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Old 02-07-2018, 01:27   #1
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Fusing

Hello all.....
I have a 150 amp alt I am installing.cables are 2/0 from alt to battery in engine compartment then to arc to start battery. Run is 4 ft from alt to house then 11 feet to start battery with arc between. I wii be using mrbf terminal fuses.

Thanks!
Greg
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Old 02-07-2018, 03:19   #2
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Re: Fusing

So, what’s the question?

To be safest, you need an appropriate fuse at every current generating device... ie alt and batt.... the fuse at the battery end needs to be sized so that a short will blow it with battery current, but without nuisance blowing.... so if your battery can generate 1000 amps, but the wire is good for only 300, then the fuse needs to be about 275 amps. The fuse at the alternator end needs to be sized to protect both the wire AND the alternator- say the alternator is good for 100 amps, same 300amp wire, you probably want a 110 amp fuse.
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Old 02-07-2018, 04:19   #3
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Re: Fusing

Its a mental block I guess I am having when it comes to picking the appropriate size fuses. The batteries currently have nothing. I am leading my new alt directly to the house bank... its a 150 amp alt so at the battery I will put a 200 amp terminal fuse. However I am a bit confused with from battery to battery switch. I have 2 lifeline agm 4D's with 2/0 battery wires.

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Old 02-07-2018, 04:44   #4
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Re: Fusing

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Originally Posted by mickt243 View Post
So, what’s the question?

To be safest, you need an appropriate fuse at every current generating device... ie alt and batt.... the fuse at the battery end needs to be sized so that a short will blow it with battery current, but without nuisance blowing.... so if your battery can generate 1000 amps, but the wire is good for only 300, then the fuse needs to be about 275 amps. The fuse at the alternator end needs to be sized to protect both the wire AND the alternator- say the alternator is good for 100 amps, same 300amp wire, you probably want a 110 amp fuse.
Dissenting view - the alternator end does not need to be fused (providing the wire can safely carry the maximum rated output of the alternator). Alternators are a limited current supply, they cannot deliver much more than the maximum rated output.

Of course the battery end of the same wire does need circuit protection.
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Old 02-07-2018, 05:10   #5
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Re: Fusing

I understand what you are saying about the alternator.... only max getting 150 amps and a 2/0 wire can easily handle that. When you say "end" you mean at the alternator power out stud and at the battery stud so that if there where a problem with the source of power (primarily battery), the fuse would blow so the wires leading from out not melt and cause a fire. Just talking my way through.... for some its a very simple concept.... I just want to make sure I fully understand and can protect where ever is needed.

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Old 02-07-2018, 15:30   #6
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Re: Fusing

^^ yes, that is what I mean by "end".

Circuit protection should be placed as close as practical to the stud / terminal of the power source.
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Old 02-07-2018, 15:59   #7
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Re: Fusing

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^^ yes, that is what I mean by "end".

Circuit protection should be placed as close as practical to the stud / terminal of the power source.
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Old 02-07-2018, 16:06   #8
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Re: Fusing

I don't get why Balmar says " don't fuse the alternator". I get it that if you were to fuse for the wire, it would be a big fuse, way bigger than the alternator output. I have a # 4 wire for a 100amp Balmar. What is the harm of having a 150 amp fuse in the alternator cable? Imagine an alternator having a meltdown without a fuse. Somebody said there is almost no chance of something like that happening, but it is a boat. I can't imagine an aircraft not having a fuse for a generator. It just seems prudent.
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Old 02-07-2018, 16:54   #9
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Re: Fusing

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I don't get why Balmar says " don't fuse the alternator". I get it that if you were to fuse for the wire, it would be a big fuse, way bigger than the alternator output. I have a # 4 wire for a 100amp Balmar. What is the harm of having a 150 amp fuse in the alternator cable? Imagine an alternator having a meltdown without a fuse. Somebody said there is almost no chance of something like that happening, but it is a boat. I can't imagine an aircraft not having a fuse for a generator. It just seems prudent.
What failure mode of a 100 amp alternator could produce greater than 150amps for a period long enough to blow the fuse? I can't see how it could occur.
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Old 02-07-2018, 16:57   #10
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Re: Fusing

I was reading on a thread and it was saying by the time the alternator got to the point of fire different components inside would have melted basically shutting it down.
In that same light the main shore charging system.... that is fuses at the battery.....

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Old 02-07-2018, 17:21   #11
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Re: Fusing

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I don't get why Balmar says " don't fuse the alternator". I get it that if you were to fuse for the wire, it would be a big fuse, way bigger than the alternator output. I have a # 4 wire for a 100amp Balmar. What is the harm of having a 150 amp fuse in the alternator cable? Imagine an alternator having a meltdown without a fuse. Somebody said there is almost no chance of something like that happening, but it is a boat. I can't imagine an aircraft not having a fuse for a generator. It just seems prudent.
A couple of points to consider.

Alternators tend not to melt down - more likely the diode pack goes open circuit thus preventing further current to flow.

If a fuse on the output does fail (open circuit) for whatever reason while the alternator is delivering current, the diode pack will blow. We all know never to disconnect the output of a working alternator .

Many alternators have thermal protection, they start getting too hot, the output is throttled back.

Really it is a case of managing potential failure modes, the likelihood of failures and the ramifications of various failures. Balmar have presumably done such analysis for us.

The aircraft generator is a completely different animal, no semiconductor devices in its output circuit, potentially large current in the brushes, field and stator windings and shaft drives (usually) to consider. All different risks to consider (and manage).
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Old 02-07-2018, 17:41   #12
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Re: Fusing

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What failure mode of a 100 amp alternator could produce greater than 150amps for a period long enough to blow the fuse? I can't see how it could occur.
How about the plastic insulator at the + lug? A loose connection will heat it right up. The diode pack could melt to ground. The bearings/bushings could fail and the rotor could explode. I don't know, I would rather replace a diode pack than the boat. My boat has a bunch of amps and that could cause a big fire.
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Old 02-07-2018, 17:50   #13
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Re: Fusing

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A couple of points to consider.
The aircraft generator is a completely different animal, no semiconductor devices in its output circuit, potentially large current in the brushes, field and stator windings and shaft drives (usually) to consider. All different risks to consider (and manage).
Sorry but I used the term generator because an alternator is a AC generator. Everything has a fuse.
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Old 02-07-2018, 19:16   #14
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Re: Fusing

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How about the plastic insulator at the + lug? A loose connection will heat it right up. The diode pack could melt to ground. The bearings/bushings could fail and the rotor could explode. I don't know, I would rather replace a diode pack than the boat. My boat has a bunch of amps and that could cause a big fire.
Hmm... interesting scenario however I still fail to see how have a fuse at (near?) the output of the alternator would save anything here. It seems (to me?) the potential failure(s) as described are all internal the the alternator and any external fusing would not prevent them. Happy to be shown wrong though!
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Old 02-07-2018, 19:48   #15
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Re: Fusing

The idea of fire scares me. A direct short from the alternator frame to the block with no fuse and 4 8D's feeding the flames seems a little out of control. My boat is flamable.
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