just an idle question

I presume on most bikes that have a battery and negative ground that the
main fuse, 10 amps or bigger, is always on the positive side, I guess to
help protect the battery from meltdown or explosion. Never having seen
a bike with positive ground and not having paid attention to father's
old A40 Austin, I'm guessing that even negative ground systems have a
main fuse on the positive side. Is this so?

(Maybe not a very important question, I admit, just something I keep
wondering about.)

 

No, positive ground systems put the fuse on the negative wire.

Les

 

Thanks for answering the question he THOUGHT he was asking !! ;-)

 

Thanks to Project Magnet, yes, but even if it makes me sound dumb, I was
really thinking that the fuse would be nearest the positive terminal in
both systems. My reason had to do with what I take to be the actual
direction of current, as opposed to the so-called conventional
direction. Perhaps it's simplistic but I reasoned that for protecting
components, it might be better to fuse them 'before', as it were, they
reach the current reaches them. (Physics is not my talent, but liking
bikes, I can't avoid electrical theory.)

 

A fuse on the grounded + side of the circuit won't protect against short
circuits on the - side.

Les

 

I read enough to know what he was asking, just not enough to read what
he was asking...

Les

 

No, current flow is essentially instantaneous, electrons/holes
moving around the whole circuit in concert. The individual charge carriers
don't move quickly but the current as a whole moves near to the speed of
light.

--
Ivan Reid, School of Engineering & Design, _____________ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD "You Porsche. Me pass!" DoD #484 JKLO#003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

Utterly ridiculous. Of course it would.
It takes a complete circuit for current to flow; doesn't make any difference
WHERE you open it.

 

That's why the fuse goes in backwards on positive grounded systems

These fuses don't protect the components, they protect the wiring.

Current is continuous all the way around. To answer without getting
long winded, for your example, it doesn't matter where the fuse is.

And you don't want to fuse the grounded side, as that just
leads to problems and lack of protection.

 

That makes a lot of sense to me (given your point about the fuse
protecting the wiring).

 

The Rule of Thumb in ALL electrical wiring is that -whatever- is connected
to "Earth" Ground is NEVER fused.

I realize on a motorcycle with rubber tires and no part of the frame
actually connected to a real Earth ground, that the concept of the frame
being at Earth potential is mainly theoretical. Also, the voltages involved
are so low that they will not kill you (ie: 12v hand to hand isn't going to
stop your heart) Thus, fuses and/or circuit breakers aren't considered
"safety" devices as such, at least, not safety devices for people.

However, the principles followed are the same

Telephone systems are also positive-grounded (ie: -48 volts DC) and
they actually do have their + side connected to a true Earth ground
(generally
a copper rod driven into the ground) Those also are NEVER fused on
the ground side.

AC power systems also follow the same rules - any Neutral line that
is connected to Earth ground is NEVER fused, it is -illegal- to fuse it.

Ted

 

Ted Mittelstaedt wrote:
....

Right, I should have thought of that in the first place. Thanks.

 

This is a hypothetical discussion, since it is virtually never done
but............

A fuse that removes the common/ground side of a circuit or circuits
completely from the power supply certainly would stop the current
flow.......completely. You cannot "short circuit" something that has no
current flowing in it because the voltage potential is the same at every
place in the "circuit".

In the case of DC vehicle systems, a very effective and recommended way to
prevent damage from an accidental short is to disconnect the ground lead
from the battery.......as if there was a fuse there that has operated.

Doesn't apply to earth ground systems like commercial AC because "you" have
no control over the power supply and it is always connected to ground.

AND.....back to the original premise.......a fuse doesn't "protect" against
short circuits regardless of where it is placed. It protects the wiring and
power supply from releasing THEIR smoke.....along with the component that
has already failed. A properly sized fuse WILL perform that function quite
nicely, regardless of where it is placed in the circuit.

 

change "virtually" to simply "never"

Unless you have a wire chafed somewhere, in which case your ground
is now reconnected and you have current flow. That's why we don't
fuse common/grounds, period, in ANY power system.

No, that is not true. The recommended way to prevent damage from
an accidental short is to disconnect BOTH battery terminals and REMOVE
the battery from the system.

Having a live battery in there will not prevent a short - all it takes is
absently setting a bolt on top of the tank with the seat removed, and
bumping the bike to have the bolt fall and wedge in between the battery
negative terminal and ground while you have the wiring hanging out
elsewhere on the bike.

Or for more fun, drop a screwdriver in there and short the battery
out alltogether.

The PRIMARY purpose of a fuse is safety of human life. The bike maker
doesen't give much of a crap if the wiring or component shorts and melts.
Wires are pretty easy to replace. What they DO care about is if the
component
shorts then gets so hot it melts and starts a fire. If that were to happen
while you
were flying down the road you might wreck and that's not very good for
sales,
you see. The secondary function is to protect the wiring and/or components.

And placing the fuse in the ground side of the circuit isn't going to
protect
anything since any component that has a supply lead going to it that fails,
can
potentially fail in a manner where the internal electric conductors short
against the chassis.

Ted

 

Some good points and some not so. Are you REALLY so dense that you don't
see that disconnecting the common side of the battery from the chassis
renders the entire "loop" DEAD......falling bolts and screwdrivers not
withstanding ?? In that case, a conductor shorted to the chassis is no more
likely to cause a problem than if the battery is completely absent.

 

The short circuit is, by definition, ANOTHER path to ground,
different from the normal path. Unless you remove the main battery
connection to ground, current can and will flow in an energized
circuit wherever insulation is parted and the conductor makes contact
with another conductor which is at ground potential.

 

Interesting that you quoted the section of my response dealing with
fusing grounds, yet replied to the issue of disconnecting the battery
common,
oh well.

If you HAD quoted the section you were responding to you would have
also read the part where I explained that if a bolt or screwdriver fell
and jammed against the negative post, it would energize the bike
again. Not a good thing when the mechanic is _assuming_ the bike
circuits are dead, and undoubtedly has the wires in a state of disassembly
with connectors laying everywhere and on everything.

The poster had said "the recommended procedure"

The "recommended procedure" is to remove the battery. That
doesen't mean we always do the recommended procedures,
naturally.

As long as the battery is in the motorcycle, unless you have covered
the terminals, there's a potential for something falling on it and
shorting the battery. This is where common sense is supposed
to kick in. Most people probably would not bother doing more than
disconnecting the battery common if they were simply replacing a
starter. Most people would probably remove the battery if they
were going to pull an engine. But, just because most people would
judge leaving a battery in the bike while replacing the starter as
OK if the negative was disconnected, does not mean it is completely
safe, nor does it mean that it's no more likely to cause a problem
than if the battery is absent.

Ted

 

The factory dealers make good money fixing this kind of problem. The
only reason the factories aren't just using straight wire instead of fuses
is
they are afriad of being sued. A straight wire is cheaper.

A lot of car electrical systems for a long time used non-fused connections
for the vehicle turn signals. My 68 Ford was like that. It saved the
vehicle manufacturers maybe 3 cents to not put a fused circuit in.
It cost me about $30 to have to buy a new steering column turn
signal switch when the old one melted because one of the turn signal
wires chafed through in the fender.

Trust me, the vehicle & bike manufacturers really honestly give a shit
about your pocketbook and safety about 2 millimeters further than their
lawyers
and the government tells them they have to. Do you really think they would
put in crumple zones, airbags, and the like if the government didn't
mandate it? Come off it. Airbags were around 2 decades before they
put them in cars, and they only did it because the government told them
they had to. And the idea that a motorcycle manufacturer even cares
as much as a car manufacturer is laughable. Otherwise they wouldn't
be selling 200HP bikes to newbies, or putting $600 worth of
useless plastic covers that shatter when the bike is leaned against
a fire hyrant. It's all about producing products that have obsolescence
built in.

Ted

 

No, it isn't. A short circuit is a connection between 2 points in the
device where there is not supposed to be a connection. It might involve
neutral/common/ground but doesn't have to.

Pay attention:

 

Do try to follow along, Ted. I commented on the part that I quoted.

What do you think happens when a fuse operates? Clue: It becomes OPEN and
breaks the connection. If that fuse is connected between the common side of
the battery and chassis ground, then the chassis is no longer a part of the
circuit. At that point, it doesn't make any difference what touches the
chassis.

What you say about the potential for accidentally re-connecting neg. to
ground is of course true but it would not be a problem unless the person
doing it is so brain dead that he ALSO simultaneously grounds the positive
terminal. Can it happen? Sure. Likely enough to mention every time the
subject comes up....not hardly.