Fuses

[Old Sport]

I am having a heated discussion with a friend about fuses. It is my contention that a 2 amp fuse in the power line to a GPS is there to protect the GPS. His contention is that fuse is to protect the wiring, he says the GPS has internal protection. Who is right?

 

[dirtrider]

Afternoon Old Sport

 

Fuses and circuit breakers are primarily intended & sized to protect the wiring, but basically protect the entire circuit should a short or overload occur in a device that is in that circuit.

 

[Old Sport]

So if the bike has a can-bus system you don't need the 2 amp fuse?

[dirtrider]

Afternoon Old Sport

 

Having a CanBus system has nothing to do with the circuit fusing. CanBus is just the way the onboard computers talk to each other (communication protocol) .

 

The actual fusing is controlled by electronic circuit breakers in the ZFE module.

 

In any case you didn't say what you are working with but the early 1200RT's & the R bikes were protected by 5 amp internal circuit breaker on the accessory plugs & the later RT's were protected by a 10 amp circuit protection.

 

Depending on the wire size you are using going to your GPS, that wire harness might only be rated for 2 or 3 amps. As an example the OEM Garmin harness wire size for the Zumo is pretty darn thin so I would suggest an inline fuse if using a factory Garmin Zumo harness.

 

Bottom line-- if your GPS came with a wire harness & that harness came with a 2 amp fuse I would find a way to use it if possible.

 

[Old Sport]

Thanks.

[Ken H.]

Who is right?

Your friend is.

 

Fuses (or other current interrupting devices) whether they be in bike, a house, a power line or most anywhere else are always always sized to protect the wiring (and wiring related components).

 

Because the current capacity of the device on the other end, be it a GPS on a bike, or the toaster in your kitchen, is unknown when the item (bike, house, car, power station) is designed and built. Over current protection of the device (e.g. a GPS, a big screen TV, whatever) is the responsibility of the manufacture of the device. Only they know what it can or cannot take. And only they can make a design decision on how (or if) to protect it.

 

Another way to think about it is delineation of responsibility. I (BMW) am responsible to build the bike to protect itself, you (GARMIN) are responsible to build your GPS to protect itself. There’s no way I BMW can ever predict if your (GARMIN) GPS needs to be overcurrent protected for 2 amps, 3, or 10 when I design and build my bikes.

 

[Selden]

Protection — which is one reason for placing the fuse as close to the battery as possible. Let's assume you add a circuit with a 2-foot run of wire, and put a fuse at the output end. All two feet of the circuit are possible failure points if a wire has a fault. Assume you place the fuse 3" from the positive terminal of the battery, and only that much of the circuit is at risk.

[Old Sport]

Let me clarify my question. If Garmin gives you a wiring harness with a fuse and you are going to simply plug the harness into a Powerlet Port on a Can-Bus system, aka R1200GS powerlet port near speedo, can you not use the fuse and allow the Can-Bus system to determine if there is a short or other problem? Better yet, will the Can-Bus system powerlet port act as a fuse? Is the powerlet port even on the can-bus system? From my experience the more advanced and computer controlled a system is the more likely you are to screw it up by tapping into it.

[dirtrider]

Afternoon Old Sport

 

You can do that BUT! If Garmin gives you a wire harness with a lower amp fuse in it there is a good reason for that lower amp fuse.

 

Your GS could have a 5 amp or a 10 amp protected outlet. Some of those Garmin harnesses have very thin wires so could heat up like toaster wire & start a fire IF there is a short or low resistance issue in the wire harness or the GPS itself. That low amp added fuse hopefully prevents that.

 

The other thing you should keep in mind is if your GPS uses a cradle with brass pins in it. A low amp fuse hopefully prevents cradle damage if your GPS has issues or something metallic falls into the pin area.

 

Personally, I would include the Garmin fuse holder & low amp fuse in your GPS circuit but if you don't chances are no issues but there is always that remote chance of problems.

 

[GRB60]

In your example the bike will allow 10 amps to the GPS if you don't use the additional fuse. That may damage the GPS or its wiring before finally tripping the "breaker" of the ZFE when the short draws more than 10 amps. If you use the 2 amp fuse it will melt the fuse before damaging wires or the GPS.

 

Always better to use the smallest fuse possible for the intended load.

[John Bentall]

I am having a heated discussion with a friend about fuses. It is my contention that a 2 amp fuse in the power line to a GPS is there to protect the GPS. His contention is that fuse is to protect the wiring, he says the GPS has internal protection. Who is right?

 

Tell us the problem that prompted this heated discussion....

[Peter Parts]

In your example the bike will allow 10 amps to the GPS if you don't use the additional fuse. That may damage the GPS or its wiring before finally tripping the "breaker" of the ZFE when the short draws more than 10 amps. If you use the 2 amp fuse it will melt the fuse before damaging wires or the GPS.

 

Always better to use the smallest fuse possible for the intended load.

 

I agree with you although I think the "engineers" think the other way.

 

The fuse should "trip" in proportion to the kind of things on the line, and certainly not the worst-case total of big and small things. I try to wire-in in-line 3AG fuse holders wherever possible. That way you can use a big-fused circuit but still have a small-fused line to your GPS.

 

That's why I think it is nuts for BMW to put the horn and the clock on one very large fuse on some Oilheads. Rad has always been stupid about fuses (size, rust, location, ID....), although my personal experience is limited to the last 53 years.

 

Ben

[dirtrider]

clip---

 

The fuse should "trip" in proportion to the kind of things on the line, and certainly not the worst-case total of big and small things. I try to wire-in in-line 3AG fuse holders wherever possible. That way you can use a big-fused circuit but still have a small-fused line to your GPS.

 

That's why I think it is nuts for BMW to put the horn and the clock on one very large fuse on some Oilheads. ----clip

 

Ben

 

What's so stupid about it? It has worked for years with no OEM wiring related fires that I have ever heard of.

 

I don't understand the issue with how they (BMW) have it fused. The circuit is fused correctly for wire size protection. The cumulative load on that F-3 circuit is well below the fuse protection limit.

 

 

If something goes wrong in the wire harness (such as a short) the fuse will easily protect the wiring & circuit. Neither the clock or horn will blow the fuse alone or together.

There is no reason to protect either the horn or clock as a stand alone as anything that goes wrong in the circuit right up to the circuit entrance to the device is protected by that circuit's main fuse. As long as the horn or clock are functioning correctly neither can or will blow the fuse. If something fails inside either that will either blow the main circuit fuse or fracture an internal circuit. In either case who cares are neither is serviceable as a normal repair item. If the clock has failed enough inside to blow any fuse (even a small one) it has failed period. Why fuse to protect an already failed device?

 

About the only time sub fuses are used in an automotive/motorcycle circuit is for safety reasons. Most motorcycles don't bother but some automobiles will use a sub fuse on things like dash lights as the main power feed comes off the tail light circuit. A smaller sub fuse can allow the dash light part to fail but still leave the tail lights functional.

Some of the older automotive radios had internal fuses but that was put in by the radio manufacturer as they actually repaired those radios. Most automotive radios now are either not (normally) repairable or have enough cheap internal electronics that a transistor or diode fails as a cheap internal protection.

 

[Peter Parts]

.... ummm, guess I've become used to the smart ways the Japanese design their car electrical systems.

 

Don't know about you, but blown fuses are a key diagnostic for me and let me turn circuits on and off for my own sometimes atypical purposes, esp. as you mix-and-match add-ons. Bridging an empty fuse socket also lets you measure amperes, IF you can figure what's connected downstream.

 

In the perfect world of BMW engineers' dreams, different rules apply compared to my real world.

 

Ben

[dirtrider]

[quote=Peter Partsclip----Don't know about you, but blown fuses are a key diagnostic for me and let me turn circuits on and off for my own sometimes atypical purposes, esp. as you mix-and-match add-ons. Bridging an empty fuse socket also lets you measure amperes, IF you can figure what's connected downstream.----clip

 

 

Morning Ben

 

Most accessories (like your mentioned Tac & horn as an example) have plugs on the wire harness where the accessory is plugged into the bike's wiring. Simply unplugging the device for diagnostic purposes has always seemed sufficient for diagnostic purposes.

 

Like I mentioned above, myself & most auto/motorcycle companies see no real need to individually fuse to protect an already failed device.

 

At what point would it end? Would you fuse each individual accessory & each light bulb as a separate circuit? That isn't feasible for complexity reasons, wire harness size, weight, ultra large fuse boxes, expense of wire use, & OEM assembly complexity. Again, for what gain? Just to protect an already failed device or light?

 

Now with (non OEM) add-on's I tend to agree with you that fusing those as a stand alone makes some sense if tied into an existing circuit as that eliminates a failed add-on effecting an OEM circuit. And definitely fusing the add-on (circuit) if a stand alone to prevent wiring related fires.

 

Some of the newer bikes & automobiles are eliminating fuses altogether with the use of electronic crowbar (type) circuit protection & even some individual circuit load monitoring to warn of individual light or accessory failures. (like a telltale to tell the driver/rider that the L/H front turn signal bulb is not working or burnt out, or headlight low beam has failed). This is possible by placing the control processor somewhat close to the accessory or light (to keep wire runs short & of small diameter) then broadcasting the failure on the CanBus back to the dash or IP processor for telltale indication.

 

[racer7]

One thing to remember in this discussion is that BMW publishes no values for most of circuits monitored by the ZFE (accessory outlet line being the only exception I know). In fact, no intrnal circuit diagrams for the ZFE are publicly available.

 

To intelligently decide to eliminate fuse one needs a few values.

1) Current capabilty of protected device

2) Wire gauges between the device and power

3) Fused capability of circuit being tapped into (the missing ZFE values limit this approach though if you're into extrapolation, wire gage on the bike might provide some approximation)

 

It certainly true that fuses must protect wires above all else. To fail to understand this is to risk fire in case of a significant short. Crisped device might also happen but its rare for one to go up in smoke- most just blow one of the solid state devices if seeing a majorr overload for which their circuit design can provide protection.

 

In cases where you have an an accessory connected to a fuse panel outlet which is used solely for that item, one can certainly move the accessory fuse supplied or use one of the same value at the fuse panel rather than in line. In such cases, the accessory must not overload the fuse panel AND the wiring to the accessory must not be smaller gage than the accessory maker selected and must be capable of safely carrying as much current as the accessory requires (Larger wire would not be an issue in most such cases but smaller might be)

 

There is no point to using wires that can carry far more current than a circuit needs. That only raises the risk of fire in cases of a short to ground in a manner that bypasses fuse protection. (If you ever rebuild a machine and need to reroute battery leads, be very careful of routing)

 

When selecting fuses, it is wise to fuse a circuit at about twice the demand load or a less, while being certain that the wire gage selected can carry that load. Err on the low side if you don't know for sure. Better to pop a fuse than burn your bike.

 

FWIW,

I find quite bit of sloppy wiring looking at accessorized bikes.

Typical sloppiness include

1) Poor routing choices

2) Poor power supply choice

3) Junk plastic crimp connectors for taps

4) Wire not cable tied well

5) Failure to use loom to protect wire bundles or exposed wires

6) Failure to use waterproof connectors when appropriate, instead relying on cheap crimp connectos and tape.

7) Failure to solder connections where that is useful for corrosion protection or mechanical strength

8) Inadequate attention to ground circuits, especially ganged grounds.

9) Coils of excess wire strewn around the bike because te installer is too lazy to reterminate, or lacks the needed tools.

10) Falire to install in line connections so wiring may be easily disconnected if it impacts service or repair work (might save you some $ on a repair bill)

 

One that I rarely see is using wire too thin to carry the expected load.

 

 

 

[kgerry]

from my point of view the fuse should protect the vehicle first and the component second.... if you are making any direct connection to the battery the fuse should be immediately at the battery, that way if the wire pinches or shorts anywhere (car or bike) it will blow the fuse as opposed to starting a fire.... if the Garmin comes with a 2A ATC and the circuit in the fuse box has a 5 or 10 amp fuse in it and the Garmin is the only device on that circuit then just swap the fuse in the fusebox for the 2A to be safe... also, just a heads up but a 2A ATC or ATS is not a very common size, so i would carry a few spares with me as you are unlikley to find them in a gas station or basic auto parts store....