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Old 16-05-2005, 12:34   #1
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Fuse Protection

Heres another basic question, If I am wiring a boat to a breaker pannel, are fuses also needed on the circuits for cabin lights, mast lights ect. I assume inline fuses should be placed for electronic equipment, VHF, Depth, GPS, ect. I dont understand why you would need both fuse and breaker? Boat is 26' to be used on inland lake.
Any clarification?
Thanks Ted

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Old 16-05-2005, 13:25   #2
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The Circuit Breakers, in a Panel, protect the downstream wiring - provided the Breaker Ampacity does not exceed the Wiring Ampacity. (Ie: a 15 A Breaker protects #14 AWG wire or larger).
You probably donít require additional in-line fuses to protect robust equipment, such as lighting, pumps, etc.
Additional fuses are often utilized with low-current draw electronics, to protect the equipment (ie: 15A Breaker, #14 AWG Wire, and 2 A Equipment would want a 2 or 3 A in-line fuse).

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Old 16-05-2005, 16:26   #3
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Ted, I would answer your question just a little differently.

Typically, breakers serve to protect the boat from the entire circuit by providing ultimate protection of larger loads - e.g. the port or starboard (or on a boat your size, perhaps the entire) lighting circuit.

However, any piece of equipment that's rated to draw considerably less (let's say 2/3 or less) of the rating of the breaker probably needs to be protected. A good example is a marine refrigeration unit. Typically, these are hooked up with heavier gauge wire (to avoid voltage drop which would affect the unit's efficiency) to a 15A breaker, and yet they will have an onboard fuse of perhaps 10A, to protect the control circuitry and onboard wiring. Another example that's perhaps applicable to your boat size is when you may have an 'Auxiliary' circuit, tied to a 10A or 15A breaker, but which supports a variety of electrical items, none of which draw anywhere near that 10 or 15 amps. E.g. you may want to power a tiller autopilot, a cockpit and/or anchor light, and an AM/FM/CD radio on this single circuit. The amp rating of the breaker would easily support all these things when used concurrently but you'd want to protect them, individually, with fuses that are each sized appropriately to the hardware. Usually, the owner's manual is the best guideline for choosing a fuse rating; sometimes I've been very surprised by what is recommended, and not always in the same direction.

BTW a great source of info for all of us 'learning' sailors/boatowners is Nigel Calder's _Boatowner's Mechanical and Electrical Guide_, which is heavily illustrated, cleanly/clearly written, and provides all kinds of practical pointers that address questions like the one you asked. If you find it a bit pricey, remember you'll be using it for years...and you can buy it second-hand for a 'softer' price. Be sure it's the 2nd Edition, however.

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Old 16-05-2005, 19:58   #4
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Think of it this way. A tree and it's branches. The root is the battery power source. The trunk is the main feed to all other curcuits. It needs protection via a main isolater/large fuse. The best choice is a circuit breaker which can be used to do both jobs. That trunk(main cable AND breaker) needs to be big enough to support all the branches when in full leaf and the wind is blowing. (full load plus a little to spare). BUT, the breaker needs to be able to protect the roots(batteries) should the wind become a storm. Thus it needs to loose the trunk before the trunk destroys the roots. Following so far?? The brances need to be fused to the trunk in such away, that if the wind blows to hard, the branch breaks off so as the trunk and roots are protected. But the branc doesn't want to be connected so stronly that the storm blows the tree over becuase the branch didn't break away first. Does all that make sense??
So yes, you should fuse every thing. Every circuit should have some form of protection that protects the cable NOT the appliance on the end of it. If a major fault develops in an appliance, then the fuse rating of the appliance would be prefered, to enable as little damage to that appliance as possible. However, it is the supply cable that is of ultimate importance. If the cable can only handle say 5A and you have a fuse capable of handling 15A, then the cable is going to become the fuse with possible disarstrouse consequences.

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Old 16-05-2005, 22:44   #5
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Fuses and breakers

Breakers, in general, are slow-blow devices in that they take perhaps minutes to open when operated at their designed current rating. Many breakers are rated to open after several seconds at 110% of their load rating and ambient temperature. Breakers are also often rated to open with a specified size and minimum length of cable or bus bar attached. The attached conductors add in to the calculation of the current versus time over which a breaker can be relied upon to open.

The two types of breakers most often encountered are either thermal (cheap) or magnetically activated (usually more expensive). It is often said that thermal breakers do not belong in a marine environment where wide ambient temperatures can affect the current for for which the breaker will open. Magnetic breakers open when the magnetic field caused by the current passing through it reaches a predesigned value.

Fuses are easier to manufacture as "fast blow" devices. Fast blow fuses are often needed to protect equipment from destruction or fire. Slow blow fuses are used for branch circuit protection (when you don't have the money or convenience of mounting a breaker). Slow blow fuses are also used to prevent nuisance blowing when operating electric motors and loads which exhibit time varying load currents.

There is a lot more to fuse and breaker technology, especially when attempting to prevent wire overheating without nuisance opening of a circuit in specific applications. Whether or not a fuse or breaker is rated as "slow-blowing" has to do with the square of the load current versus the time of operation before the device is rated to open the circuit. This is called "the I-squared-tee" curve which describes how to predict fuse or breaker operation with various loads and times.


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Old 17-05-2005, 10:07   #6
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Well...I'll bet Ted is r-e-a-l glad he asked us this question!

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