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Old 01-11-2008, 01:30 PM   #1
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Fuse on battery negative

I am getting ready to change the distribution panel out since my old panel is having problems.
Is there a reason that the cable coming from the battery would have a 40 amp fuse (circled area)?
There is a 50 amp fuse on the positive cable (red cable)- which makes sense.
Just trying to make sense of everything.

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Old 01-11-2008, 03:22 PM   #2
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Quote:
Originally Posted by crispyboy
I am getting ready to change the distribution panel out since my old panel is having problems.
Is there a reason that the cable coming from the battery would have a 40 amp fuse (circled area)?
There is a 50 amp fuse on the positive cable (red cable)- which makes sense.
Just trying to make sense of everything.

Attachment 52404
There is a very positive reason why that "ground" fuse was installed.

It protects the trailer 12 volt electrical sysytem, in the event that someone however accidentally, installed the battery "BACKWARDS",
as well as someone perhaps reversing the polarity at the tow vehicle.

"LEAVE IT IN."

Andy
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Old 01-14-2008, 07:29 AM   #3
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Thanks Andy!! That's what I had suspected.
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Old 01-14-2008, 07:43 AM   #4
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Another reason

While Andy is absolutely correct, there is, in my view, a more important reason for a fuse on the negative post.
In a recent maintenance cycle on one of our large uninterruptible power supply (UPS) that serves one of our telephone switches which have two 220 amp hour batteries, the tech inadvertantly dropped a wrench on the positive pole of one of the batteries. The wrench found a ground and immediately welded itself to the case. He had the presence of mind to pull the negative connector off - fast. The battery had already started to overheat. If that had not been possible, the battery could have overheated and exploded.
We now are installing fusing on all of the negative posts in our UPS units.
In an accident where the battery compartment were to be distorted, the negative fuse could prevent some additional problems.
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Old 01-14-2008, 10:13 AM   #5
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This leads me to more questions.

The positive fuse is 50 amps. The negative is 40 amps. The previous owner allowed the owners manual (schematic diagram) of the electrical system get wet so of course I cannot read this section of the manual to find out what the exact amperage should be. Should they both be the same amperage?

Can these fuses be safely replaced with circuit breakers? The reason I am asking is because the original board is shot and must be replaced. I have a replacement board purchased but I need to do something with the large fuses and was looking for something more modern. I have seen 40 amp ATC fuses but not 50 amp fuses yet.
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Old 01-14-2008, 11:18 AM   #6
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My first suggestion would be no. A circuit breaker contiunes to reset, and may cause more damage to sensitive equipment that might be connected. Most items in the AS, (pumps, fans, incandescent lights) are not too sensitive, but radios, lights with ballasts, are. A fuse is a bit of trouble to replace, but stays blown until you find the problem. Although finding the problem might be quicker with a circuit breaker, you can follow the smoke.
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Old 01-14-2008, 11:56 AM   #7
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my solution

Our local marine store has some really cool surfac3 mount fuse blocks. They are waterproof when they are properly closed and installed. Those are my picks.
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Old 01-20-2008, 08:51 PM   #8
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Your local truck and commercial (flatbed) trailer service center will have a modern 50-amp circuit breaker specifically made for commercial haulers. They cost about $60 but when they trip, they stay tripped until manually reset. And a little arm pops out to show you that it has tripped. I would think this is the best way to go and I am installing them to protect my batteries. They have threaded posts so that you can screw down your battery cable right to the breaker.
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Old 01-21-2008, 07:28 AM   #9
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While the focus of this link is for Mobile two way radio (Ham / Amateur Radio) it gives a bit more detail on why you should have Fuses, positive and negative wires, close to the battery.

Radio Telephone / Mobile Radio Installation Guidelines


--Dwight
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Old 01-21-2008, 07:36 AM   #10
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Quote:
Originally Posted by jdalrymple
My first suggestion would be no. A circuit breaker contiunes to reset, and may cause more damage to sensitive equipment that might be connected. Most items in the AS, (pumps, fans, incandescent lights) are not too sensitive, but radios, lights with ballasts, are. A fuse is a bit of trouble to replace, but stays blown until you find the problem. Although finding the problem might be quicker with a circuit breaker, you can follow the smoke.
Jeff,

You're thinking of the 'auto-reset' type of breaker, like the ones used on the power feed from your TV battery to the brakes. Most 'normal' DC breakers do not re-set when they are tripped.
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Old 01-21-2008, 09:13 AM   #11
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The fuse to the battery negative lead is often termed a "catastrophic fuse", as in, something goes catastrophically wrong and that fuse saves your bacon! It should definitely be there.
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Old 01-21-2008, 09:37 AM   #12
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not really paranoid...

After speaking to the head electrician here, he opineed that having the "Catastrophic Fuse" (CF) as close to the negative battery terminal as possible, is the best practice. Apparently, the best protection is a fully sealed negative battery terminal terminal connector that has the fuse right in it. The fuse is replacable. I asked him where I could find a source and he had no suggestions. Dang, I thought this would be easy.
Still, the most conservative approach from above would only come into play in an accident where the battery compartment becomes distorted. The compartment in the Arosy is plastic. So, the fuse on wire between the fuse block and the negatie battery terminal is completely adequate for just about any senario of failure of the system inside the camper. Whew! I feel much better now!
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Old 01-21-2008, 09:45 AM   #13
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I owned several RV and Airstream is the first one that doesn't have the negative cable bolted directly to the frame.
So IMHO 99% of manufacturers consider "Catastrophic Fuse" unnecessary.
You can look on it from at least 2 angles:
-is Airstream manufacturer concern about owners safety more than others
-or does the manufacturer consider the owners idiots?
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Old 01-21-2008, 03:13 PM   #14
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Originally Posted by Kajtek1
I owned several RV and Airstream is the first one that doesn't have the negative cable bolted directly to the frame.
So IMHO 99% of manufacturers consider "Catastrophic Fuse" unnecessary.
You can look on it from at least 2 angles:
-is Airstream manufacturer concern about owners safety more than others
-or does the manufacturer consider the owners idiots?
But do they have a product that is so able to conduct electricity to so much surface area? Body shell inside or out . lots of area to get a shock from. Better to cover all area's nothing is idiot proof they always make a better idiot
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