Battery Cables & Bow Thrusters

[TerryKing]

Frosty, I think you're basically right, when the device is one you depend on working, even if it's a bit overloaded right now...

On the other hand, Smoke and Fire is Bad...

What do the big automobile manufacturers do?? They don't exactly Fuse the biggest loads, like the starter solenoid, or some A/C stuff, or the Main Power Panel feed. But they don't want uncontrolled fire and smoke, so they decide where to PUT the fire and smoke. Right at the battery positive terminal, they connect the wires for big-load devices. But they put a short (2 inches or so) piece of SMALLER wire right at the beginning. Short enough so that the added resistance is negligible. Small enough so that if there's a major short circuit upstream, that smaller wire will be the one to smoke and melt. These days they use some kind of high-temperature self-extinguishing insulation on these. In the shop manual these wire ends are called a "Fusible Link". Often the dealer will replace the whole wire, but there are aftermarket replacement ends. Maybe we should look at using these on boats??

Take a look at: http://www.madelectrical.com/catalog/fusible-link.shtml
for a good overview. A quote:

Installation and function of a Fusible Link is similar to the “cartridge type,” in-line fuse. But the Fusible Link does not deteriorate or have meltdown problems with constant use in heavy-duty systems, as the cartridge type fuse is famous for. (Chevy began installing Fusible Link wires in the main-power circuits with ’66 models, and of those old cars that are still in use, most of the original Fusible Link wires are still in place. If a Fusible Link burned out then it saved the car!)

[Frosty]

That is for "full overload ' (a short to ground). Providing the shorted wire is bigger? If the shorted wire is smaller than the fusable link then you still get a supply and a fire.

Automotive ac is fused, compressor clutch and fan for the evaporator is all
there is to it.

A compressor clutch would be protected by 10 amp max.

Head lamps and wipers are the biggest draw along with a cigarette lighter socket.

A winscreen wiper motor would burn up with 15 amps and the "fusable link" would'nt even notice it.

[masalai]

Well thought out wisdom, Thanks Frosty

[Moosemiester]

Although I don't have a good detailed technical explanation handy, the fuse both ends method is what is taught to installers of high power radio equipment in automobiles.

QST magazine (A Ham Radio publication) has had several good explanations of why this is done and I'm sure there are other places.

In my 2001 Dodge truck with a Cummins Diesel, yes, Frosty, the starter is fused. They are on all modern automobiles, on both ends. It's not a traditional fuse, but rather a fusable link, a piece of wire that melts under the right load. Accidentally ground the back of the starter, causing a short, and you discover this real quick.

The power that comes into my house in the USA is setup this way too; there is a fuse-a-ble link on the pole, and a breaker at the box. I think my 100A 220v house panel qualifies as heavy equipment...

[Frosty]

"fusable link" the short word for this is fuse,--its nothing new .

The fuse you are refering to is now called a maxi fuse and is not on the starter circuit.

Maxi fuse 3--70 amps how can you fuse a starter motor that can draw 300 amps. and then when the engine is warm maybe only 200amps.

A cold morning start could see more than 300. How do you fuse a variable load.

A house is nothing like a car.

[TerryKing]

Originally Posted by Moosemiester

Although I don't have a good detailed technical explanation handy, the fuse both ends method is what is taught to installers of high power radio equipment in automobiles. QST magazine (A Ham Radio publication) has had several good explanations of why this is done and I'm sure there are other places.

I've seen various references to this issue, in QST and also in some online Ham websites talking about high power mobile. There are two considerations:

1. Fusing BOTH the positive and negative power feeds to a radio (or other) unit. This is to protect against the negative lead (which is also connected to the radio's chassis, which is usually connected to the frame/body of the car) becoming a conductor of some OTHER device's ground/negative current. The obvious example is a failure of the large cable from the battery negative terminal to the engine block/starter. If that cable is disconnected, current from the engine/starter will flow from the (Radio) negative conductor, through the radio, to car chassis/ground. This has actually happened and smoked some radio or two.
2. Protecting the WIRE from overcurrent. This is one reason for fusing BOTH ENDS of the positive cable to a radio etc. IF the cable gets shorted to ground, the fuse at the SOURCE end will open, protecting the cable. IF the radio draws too much current, one of the fuses will open. Which one?? One practice is to fuse the source end with a value equal to the max current carrying capacity of the cable. There seems to be no reasonable scenario for fusing BOTH ends of the negative lead, however.

The Kenwood VHF-UHF radios I have, have fuses in both the negative and positive leads at the radio. I guess to be really safe, I should put a fuse or fusible link at the battery end of the positive wire. Using the example of a #12 positive wire, and a "4 wire sizes less for a fusible link" I'd need a short section of #16 at the battery.

I think a few of the online suggestions confuse both WIRES with both ENDS.

[TerryKing]

Automobile manufacturers started doing this because high-current fuses used in earlier cars suffered from poor connections and failed from overheating.

Regular fuses "plug in" and are dependent on a spring pressure for a good connection. Over time, temperature cycles, vibration and environmental contamination, the contact between the fuse and its holder gets worse and worse and the contact resistance goes up. That causes heat, which makes contact/corrosion worse. An overheated fuse blows before its normal current rating. Replacing the fuse in a corroded holder doesn't last long. I've seen "3AG" type fuseholders on older boats that were green with corrosion.

"Fusible Links" solve this problem two ways:

1. The connection is made with a good crimp connector that makes a high-pressure large-area gas-tight connection
2. The joint is covered with a water-tight shrink covering. The newer ones have a remelted zone over the wires.

In an automotive or Marine environment this is a much more reliable years-long-life solution.

Some Boats have a large (100 to 200 amp) 'fuse' near the battery that feeds a main cable to the main circuit breaker panel. Good fuses have large LUGS that bolt to lugs on the cable, with lots of pressure and area. They do not rely on spring pressure in a fuse holder. I hear they are reliable.

For my own DIY stuff, I guess I'll do a few 'fusible links' when I refit my boat this Spring in Vermont.

We're all learning stuff!

[Frosty]

Wow, we are jumping from cars to houses to boats to radios . I cant keep up with you all.

But basically the point of a protected from the environment type fuse is a good idea but you cant protect a 5 amp circuit with 30 amp fuse. Yes fuses can corrode of course and cause a problem, but its not an every day occurence.

[Landlubber]

ANL type fuse holders are the ones to use for high current applications in boats. They are available in many sizes up to about 750 amps, so there would be very few applications that could not be covered. If the boat is drawing 750 amps through the system, it may be time to go to 24 VDC instead of 12VDC electrical systems.

Most boats using say the Vetus bow thruster of about 8hp use a 350 A slo blo fuse. The motor draw on these motors is actually about the 500 amps, which is very high indeed. Fortunately they only have a capacity to run for two minutes TOTAL in an hour, something many, many owners ignore. They seem to think that they can run them for two minutes whenever they like, but the poor old DC motor really gets to heat up with such high currents running.

[nordvindcrew]

just another idea on the problem. I install heavy automated rolling access gates. They all have a stress circuit that shuts down the operator if too much stress is felt. No idea how they do it but they work every time. Have been told it's done two ways, one is mechanically with a switch that senses movement of the motor if it torques up too much, and the other electrical that senses the load on the motor and de-activates the solenoid. You might want to check out these ideas and see if they will work. In line with this general problem, how do you calculate wire size needed foe a piece of equipment. My brother wants to move the batteries in his boat forward to help weight distribution and can't find a clear cut way to figure the wire size.

[Frosty]

How do you think your electric windows on your car know when the glass is at the top.

See how much the window switches are????

I bust my door switches and a new set for 1 door x 2 windows was 200 dollar.

Because its got fancy stuff in there.

[TerryKing]

Here's what those ANL type fuses look like. They are bolt-down types which are more reliable in Marine Environment than types that only use spring pressure.

This is from:
http://www.texasindustrialelectric.com/breakers.asp

Landlubber, or anyone, have you seen a supplier for those high-current fuses that look like a large cylindrical fuse, but they have side lugs for bolt-down holders?

And I agree that any boat running motors at 300 amps on 12volts should better be running 24 or 48 volt systems. With inexpensive point-of-use switching regulators becoming available, you can have 12 volts wherever you need it, from the 24 or 48 volt bus, and the output is a clean regulated voltage that won't have dips and surges during engine starting etc.

I'd like to find a decent 12 volt output device that runs from 18-50 volts, and has a NMEA network port so you can turn it on and off remotely, and read back the status and the current being drawn. Maybe, in my Other Spare Time I'll design one...

[Landlubber]

Terry,

You can buy them almost anywhere in China electrical suppliers, they are regularly used here still.
I definately saw them in stores in Ningbo the other day, and also the old ones that look like confetti blowers for a party too.

[TerryKing]

Thanks Landlubber, I'll look thru some stores near the Shekou Port..

Here's another fuse solution, especially good for Retrofit and DIY:
http://www.jackrabbitmarine.com/Detail.bok?no=3859