Bigfoot928

19psi

If it's carrying current going to the starter, which is the highest load any vehicle will see, that's all that matters when trying to determine a good fuse size for a 928 that won't pop when starting during cold weather. I'd bet most European V8s are using a similar size fuse.
As far as instrument clusters and other components, no main fuse is ever going to protect against that. The point of Carl's fuse is to keep a large gauge wire from acting like a welder and getting hot enough to burn holes through metal.

Carl Fausett

Thank you for sharing, Perry. Your comments are spot-on.

Bulvot

To determine appropriate fuse rating for a starter, I recommend using a clamp meter. No guessing with that.

MEGA fuses have an interrupt rating of 2,000 amps. MEGA fuses also supposedly produce a spark when they blow (I've never used these types of fuses because they aren't typically considered appropriate for automotive applications, so I've never seen one blow and can't personally confirm that they spark).

ANL fuses definitely don't spark when they blow, they have a clear view of the fuse element to determine status, and they have an interrupt rating of 5,000 amps.

If the problem that you're trying to solve is a short to ground from the battery with the primary large gauge battery cables, the 2,000 amp interrupt rating is probably inadequate.

I prefer ANL fuses for all of my high current automotive applications.

terry gt

good discussion , I will be checking my wiring tonight . I also have a MB S600 coupe with the V12 , parking with the battery disconnected is a good idea

Rodrv6

Looked at my 88S4. The wire that caused the fire on the car in Carl's shop is in good condition and not rubbing anywhere. However, the bigger feed wire at the jump post does have cracking in the insulation. I've got some high quality shrink wrap on it now. I've also ordered the battery fuse kit. I'd replaced the long battery positive cable under the car a couple of years ago, but the fuse will be good insurance and peace of mind. Thank you Carl for sharing with us!!

captainOCD

As another data point for cars with batteries in their trunk, my Charger doesn't appear to have a fuse from what I can tell. What it does have is some "sensor" between the battery post and negative wire. Some research suggests that it monitors current, voltage, and temperature. Nothing seems to specify what all it does with that information, but perhaps it has the ability to isolate the battery. It would make sense that given that all the computers are controlling most functions that it should have some idea of how much current should be being drawn and could perhaps act if something starting drawing unusually high current. Kind of like a GFI.

Something like that that knows about how much power should be being drawn and compares to the actual could be pretty interesting, but would be hard to retrofit to a vehicle with mainly analog controls.

Carl Fausett

928SG: Before you go to all that trouble, consider this: If I read my WSM wiring schematic right, those small feeder wires are 4.0mm/squared, or about 11 AWG. Depending on the length of the run, that wire is rated to only carry between 8 and 12 amps. (PLEASE Fred and DocMirror check me on this!) If you were to take a piece of that 4mm wire and connect it post-to-post of a test battery on the workbench, it would burn through in no time at all. I know those feeder wires are not fused, but they are pretty small and I wonder if they are a real threat. Those circuits wired directly off the battery do not seem to be giving us trouble.... but the larger AMP-carrying wires seem to be coming into our focus. They are the ones large enough to destroy the car if they should ground out.

Anyway, that's why I'm focused on the large primary cables, and less so on the smaller feeder wires.

Speedtoys

Any wire that can short and catch fire..just being a _wire_..for 12 feet, is a threat.

Ive towed any number of race cars that burnt to the ground because they rewired the sucker to be "light" and used no fuses.

All it takes is -1- wire...of really any gauge, and anything near it can light up too. Not the same as a 200A welding torch size short..but a hot burning 12' stretch of 12ga wire will make a mess too.

In for one, in for all I say.

GregBBRD

The "fuse" solution in a 928 seems complex. There's a bunch of spots that will probably need fuses. Your new fuse will certainly protect the wiring to the starter, the alternator, and the front jump post.

And that's a good thing!

Beyond that, will it protect anything?

Getting right down to it, would your 250/300 amp fuse protect the car that burned in your shop?

And even if there were multiple fuses for the smaller red wires that go to the relay board, would those fuses protect the car that burned in your shop?

If I understand the source of the fire correctly, one of the two 12 gauge wires that go forward from the front jumper post apparently rubbed through and shorted (doubtfully both shorted out at the same time...so I'm assuming that one shorted and the other eventually melted the insulation off of it.)

If that is the case, even if there were multiple fuses in the rear (a 250/300 amp fuse and multiple smaller fuses for the smaller red wires), the current from the short that burned the car in your shop would have only came through the main 250/300 amp fuse....the wires to the relay board would not be involved.

So, the question becomes, will a short on a single 12 gauge wire, blow that 250/300 amp fuse?

If not, not only will the small red wires in the rear of the car need smaller fuses, but those wires that go forward from the jump post will also need smaller fuses.

Complex problem.

GregBBRD

Exactly. When I bought my '63 Nova, it had nary a single fuse...anywhere.

I disconnected the battery and re-wired the car, before I started it.

Christopher Zach

Contactor. Solves problem. Really. You can even get one that can hot break a 200a load if the contactor drops out (which it will if the car is running and dead shorts the entire electrical bus).

Add a 10 gauge snooper wire fused 15 amps or so, a 14 gauge wire to key positive, and you're done. Heck with this even the ABS would continue to work if the contactor dropped out for some reason (rest of car would probably work too for awhile)

Next problem?

(Maybe I'll take the driver's side door skin off this weekend since I didn't get my passenger door handle thingie)

h2pmr

you mean something like picture below?
fits directly on top of the battery, main starter cable comes off one of the large fuses and there is provision for the 3rd smaller fuse
it is off a vehicle that has the battery under the floor in the cab area so has a long'ish main starter cable
cheers
Phil

FredR

Carl,

First of all much kudos to you for posting about this incident which you could well be forgiven for not wanting to disclose publicly.- as you have no doubt learnt the hard way it could happen to anybody at anytime. As someone who has been involved in many engineering investigations the prime take away from this that I would recommend to you and any of your fellow professionals would be to systematically disconnect the ground strap on any car that is sitting in your workshop that is not being worked on and for sure to do this last thing at night before you lock the premises up.

A 4mm2 cable [approx 11AWG] should be good for about 40 amps according to the sizing criteria I would use and if a safety factor of 100% were applied the same cable will carry 20 amps with consummate ease. I get the impression that Porsche used a 100% safety factor when sizing their cables in this car.

When a cable shorts to earth the current flow is only limited by the resistance of the circuit to earth. If we assume negligible resistance at the point of shorting out, a 2m length of 4mm2 cable would choke at about 720 amps. Unfortunately one can initiate ERW welding with as little as 30 amps and when the short is made the residual resistance is a crap shoot. If there is major leakage then a 250 amp fuse may indeed do some good but if the leakage is less than 250 amps nothing will happen protection wise. Thus if 30 amps can strike an arc, it is not difficult to understand why fitting a fused link may be little more than a false sense of security [sad to say]. This in principal is why I concluded that Porsche designed the system the way they did. The only practical mitigation is to ensure that the integrity of the cables is sound. The 30mm2 cable from the positive post to the starter motor is physically well protected. The 16mm cables from the starter motor to the alternator and from the alternator to the hot post are invariably screwed if still OEM fitment as is the 10mm2 cable to the ABS post. Any example running them should be changed out unless the car has been stored in nitrogen.

On the example you cited, the troubling bit is why one of the two 10mm2 cables from the hot post to the CE panel shorted out. Those cables run inside a heat shrink outer sleeve and presumably two layers of protection had to be breached to cause what happened. We can see the burn through marks on the steel wall and presumably that caused secondary fires as the wheel liner on the other side went up in smoke.

After studying the wiring layout I decided to run my home brewed wiring harness with the two power feeders run separately from the cables emanating from the 14 pin connector. I used two layers of heat shrink over the basic cable and where possible, I reused the original orange coloured silicon rubber impregnated fibre glass sleeves over the power cables. That way I can inspect them. I also ran the power cables a bit longer than stock to help facilitate dropping the alternator when on my axle stands. I will also be fitting additional nylon woven sleeving over these cables during my upcoming annual inspection/maintenance work I intend to start shortly. I also did some more inspection of the two 10mm2 cables from the hot post to the CE panel following your post.

If nothing else your little kit is worth the utility of getting those various smaller cables off the positive post of the battery.

Finally, for the benefit of those following this thread who may not fully appreciate what we are dealing with here - a note of some amusement. Some 18 years ago I was driving along a local motor way at night with my wife and two [young at the time] daughters in the back seat. It was during rush hour and I was doing about 80 mph on a road where all three lanes were chock a block. All of a sudden my engine died, the lights and dash panel went out and my windows were locked closed. I could not indicate my need to move over and I decided to just cut across the cars on the inboard lanes and make a bee line for the hard shoulder where fortunately I managed to stop safely. I was stunned as to what had happened so dramatically and instantaneously without warning. Turned out I had forgotten to install the battery hold down clamp and the positive post of the battery welded itself to the underside of the compartment lid. I could not get the lid open and after towing the car home, I had to drill small holes in the lid. That is the power of what a short can do. The nearly new battery was also toast. After cleaning the terminal up, I took it back to the shop I purchased it from together with the warranty slip- they tested the battery, said it was stuffed and gave me a new one. They never thought to ask what caused it to happen [ha ha]! Nowadays I always run with a sheet of rubber sandwiched between the positive post and the battery compartment lid.

If my car were kept in a timber built in garage I would isolate the battery every night. If I leave our home on vacation I always disconnect the battery and I have identified a project to install a quick isolation switch on the rear tailgate cover panel.

mkriete