Electrical fuse question

[48pan]

I've heard of people putting a fuse between the neg side of the battery and the frame. What's the story behind that and how big of a fuse would you use on a "76 FXE?

[Lew]

I've not heard of that.  I removed the stock main circuit breaker and installed a fuse under the seat of my shovel though.  That main circuit breaker hanging out in the rain and elements can cause problems without warning.

Lew

[Mule]

 The fuse or circuit breaker is on the positive side of the battery. It is a fail safe measure! If any wires short to ground under the tanks, in the dash or headlight etc, it will blow before the wiring harness melts.   Mule...

[48pan]

Hey Mule

[rbonner]

On a kick start only bike it wouldn't be a problem; it's just not standard practice and serves NO useful purpose.  The new owner of the bike would blow the fuse and post on HTT and drive us all crazy for three months until he found the fuse on the negative lead.

ON an electric start bike it would prolly need a 300 A fuse.

BOB

[76shuvlinoff]

........one of these days I need to put that 30amp breaker back on the ignition power supply that's on the schematics. It wasn't there when I bought the bike and I have never gotten around to installing one. 

 

[MMOCGuy]

I agree with Bob.

A fuse/circuit breaker on the negative terminal serves no useful purpose whatsoever other than to drive one crazy trying to figure out what is wrong if the dang thing fails. There is nothing that can happen that a fuse on the negative terminal would protect. the only thing that would cause it blow would be the electric starter and it would take about a 300 amp fuse to handle the electric starter.

Hope this helps.

Norm.

[twincamzz]

I'll be the guy that begs to differ. Recently wired up my '61 Pan & had fuses in all the usual spots ( headlight, tail light, brake light & generator return to battery), 30 Amp main breaker to ignition switch. Thought everything was covered until I got a short between the  generator fuse & the battery which would never blow as the short was on the wrong side of fuse. Burned the ground wire completely in half before all the smoke came out. lol.
Now I have a 20 Amp fuse on the ground side & feel that if I ever get a short in an odd location on the positive side the ground fuse will blow before I burn anything else up.

That's my story & I'm stickin' to it...

[rbonner]

Geez Twin, it still shouldn't matter.  You just didn't have protection on the positive side of the generator.  You need a path between GROUND and the HOT to form a short.  They just don't happen with one leg.

But that's fine, you can put the fuses on any side of the circuit...  If you ever sell your prized pan make sure you let the guy know where that fuse is at and what it does.

OH, you wanna see the dreaded smoke roll out of your Shovelhead...  Sheesh, my 78 1/2...  The primary drive of the starter bound up and the Whole wiring harness for the starter started poofing out white smoke.  I threw out the kicker and started cranking it over until the drive freed itself...  Melted the solenoid really good.

I suppose a 400A fuse wouldn't have held that nightmare in...     BOB

[twincamzz]

Geez Twin, it still shouldn't matter.  You just didn't have protection on the positive side of the generator.

Yes I did. I had a 20 Amp fuse between the return wire ( positive side ) of the generator & the battery. Short happened between the fuse & battery in my case, rendering the fuse useless. ( soldered connection poked thru shrinkwrap causing short ) Had the short occured between the generator & fuse the fuse woulda did it's job. The way I see it, if this were to happen again the fuse on the ground wire will blow before wiring gets burned up.

[Lew]

Sounds like the fuse was in the wrong spot.  If you want to protect a circuit with a fuse or circuit breaker you need to locate it at the beginning of the circuit.  Placing a fuse in the negative side is not the way to do it.

Lew

[rbonner]

Oh yeah, sure any unfused portion of wire is at risk.  I see exactly where your's shorted.

Now here's horror story for all of you...  I'm really good at telling stories about stuff that's happened.  With turbine airplanes we use NiCad batteries instead of lead acid.  NiCad's are prone to shorting INTERNALY when you aren't looking.  "Can't fuse for that!!!"  The Aircraft manufacturers place tempurature sensors on the batteries.  I've flown several different turbine planes, many have two levels of sensors on the NiCads.

If the first level was to go into alarm we first had to stop Charging the thing.  Turn off the Alternators and unload all the items off Buss B.  Just the opposite of Spock "Cross Circuiting to B" in a Star Trek emergency.  We had to wait several minutes to see if the alarm would go off and then we could try putting the alternator back on.

If the battery got damaged and went into thermal runaway...  Yes this is just like a phasor in overload...  A second high temp alarm would go into alarm...  If we ever got the second alarm we were supposed to Get the aircraft on the ground as soon as possible as it was going to be ON FIRE any second.

This also included crashing the plane.  These kind of batteries are really dangerous, especially ones that weigh a couple hundred pounds..  Ever feel how hot your cell phone gets when charging?  Now picture a 200 pound cell phone.

At the end of runway 26 at my home airport in Anoka County, MN there is a crator.  It's a small moon sized jobby maybe 75 feet across.  It was created one morning by a Sabreliner coming out of MSP International.  The NTSB determined it to ba a NiCad explosion that blew the tail off...  the engines at the back of the A/C were 8' below the surface when they dug the plane out.  My old timer buddies heard a sonic boom before the crash noise.

I thought how often do these things actually go into thermal runaway?  CAn't be very often.  Then one day while flying the University of Minnesota king air; My buddy had the pilot's job for Uof M..  There came a NiCad warning.  He did the emergency proceedure but was only 15 minutes out of anoka.  He got home and told his boss, he say...  Get out there and pull the battery!  $2million dollar plane...  They sat it on a 2x4 in the middle of the hangar then next morning there was only ashes...  Who moved the battery?  HA what battery.

A Harley electrical system isn't much different, except we don't use NiCads, also as close to the ground we are already crashed before the electrical failure.

Don't feel very safe on your airline flight next time on the way to grandma's them things are dangerous.

BOB

[FSG]

Sounds like the fuse was in the wrong spot.  If you want to protect a circuit with a fuse or circuit breaker you need to locate it at the beginning of the circuit.  Placing a fuse in the negative side is not the way to do it.

Lew

That all depends on what your trying to achieve, there are times when fusing the negative is the most logical method.

[Lew]

This is true.......
Standard electrical practice places the protection at the beginning of the circuit on the positive leg. Technically circuit protection could be installed on the negative leg and it would work fine but it will most likely cause (more) confusion to someone trying to work on the circuit in the future.  The poor guy is probably already confused as is.....
Typically the negative lead is not fused unless it is some type of sensitive electronic circuit or a positive ground system.

Lew  

[twincamzz]

Sounds like the fuse was in the wrong spot.  If you want to protect a circuit with a fuse or circuit breaker you need to locate it at the beginning of the circuit.  Placing a fuse in the negative side is not the way to do it.

Lew

I had wired an in-line 20 Amp fuse in the wire that runs from my generator to the battery ( at the suggestion of Cycle Electric as I am using their 12V  gen/ reg combination on my '61 Panhead ) with the last soldered connection approximately 3" from the positive terminal on the battery. Unfortunately for me about 7 or 8 miles into the ride, I let the smoke out of the old girl. Seems a spot of solder managed to poke through the shrink wrap on that last connection between the fuse & the battery & shorted out on the battery box/ oil tank. Just my luck. Not certain why the fuse did not blow... the ground wire burnt in half instead. It happened quick, too. The charging system works fine , so thankfully no real damage was done.  I simply added a 20Amp in-line fuse on the negative cable for the battery. It would be hard to miss if one were doing any diagnostic work as the wire is only about 6-8" long with an in-line fuse installed. Besides, my son knows where the fuse is located & one of these days she will be his. So no problems there.

** I got the ground wire fuse suggestion from a very good electrician I know. His logic was that since the 20 Amp fuse in the negative connection was protecting the entire circuit that no matter where the short would happen to occur that the ground fuse should blow, regardless. He said it's done in race cars all the time, I dunno. Made sense to me anyways. But then again, I'm not an electrician, I'm a millwright. lol.