Technical Relays, Fuses and Circuit Breakers

My sprint cars had no fuses or relays. However, with the exception of the plug wires, the entire wiring on the car consisted of a wire and switch to kill the mag. They had maybe 8 inches of wire total.

 

Yeah, I bet they were dim! Nice one!

 

I hear ya. How do you like running the inline fuses? Can it be a pain sometimes?

 

I try to plan them as much as possible. You can lay things out where you can get to them easy enough and that makes a real difference.

I think that if I was running lots of circuits I would opt for a fuse block or panel of some sort. My stuff is generally bare bones and that makes the inline fuses more plausible I think.

 

The problem with in-line fuses is guys tend to install them at the device, not the point of origin of the circuit. The factories also did this until they finally switched to fuse panels in the early 60s. This practice is a major cause of wiring fires. I've said this before but I can't stress it enough; the fuse is to protect the WIRE, not the device. If the device is going to fail, the fuse will have zero effect in preventing it. At best, the fuse may limit damage to the device and prevent it from starting a fire.

Think it through... So you grab power off your ignition switch and run it to a device, whether it's a clock, radio, wipers, electric fuel pump, or whatever. You install the fuse at the device. Convenient, no question about what the fuse feeds and easy to locate. If the device fails, the fuse blows and all is good. But what happens if the WIRE gets damaged before it gets to the fuse? The wire has a path to ground, it doesn't 'see' the fuse at all and you now have an uncontrolled short circuit that won't clear until something burns in half. If it's in a wrapped harness, it damages the other wires, possibly compounding the problem if those unfused wires also go to ground but at the very least requiring major harness repairs.

Fuse panels offer point of origin protection and were a huge leap forward in terms of safety/reliability.

 

Thank You, Steve.

Ben

 

Thanks Crazy Steve. I think you described the way a circuit works and the way a fuse protects the circuit in a way that those that might have a lesser understanding of how electrical systems work, can see why it makes a difference how things are done. You share your knowledge often and I think many people benefit from it. You make this site a better place with the information you share.

 

Beaner also mentioned fusible links. These can be a solution but the problem is a complete lack of information on sizing them. I don't like them for multiple reasons.

The factories started using these in the late '60s, primarily as a way to avoid using larger wire sizes in some places. They're basically a very slow-blow fuse. Where fuses blow as soon as current reaches their maximum rating, a link will 'hold' for a period of time. This is useful if the operation of an intermittent load briefly exceeds a fuse rating, but where the problem comes in is I've found zero information on their 'current slope'. The 'slope' defines how long it will 'hold' before melting. The time varies with current; it may allow 200% for one minute, with the time increasing as current decreases. This is just an example; I haven't found any actual info on this. So you're left with taking somebodies 'advice' or simply guessing. The factories have all needed data/equipment to determine this, we don't. Too small, you'll be replacing one regularly. Too big, not enough protection or maybe none. I've heard of a 'rule of thumb' that says the link should be two wire gauge sizes smaller than the protected wire, but that simply points out the importance of sizing the wire right to begin with. The hot rod aftermarket electrical vendors sell a lot of stuff that's basically to correct the inadequacies of their designs IMO.

Two other issues. One, they melt when clearing overcurrent. Unlike fuses, these don't have any enclosure to contain them, so care has to be exercised so that if it does melt, it doesn't cause damage to surrounding stuff. Two, they introduce a higher resistance into the circuit, increasing circuit voltage drop which is the main issue with 12V systems.

 

Well... Fuses also have a "delay", it takes time for them to heat up to their melting point, and it varies with how far above the rating the current is. Looking at this data about the common bladed fuses found in many modern cars they can last >10 000 seconds (~three hours) slightly over the rated current, but at double the rated current they are supposed to blow in tenths of a second.
Different fuses have different caracteristics, some are available in different types from slow blow to very fast. You are quite right that such data can be very important when designing a system, as the wrong type may blow before a electric motor has reached normal operating rpm, or not blow at all despite there is a fault.

 

Yeah! Such an archaic design. Why can't they just trip at a fixed limit? No matter the timing? Makes a lot of sense. Welcome smart switches
https://www.jalopyjournal.com/forum/threads/no-more-fuses-or-relays.1246721/

 

Such overcurrent devices exist, but they're far more expensive than pennies per unit. Fusible link-style (AKA fuses) overcurrent devices are prevalent because for the cost, they're by far the least -expensive option. As well as being the most reliable and generally the most accurate. The more complex the mechanism, the more opportunities for failure/malfunction exist.

That graph also represents the ideal and to the uninitiated who isn't aware of all the other factors at play may lead some to wrong conclusions. These are mass-produced bits, so manufacturing tolerances also apply. This is one reason almost all overcurrent devices have their actual connected load lowered to 80% of the overcurrent device rating. Virtually every circuit (not just motor circuits) has current inrush, that 'slope' is what prevents nuisance tripping. Even a single 1157 brake light that draws about 2.2 amps will have inrush of over double that amount.

 

It’s just an older way of dealing with a problem. Technology advances. The slew rate of that fuse plot is not by design, but by the materials property. It’s a nice consequence, for sure.

To your point, automotive-store bought fuses are relatively inaccurate, unpredictable in their behavior around the failure point and expensive for what they are — all because they are mass-produced, batch tested and sold in limited quantities to the consumer. This is all in comparison, obviously.

This is always a sensitive topic for folks. Everyone has their own way of ultimately, with varying extents, solving the same problem — staying safe.

I’d be happy to send you one of my power switch products to try out and give an honest review. I know it won’t disappoint.