter relay, and then to the starter terminal. (Remember that the starting
current then flows through the starter housing, into the engine block, through
the engine ground strap, and back through the frame (hull) to the "grounded"
battery terminal.)
The FL connects to the "hot" battery terminal to a power distribution point
(typically, a threaded stud on a plastic block insulator). I have seen some
battery cables that have two cables molded into the female battery clamp; one is
a huge wire (for the relay/starter circuit), and the other is a smaller (about
the size of a #10 household wire) and has a connector that mates with the FL.
In an Amphi, I would run a single heavy battery cable from the "hot" battery
terminal to the starter relay. I would attach a ring lug to the same starter
relay terminal, and connect the FL there. The other end of the FL would be
connected to another insulated stud terminal (you add this yourself) that would
supply all other Amphi loads (switched and unswitched). This arrangement would
not protect the starting circuit, but would protect all other circuits.
Subsequent branch circuits should all be individually fused, with fuse ratings
only a bit higher than expected current. (This is the job of the existing fuse
block, which provides for fusing of various circuits at the safest level. The
fuse block concept also serves to isolate a fault from the other vehicle
systems, so that fault in say, the wiper motor, doesn't shut off the headlights
and ignition.)
The rating of the FL should be sized to just allow all normal electrical
loads. Since an Amphi doesn't have exotic electrical gadgets like a compressor
clutch or active ride control, the current rating should be about 30 amps. (This
would allow for high headlights, wipers on, fan blower on, bilge pumps running,
cigarette lighter pushed in, marine lights on and stoplight on. Unlikely, but
possible.) Also remember that the FL has to carry the generator/alternator
charging current back into the battery. A dead battery will take as much charge
as the gen/alt can source. The original generator can source about 20 amps, but
you may have replaced it with an alternator that will have a higher capacity.
Still, a charging current of greater than 30 amps for more than a few seconds
would be rare (and maybe a sign of an internally shorted battery, in which case,
the FL should be expected to blow).
One other thing to remember about your electrical system is that the battery
typically supplies only the starting current; once the engine is running, all
electrical loads (and battery charging current) come from the gen/alt.
Automotive batteries are designed to produce a high peak current for a short
duration (like a few hundred amps for several seconds during cranking). Although
the current is high, the duration is short, so the total power is not really all
that much. The gen/alt replaces that power into the battery at a much lower
current over a long period of time. For instance, power removed from the battery
at 200 amps for 5 seconds can be replaced by a 5 amp current with a duration of
200 seconds (OK, ignoring losses and some other teensie factors).
Sorry for the long discourse, but the electrical system has to be thought
about as a set of related subsystems, and what affects one part usually affects
all the other parts too.
Ed
El Cajon
67 Rust Guppy
----- Original Message -----
From: DouglasK
To: amphicar-lovers@yahoogroups.com
Sent: Tuesday, October 15, 2002 7:42 AM
Subject: Re: [amphicar-lovers] Main harness fuse and fires and suggestions
In aircraft, we call these current limiters.
There are two main ways to do that.
One is a resetable curcuit breaker on the dast.
Another is a fuse, that is installed in line with the main
generator side of the bus.
I would suggest to any amphi,
to use a resetable curcuit breaker on the dash,
connected to the coil of the battery relay, if you have one.
A resetable curcuit breaker also on the dash for the generator side of the
bus.
You would have two toggle switches on the dash.
Simple wiring job, and it would clearly increase the safety of the car.
These little cars are now quite the investment.
Installing two $25 breakers would clearly be worth the money.
DogulasK
----- Original Message -----
From: Dave Derer
To: amphicar-lovers@yahoogroups.com
Sent: Sunday, October 27, 2002 11:36 AM
Subject: [amphicar-lovers] Main harness fuse and fires and suggestions
Like the steering brace, it is time for ALL Amphi owners to install a
main fuse. There is no fuseable link or main fuse in an Amphi. I would
like some suggestions on amp rating for it and a design for it. Just
recently Joel from Florida almost lost his car due to fire while in the
water. His harness shorted inside the frame. Eric from Illinois: last
year main ignition wire cracked and went red hot. Steering braces
usually give a warning this type of trouble does not. Check your fire
extinguisher too. Later Dave the Wave
[Non-text portions of this message have been removed]
Yahoo! Groups Sponsor
ADVERTISEMENT
THE AMPHICAR-LOVERS LIST
~~~~~~~~~~~~~~~~~~~~~~~
Before posting requests for information, please check the List
Archives:
http://www.escribe.com/automotive/amphicar/search.html
For more information about this List and other available services
visit:
http://groups.yahoo.com/group/amphicar-lovers/
To UNsubscribe from this List, just send a blank email to:
amphicar-lovers-unsubscribe@yahoogroups.com
Any other issues may be addressed to the List owner (Mike Israel) at:
amphicar770@yahoo.com
Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
[Non-text portions of this message have been removed]
