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C-Dawg



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PostPosted: Tue May 23, 2017 5:42 pm    Post subject: Inline fuse Reply with quote

I'm wiring in some new downriggers. The manual says to put an inline fuse no more than 7" from the battery. Does it matter if the fuse is closest to the battery or the downrigger plug? If so, why?

In the old wiring I had the fuse close to the downrigger. I'm wondering if I was wrong all those years.

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Marco Flamingo



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PostPosted: Tue May 23, 2017 6:40 pm    Post subject: Reply with quote

The fuse can save that length of wire from overheating. Generally, overheating in the area of the battery is safer than a wire that's threaded through some inaccessible area overheating (and catching fire).

Mark
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Wandering Sagebrush



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PostPosted: Tue May 23, 2017 9:49 pm    Post subject: Reply with quote

What Mark said!!
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SEA3PO



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PostPosted: Wed May 24, 2017 12:50 pm    Post subject: Reply with quote

I always dislike in-line fuses as you always forget the location in a panic...but as Mark says...close to the battery is best...and probably where I would look in that panic.. Other than to my wife..
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thataway



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PostPosted: Wed May 24, 2017 1:52 pm    Post subject: Reply with quote

Even though many appliances say to go directly off the battery, there are reasons not to have any wiring directly off or on the battery terminals, except the battery charger, bilge pump, engine start and wire to house battery system or a battery switch --then feeding house loads--including down riggers.

There will be a main 40 to 60 amp breaker for the house wiring. Then you can put the inline fuse as close to the bus bar which that is taken off of. The 7" is as per ABYC standards--it can be longer if fully supported in a tray (42" or enclosed in a conduit 70".

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C-Dawg



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PostPosted: Wed May 24, 2017 9:54 pm    Post subject: Reply with quote

Thanks for the replies and help. I'm connecting them to the battery selector switch, and I'll put the inline fuse near the switch.
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smckean (Tosca)



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PostPosted: Thu Jun 01, 2017 12:55 pm    Post subject: Reply with quote

Quote:
The fuse can save that length of wire from overheating.


This is something I've always wondered about. Can anyone explain it to me?

I've never questioned the wisdom of this advise, but I've always wondered how it can be so. I assume that the electrons in a DC circuit move very quickly indeed.....capable of going a long distance in the time it takes a fuse to melt and break the circuit. So if the too high current is flowing in a circuit that would cause a wire to heat up, or even to burn, why doesn't that occur every where along that wire regardless of where you place the fuse? I assume it's not like the "break the camel's back" excess electrons are flowing slowly up to the fuse which then melts leaving the rest of the circuit OK. I would have thought that the current is essentially the same every where in the circuit at all times; and therefore if that current is high enough to heat the wire any where on the wire, it is high enough every where in the circuit to heat all the wire in essentially the same way (at least with boat length circuits).
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thataway



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PostPosted: Thu Jun 01, 2017 1:30 pm    Post subject: Reply with quote

You want the fuse to be as near to the source of the current as possible. (For a lead directly off Battery ABYC suggest 7" for wire not supported in a tray).

When wire overheats and burns the insulation it generally involved the entire wire, and then is likely to start a fire. If the length of wire is short, before the fuse, there is less likely a chance that a fire will start in that few inches behind the switch. The current load, and size of fuse dictates the size of wire. (Plus the main circuit should be protected as I noted above.). Generally one is worried about a dead short, rather than an overload and heating of the wire.

Although electrons at a relatively low speed (Drift speed), in a typical 10 amp circuit about 1/4 of a mm per second. Wire is more like a water pipe. (Not at the speed of light as you might think, 97% only in an absolute vacuum ). Wire takes some time to heat up and reach the temperature of ignition of the covering. This depends on a number of factors, including the size of the wire, type of metal, type of insulation etc.
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Marco Flamingo



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PostPosted: Thu Jun 01, 2017 1:43 pm    Post subject: Reply with quote

Good point. If over current of the electrical draw pops the fuse, why does it matter which end? If the fuse is placed right at the bilge pump, but the wire shorts out down in some locker before the fuse (tight bend, broken strands, corrosion, worn insulation, tight bundling of live wires, heat, etc.), the fuse doesn't do anything. The fuse can protect the pump, but can't protect that length of wire and it doesn't respond to the short. The sooner the fuse, the more protection of the circuit.

Mark
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Kushtaka



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PostPosted: Thu Jun 01, 2017 1:56 pm    Post subject: Reply with quote

It's like putting a breakaway link in a towing chain. It fails before you rip your tow hitch off.

The whole thing will heat up, but a properly spec'd fuse will heat up and break before the wire heats up to a dangerous temperature, then the circuit is open and no more heat.
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smckean (Tosca)



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PostPosted: Thu Jun 01, 2017 4:09 pm    Post subject: Reply with quote

Well, these somewhat conflicting answers have prompted me to do some research on the topic. Here's what I think I've learned (YMMV Smile).

I'm pretty sure that the common belief that a fuse protects only the wire down stream of the fuse is wrong. My gut feeling that the current has to be the same at all points and at all times along the wire is correct; and therefore that the fuse can be anywhere in the circuit is correct also. However, and this is a big "however", there are plenty of other reasons for putting the fuse near the battery.

A big reason is that once a fuse blows the wire is still energized from the source to the fuse location. In higher voltage applications (e.g., 120v home applications) this is obviously important since a person can be shocked, or worse, any where along the wire btwn the source and the fuse even after the fuse blows....hence keep the fuse near the source. In 12v applications, it doesn't seem to me that this matters much.

A reason that does matter that the fuse be near the battery in 12v systems is where there are multiple devices on the circuit or at least multiple locations where the circuit could be shorted. So let's say a device shorts somehow to its return wire. If the fuse is further down stream along the hot wire than that device, the fuse provides no protection since the current is now flowing via the short instead of down the wire beyond that shorted device. Perhaps a better example is if, say, one drives a screw into a panel behind which exists wires and somehow that screw shorts the source and return wires. If the fuse were down stream of that location, once again no protection.

So I think in 12v applications the idea that a fuse disallows the wire downstream of the fuse to not heat up in case of an overload is simply wrong; however, what the practice does accomplish is to minimize the length of wire along which a short is not protected by the fuse.

IOW, a true overload (e.g., too small a wire for a given fuse capacity) that burns a wire before the fuse blows (if it ever blows) is going to burn anywhere, and perhaps every where, along that wire regardless of where the fuse is located. But by keeping the fuse near the positive battery terminal, one minimizes the odds that a short will occur upstream of the fuse since the wire from the source to the fuse is not long, and since any short will burn the wire up to the shorted location if the short is located up stream of the fuse.
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thataway



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PostPosted: Thu Jun 01, 2017 5:22 pm    Post subject: Reply with quote

Don't see where you said much different that what others posted but some basic concepts may be misunderstood, or it may be semantics. Basically the fuse is there to protect the boat from fire. The closer to the source of power, the more wire is protected. This is why I put my peripheral fuses right next to the switches and the main fuse within 7" of the battery switch.

All wires will have some degree of resistance, and there will be some heat--in most cases the loss of voltage is minima. (we try and keep voltage drop to less than 3% for electronics and 10% of resistive loads--and that has to do with the size of wiring)

An aside some fuses may protect the internal circuits of a device.. But that is a different concept.

Fuses from battery chargers should be have a fuse or breaker at both ends of the wire. There are two sources of the power--charger and battery.

You may be confused about current (amps) and voltage. Household mains power is alternating current, and one might get a shock from the neutral side of a circuit in some instances.
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smckean (Tosca)



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PostPosted: Fri Jun 02, 2017 1:44 pm    Post subject: Reply with quote

Quote:
The closer to the source of power, the more wire is protected.

Only from shorts located upstream from the fuse, not from overloads (as I have often heard said).

Quote:
You may be confused about current (amps) and voltage.

Nope
Quote:
Household mains power is alternating current.....

My point was that shocks, especially dangerous shocks, are all about voltage. 120v is dangerous and 12v is not.....AC or DC.
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thataway



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PostPosted: Fri Jun 02, 2017 7:02 pm    Post subject: Reply with quote

Quote:
Only from shorts located upstream from the fuse, not from overloads (as I have often heard said).


Sorry but overloads will cause fuses to blow. The fuse protects wires downstream, not upstream.


Quote:
My point was that shocks, especially dangerous shocks, are all about voltage. 120v is dangerous and 12v is not.....AC or DC.


From a basic article in Emergency medicine journal: Some elementary basics , which you are apparently not aware of"

Quote:
Pathophysiology
Electricity is generated by the flow of electrons across a potential gradient from high to low concentration through a conductive material. The voltage (V) represents the magnitude of this potential difference and is usually determined by the electrical source. The type and extent of an electrical injury is determined by voltage, current strength, resistance to flow, the duration of contact with the source, the pathway of flow, and the type of current (ie, direct or alternating).

Voltage

Electrical injuries are typically divided into high-voltage and low-voltage injuries, using 500V or 1000V as the cutoff. High morbidity and mortality has been described in 600V direct current injury associated with railroad "third rail" contact. [2] In the United States and Canada, typical household electricity provides 110V for general use and 240V for high-powered appliances, while industrial electrical and high-tension power lines can have more than 100,000V. [3] Voltage is directly proportional to current and indirectly proportional to resistance, as expressed by Ohm's Law:

V = I X R; where I = current, V = voltage, R = resistance.

Current


The volume of electrons flowing across the potential gradient is the current, which is measured in amperes (I). It is a measure of the amount of energy that flows through a body. Energy is perceptible to the touch at a current as low as 1 mA. A narrow range exists between perceptible current and the "let go" current: the maximum current at which a person can grasp and then release the current before muscle tetany makes it impossible to let go. The "let go" current for the average child is 3-5 mA; this is well below the 15-30 A of common household circuit breakers. For adults, the "let go" current is 6-9 mA, slightly higher for men than for women. Skeletal muscle tetany occurs at 16-20 mA. Ventricular fibrillation can occur at currents of 50-100 mA.

Resistance

The impedance to flow of electrons across a gradient is the resistance (R) and varies depending on the electrolyte and water content of the body tissue through which electricity is being conducted. Blood vessels, muscles, and nerves have high electrolyte and water content, and thus low resistance, and are good conductors of electricity–better than bone, fat, and skin. [5] Heavily calloused areas of skin are excellent resistors, whereas a moderate amount of water or sweat on the skin surface can decrease its resistance significantly.


The skin's high resistance protects us from serious damage from 12 volt, for the most part. However, I have seen some very serious direct injuries from 12 volts. (As a Diener for Los Angeles Co; Coroner during college, working in Emergency rooms for over 35 years, and also during my academic and private practice of medicine.)

Under the correct circumstances even the power from a D battery can kill.

The reason I spend time correcting, is that people can be seriously hurt, and property damaged, if there are misconceptions. If you want to take the time to educate your self, I will be more than happy to give you in a PM, the names of good basic text books, in physics, biochemistry, physiology and various fields of medicine.
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smckean (Tosca)



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PostPosted: Sun Jun 04, 2017 2:40 pm    Post subject: Reply with quote

Quote:
The reason I spend time correcting, is that people can be seriously hurt, and property damaged, if there are misconceptions.


Yes, folks reading stuff on the internet can pick up misconceptions.....but I think most folks realize that (at least they should!).

There are some misconceptions, or perhaps more likely some misunderstandings, in this thread which probably ought to be cleared up (for the reason you stated). I am leaving today on a 4 day cruise on Tosca and don't have time to respond, but I will when I return.

I assure you I am very well educated on such matters (indeed my college degree is in physics). It is surely possible I've made errors in my posts above, but I don't think so. I will read what I've said more carefully when I return.
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