Making Fusible Links

[Zed Head]

If you prowl the salvage yards you'll see many fusible links on pretty high-end luxury cars, like BMW's, up in to years in the (edit - not sure of year)s. And if you're familiar with home fuses, you'll know of "slow-blow" fuses, used to take a surge so that fuses don't blow unnecessarily. If they didn't look like scabbed-on backyard fixes even when brand-new, people would probably be fine with fusible links. And it they rated them by current flow, like a typical fuse, that would help too. But, instead, they're ugly looking, confusing scraps of wire. Unfortunately, even if you want to swap to fuses, yous still have to calculate the desired upper limit for current on the circuit. Just one of those primitive things that has stuck around, like the AAR valve.

Edited May 16, 2014 by Zed Head

[EuroDat]

Zed beat me to it.

If you deal with electro motors, you can get up to ten times the current when they start up from rest. Its only a very short time, but its enough to blow a set of noraml fuses. In that case the fuses are "Slow blow". Google it and you will find all kinds of info on them.

I think modern cars use them on the primary circuits. You generally find a couple of 80amp fuses, which are most probably slow blow to cope with surges.

Like Zed Head mentioned, calculating the size fuse required is the problem.

Chas

Edited May 16, 2014 by EuroDat

[madkaw]

I guess I forgot my little bit of electronic knowledge from school. I wonder if the locomotives I drive have these type links for those 1200 amp surges

[TomoHawk]

When I first started I researching fusible links, I found a lot of talk about them on websites for modern Mustangs.

If you really wanted to replace the fusible links, IMO the best thing there is out there are manually-resettable circuit breakers. Power windows employ automatic-resetting circuit breakers.

I wonder how many people installed the MaxiFuses,only to find that they keep blowing, until they put in an overly-large fuse?

Edited May 16, 2014 by TomoHawk

[Mark Maras]

I use auto-resetting circuit breakers when working on specific circuits on my cars. They replace the existing fuse & are avail. at NAPA.

[Bonzi Lon]

My experience with a fusible link (73) was during refurbishment. With both battery cables being black they were placed on backward. I noticed a larger spark than normal, then saw below the fusible link starting to smoke and could see red heat going from right to left. I quickly disconnected the post that was in my hand. The fusible link was about half burned, then I noticed what had happened. I figured it burned about 3 seconds for the 6 inch length.

The next day I found a replacement at the auto parts store for a Ford. Same amperage and diameter as the FSM called for. Worked fine. A red battery cable was installed at the same time.

It sure smoked, but saved my wiring.

Bonzi Lon

[240260280z]

After a lot of thought, I think this is the essence:

[*=1]Fusible links protect the wires in the harness.

[*=1] Fuses protect devices that are connected to the harness.

[SteveJ]

I'll beg to differ with you on point #2, Phil.

Most fuses in a car are are there to protect wiring. Most circuits in our cars have several components wired in parallel. A great example of this is found with our running lights. There is a 20 A fuse for the circuit. It is designed to protect the wiring. Fusing for a component actually only exists to protect against an overvoltage condition, and it is usually downstream of a fuse for the wiring.

Why do I say this?

A component will have an impedance or an impedance range. Unless the component has failed, it will maintain at least the minimum impedance. Since current is voltage divided by resistance (impedance), the only way for the current to go up is for the voltage to increase. If the designer isn't suffering from cranial/anal insertion syndrome, the component is designed to handle up to about 15VDC. If you have a voltage regulator go bad, the voltage may spike. To protect the component, it has a fuse rated for the current that the component would see at a lower voltage, say 12 to 14 volts.

[240260280z]

I partially agree. I was only thinking of radios, horns, and electric motors (starter, fan, blower, antenna, wipers).

After further thought I think fuses are to protect in anomalous conditions where the nominal expected load is exceeded. This would prevent fires when something shorts downstream.. only 1 spark and no time to heat up.

If fuses were to protect wires then wouldn't the values be much higher? I guess for the defroster this is true as it is wire but for most devices I think the fuse is sized for protecting an unforeseen condition where the nominal current drawn from the device or ganged devices (such as lights) is exceeded?

Edited May 17, 2014 by Blue

[TomoHawk]

I think I would reverse your #2, to say fuses protect the wiring from (overloads caused by) devices. But that's just me.

Still, I think it reinforces the idea that Maxi fuses aren't a good choice to replace fusible links. Could you replace your tires with rubber bands?

Edited May 17, 2014 by TomoHawk

[beermanpete]

The primary purpose of a fuse is to prevent fires (this is more important in homes, but who wants a car fire?) Think about it. If a component starts drawing too much current it is already bad so there is nothing to protect. Without a fuse the high current from a failed component could cause the wiring to overheat and melt, smoke, and eventaly the insulation or other flamable material near the wire will catch fire. The purpose of a fuse is to interupt the current flow before the circuit it is in can get hot enough to start a fire.

Fuses have a rating for I2T (Current Squared times Time), which is essentially a heat vs. time rating. This helps the engineers designing the product (cars in our case) select a fuse that will provide the required level of safety while avoiding nuisance tripping from in-rush currents (turn-on surges) and other expected, short term high current conditions. Also, the fuses used in cars will only handle about 80 percent of their current rating indefinitely. At their rated current they will open in about one hour. I have never found any ratings, running current or I2T, for fusilble links so it is difficult to make a fair direct comparison. I suspect the I2T for a fusible link is rather large (compared to regular fuses) since fusible links have a much larger surface area (providing more heat dissipation) and seem to be made of copper rather than a special low melting temperature alloy.

[jwtaylor]

Beermanpete is correct, Fusible links, fuses and circuit breakers are there to protect the wiring from burning and starting fires. They are not there to protect the equipment (the load) at the end of the wire, but to prevent fires from an overloaded / shorted condition.

I have seen the results of not using fusing in a large multiple motor control system that had a short in one motor that caused burnt wiring and damaged controllers throughout the system. The repairs cost many thousands of dollars and hours of work to repair.