M. Requin

Has anyone added one or more of the above to their electrical system? If so, I would very much appreciate hearing the details!

Alan

I have fused the feeders from the battery - using a high current ATC fuse holder block. Fusible links are fuses that are not easily changed usually buried somewhat randomly in the wiring IMO they are a quite stupid idea.

You should not scatter fuses randomly about the car - collect them into logical places, and try to use a single type matching the existing CE panel (this isn't always possible for high current cases).

Porsche tried to collect all fuses solely at the CE panel - but this didn't really fully work since they also decided on dedicated feeders for low noise - so they ended up with some unfused circuits and some circuits fused in the wrong place.

It's actually not easy to do a good job of it - generally I wouldn't recommend changes here to most - it certainly creates new potential failure points. Without major restructuring of the primary wiring it is also not easily possible to fuse everything that ideally should be fused.

Alan

M. Requin

I agree, not easy to do a good job of it. I queried the use of fusible links because the V10 in my F350 uses them in ignition circuits (I have no idea why, but since it is current art on that motor I am interested in why, and if it is an applicable solution to circuit protection in my 928).

The circuits I am particularly interested in are the B+ leads from the alternator through the "front engine harness". it seems to me these are particularly vulnerable. I also agree that doing this is not easy. One can document changes all one wants, but that documentation will ultimately be lost, and the future tech will be much aided by a logical and consistent approach by previous meddlers. Having once spent some time installing 1200' of wiring in and creating a new panel for a 37' yawl I can appreciate these problems, whether I solved them well or not!

Any particular solutions you have come up with you would be willing to share?

dr bob

There's no good place to add 'protection' to where the alternator connects to the front-of-engine harness. Anything that would protect the front wiring would me mounted right at the alternator. While correct electrically it would be a disaster mechanically. On your 'murican truck, fusible links are in the engine bay between the battery B+ junction and the individual load centers; no protection on the primary to the starter obviously.

I bought a large-capacity (60A) DC thermal breaker to put above the CE panel, between the wiring from the jump post and the positive bussbar. Protection, but also a way to disconnect power for hibernation. On the S4+, that leaves fans, fuel pump, EZK and LH, oxy sensor heater and injectors feeding separately from the battery. Those all go through relays that can only be energized via the front feed so there is still isolation unless a relay welds. It also leaves the ABS unit powered up. I have not yet installed the breaker.

RFJ

Alan, very nice work(of course)Ray

M. Requin

I like your breaker idea, but on fusible links, by your own reasoning, since "There's no good [italics mine- MV] place to add protection..." (like in my 'murican V10), it would seem like fusible links would be a good application for the alternator-front engine harness leads. Worth trying?

GlenL

A functional problem is having the fuse match the need.

Some of the unfused wires are small and you need a 20A fuse to protect them. A fuse at the battery that doesn't blow when the car is cranking will allow those wires to burn if they short out. Generalizing, you need a smaller fuse every time the gauge steps down. That's impractical.

Something I noticed was that the power to the rear wiper was unfused and tired that into an unused fuse on the CE panel. Really a head-scratcher as to why that was needed. Perhaps there are more cases of that.

It might be simpler and better to replace old wiring rather than protecting for when the old wiring shorts.

M. Requin

A very good point indeed, especially since I am working on a new front engine harness, which prompted the thoughts about circuit protection. But lily-gilding is sorta fun.

And Alan, I almost didn't say "nice work", since it's just what we all expect from you, but- nice work.

dr bob

So, you add a 75A fusible link to the alternator end of your new front harness. Shrink-sleeve it to protect the splice to the new harness, of course. What happens if it does in fact melt/open? You get to remove the whole harness, or at least the alternator end of it, to replace the fusible link.

Decide where you might expect a fault to ground, and size the conductors and the protection accordingly. I picked a 60 amp breaker/disconnect based on the expected loads in the CE panel that are fed through that front harness and the smaller conductors via the jump post.

But I want to have a primary larger conductor in the front harness, perhaps with a fusible link at the jump post end to protect against an accidental fault to ground at the jump post itself. That would add protection from a reversed-polarity attempt to jump-start the car, at least for the alternator and battery; the CE panel and electrically-controllers would still be on the wrong side of the fusible link. Maybe the jump post itself needs to be isolated from both the CR feeders and the FOE feeder from the alternator by a fusible link rated for less than the FOE feeder's current capacity.

Point is that there are more than a few places where one would install fault protection. But you need to consider the possible causes of the fault and probability of that fault occuring. Decide what the consequences of that fault might be, and whether the protection needs to be readily serviceable. Protection needs to be sized based on available fault current, and placed where it will protect wiring that is rated for less than that available fault current.

There are numerous examples of 928 fuses sized for specific loads, while parallel loads have no protection at all. There are dedicated feeders for fans that are fused at the CE panel, fuel pump -- fused at the CE panel for the pump but not the oxy sensor heater, injectors with no fusing. And many more. The AC system as a whole is protected by a big fuse, yet has load-side wiring that's nowhere near up to the rating on the fuse. Maybe the car needs more fuses, so every conductor can be protected adequately; maybe a double-layer CE panel, hinged so you could open up a couple wing-panels that would hold some of the additional fuses.

To your original proposal, yes you might want to add a fusible link to your new FOE harness. Make the primary conductor bigger and size the link accorcingly. Include provisions for replacement in your design, and make sure your docs package for the car included details of what you installed and how to service it. Otherwise, the next owner will end up buying a whole new harness when the link opens, not knowing any better.