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Three purposes of a fuse are

Old 04-17-2003, 09:37 AM
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Default Three purposes of a fuse are

to protect the wire, to protect the wire and to protect the wire. It is not to protect the device that is being powered through the wire. This has got to be one of the most misunderstood subjects on this and all the other boating message boards.

Wire size is determined by the load that the wire needs to carry and the total length of the wire. An appropriately sized fuse is then installed as close to the power source as practicable but no greater than a specified distance (I forget what it is).

If the device fails and draws excessive current then the fuse will open the circuit before the wire overheats and becomes damaged. Similarly, if the wire chaffs to a ground source (known as a short circuit) then the fuse will open the circuit to prevent wire damage.



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Old 04-17-2003, 05:43 PM
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Default Three purposes of a fuse are

Absolutely correct! Welcome aboard. Why the statement?

Boston Whaler, "MUMBLER", 24' Outrage, twin 175 HP Evinrude Ocean Pros. Snowball, the cat...
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Old 04-17-2003, 06:41 PM
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Default Three purposes of a fuse are

I disagree… tbe fuse is there to protect the insulation on the wire. A wire with 90C rated insulation can be fused higher than the same size wire with 70C insulation.
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Old 04-17-2003, 06:48 PM
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Default Three purposes of a fuse are

protect the wire, protect the insulation, protect the vessel from fire!!!
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Old 04-18-2003, 05:23 AM
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Default Three purposes of a fuse are

NightFisher...quote:tbe fuse is there to protect the insulation on the wire. A wire with 90C rated insulation can be fused higher than the same size wire with 70C insulation Then the insulations melts and what happens; the wires touch and the fuse blows, preventing a fire. I understand what you are saying as to higher temp to melt insulation (more current) but after that ...poof.

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Old 04-18-2003, 05:55 AM
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Default Three purposes of a fuse are

SO here's a delima for the statement.

The smallest allowable wire on boats is 16 gauge. It's nominally rated at 25 amps of current capacity.

If your fuse is rated for 25 amps to protect the wire, it's quite possible that your 5 amp CD player could be in flames before the fuse blows.

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Old 04-18-2003, 06:40 AM
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Default Three purposes of a fuse are

And that is a case where you should change the fuse down OR add another fuse for that equipment, and maybe close to it.
What I want to know is who almost stamps the ratings and size markings on the old style fuses.
The poor marking of these most likely has caused more blown equipment than selecting the wrong size.
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Old 04-18-2003, 06:57 AM
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Default Three purposes of a fuse are

Mumblerone, you are right… It always comes down to temperature, you have to keep the conductor temperature below the melting (or worse yet ignition) point of the insulation and surrounding material. In the low amperage circuits we deal with on boats it is rarely an issue because the wire is pretty small and easy to work with so the recommended fuses are very conservative. When you are working with a 400 amp feeder you have to take into consideration the insulation temperature rating and ambient temperature to determine the correct wire size.

When you are pulling a wire for your GPS go with the manufacturers recommended fuse size. Lets say I ran a 16AWG wire to my GPS, I don’t have my tables here but for the sake of argument I can fuse that wire safely at 10 amps. How do I connect to my GPS? I splice that 16awg wire to the pigtail the manufacturer included with the unit. The wire in the pigtail is 22awg. That is the wire I need to protect so I need to fuse the circuit with a lower rated fuse, maybe 2 amps. You are still protecting the wire.

Another reason to use the manufacturers spec is reverse polarity protection. Have you ever hooked something up backwards? A lot of 12V electronics have a reverse polarity protection diode on the input. If you hook the unit up backwards the diode causes a high current to flow, hopefully blowing the fuse before the electronics is damaged. If you have the fuse rated for the wire the diode in the device can’t handle the current and it fails. I made a lot of money back in the CB craze days replacing diodes in radios where people had not used the recommended 2A fuse and connected the radio up backwards to a 15A circuit.

A final trick I stole from the automakers is a fusible link. Since a fuse is nothing more than a smaller conductor that melts before the wire gets hot you can perform the same function with a short length of smaller wire. When I put a second battery up forward in my boat I wanted to use a 4AWG wire to tie it back to my battery switch to keep the voltage drop low. I wanted this wire protected but I didn’t want the problems associated with the connections on a big fuse or circuit breaker in that location. A rule of thumb is drop down two wire sizes for the fusible link so I spliced (soldered connection) a short piece of 6awg wire to the end of the 4awg wire connecting to my battery. This way I know where the wire will melt in a short circuit situation. You have to make sure the fusible link won’t cause any damage if it melts and that the heaver wire is held in position so that its end can’t fall onto anything after the links melts but you will have reliable protection.
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Old 04-18-2003, 09:42 AM
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Default Three purposes of a fuse are

You may agree that a fuse/CB is current limiting device, its purpose is to conduct current up to it rated limit then self-destruct/trip.

By saying the purpose is to "protect the wire" that implys you first determine the anticipated current requirements, size the wire, them select a fuse to protect the wire. This is certainly true when the circuit load can vary, as in residential outlets and overloading the circuit with too many devices.

With Dedicated DC circuits the device manufacturer sets fuse/CB size. Then you size the wire based on run length/current requirements.
Its primary purpose in a dedicated circuit is to limit current to the load device. Do agree in the event of a circuit wire short the load device is the wire.

[This message was edited by HookMan on 04-18-03 at 12:58 PM.]
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Old 04-18-2003, 10:06 AM
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Default Three purposes of a fuse are

Just my opinon,
I would be very leary about installing a smaller gauge wire in line with a larger gauge wire for a fuse. If there is a problem and it fails you will be creating a lot of hot material possibly molten and it will happen over a much longer period of time vs a fuse or a fusible link or cb. Sounds like a fire waitng to happen. Not only that, but you just derated the whole circuit two wire sizes.
Why not just use the over current devices rated for the circuit you are trying to protect, it could save your boat and possibly your life.
Dave

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Old 04-21-2003, 08:14 AM
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Default Three purposes of a fuse are

I used to have an AlfaRomeo Spider. One day going down the highway the cabin filled with toxic smoke in about 5 seconds. I screached to a stop and jumped out expecting to watch the car go up in flames but it didn't. Under the console I found what appeared to have been a 14 ga wire with no insulation on it, all melted. Run straight off the hot terminal, so it was unfused. It had sliced into everything it touched until the wire had actually melted in two. Alfa had run the wire to feed the accessory switches and even stripped back the end, but I didn't order any accessories so they didn't connect it to anything. The bare end got up against a screw head and the circuit was made!!! This situation could have had grave consequences

Product Engineer, SMITH Marine Products
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