Jana wrote:We came to this conclusion about the fuse by extensive ellimintion. There was simply no place for a short. The ground and cables to switch were 4GW commercially crimped. The cables tested out fine. By the end, the only thing connected to the battery was the ground, the fuse, and the switch. Every time the switch was turned on the 100 amp fuse blew. This happened 6 times. We had the battery tested and he said it was a really good battery. Our neighbor who is an electrical engineer said everything looked like it was hooked up right. I drew out the diagram of the system for the guy at the battery store. He was the one who suggested that with this large of a battery that maybe we needed a 200-250 amp fuse. When we used the larger fuse the fuse did not blow. we then hooked up the blade fuse box the ground busbar and all the electricity worked with nothing shorting out. Our battery is designed as a large solar storage battery, it seems like it is designed to move a lot of energy. We have 2 friends who are electrical engineers and they both initially thought there must be a short--neither of them seemed concerned about a possible fire when we told them what finally worked.
This is meant to be helpful....
The only thing that will 'blow' a fuse is a 'shorting bar'....let me explain.....on your battery you have two posts....or connections.....as long as something is not connected 'across' those posts....i.e. 'shorting bar' for ease of visualization.....there is no 'current' flow.....all the 'potential' i.e. big battery that you can possibly put in the trailer won't blow 'any' fuse.....unless something creates a 'circuit'......a closed circuit.....i.e. a short......or a circuit approaching a short....resistance is inversely proportional (as shown in formula below)....so resistance has to be decreased to blow a fuse....conductivity has to increased.....and the shorting bar has to survive the process.....a little wire won't survive 100 amps....it will burned 'open'....and become your new 'fuse'....(that is how fuses are made by decreasing the size of fuse wire inside the glass allowing the smaller diameter wire to burn open at a lower current 'amp rating')...this is why one does not do what you were told to do......you have just created a 'whole' system of 'fuses'......while, your 200 amp fuse will definitely survive up to a new level.....either 'all' of your wires burning open....or if you increase the wire size in more places, enough to carry more current than is safe.....everything will burn.....to then create an open circuit......leaving the battery and the 200 amp fuse possibly intact.....
The capacity 'how big the battery is' won't 'let out' a 'big' current......different between blowing a 100 amp fuse or 200 amp fuse.....unless the resistance of the 'shorting bar' is sufficient to change the formula (E = I/R).....E= Electromotive Force 'EMF' (Voltage), I= Current (Amps), R= Resistance (Ohms)
Potentially exaggerating....the only thing on a trailer that will make a different between a 100 amp fuse and 200 amp fuse blowing 'opening up' ' burning up would be to put a section of the trailer frame 'as' the 'shorting bar'.....(that short only has to be momentary....like pressing against a switch and something on the back of it touching something it shouldn't....)
I would look for some evidence of arching.....some little black spots somewhere on the trailer frame....probably under something electrical.....near 'like a hair' away from a connection that is resting on the trailer frame....
More info:
The battery should never determine the fuse size, neither should the total of the loads.....the wire size of each branch is the limiting factor, as the wires will become fuses.....this is why in your house.....you have different wire size and outlet hardware for 15 amp or 20 amp or 30 amp circuits......these are branch circuits.....lets say you have either a 100 amp or 200 amp box on your house.....if you total up all the 'branch' circuits, the smaller circuit breakers.....i.e. fuses....they usually add up to more than your distribution panel main breaker......it still is totally safe.....as each branch is protected for what it will handle, and the 'main' is protected for 'too' many being added as 'load' at one time.....this way 'no' wires burn up......
So......fusing.......fuse size or breaker size......starts at the branch load......NOT at the power source....(battery)
If you only protect the battery.....you are not protecting the loads......but.....if you protect the individual loads......you are then protecting the battery.....
a little background.....I worked with A/C up to 3 phase 440, and 12v automotive and 28v aircraft....in past lives.....I would recommend that you have someone 'who' knows, not thinks he knows about proper fusing....
Now.....why is the big fuse a NO-NO......in the far off chance that a wrench or something 'that' can actually pass close to 200 amps.....should that 'shorting bar' fall across a positive and negative potential anywhere in your system.....it will create a 'new' branch circuit that your system of wiring will attempt to provide all the amps that your wrench will pass....(how much it will heat up).....and chances are the wiring in your camper will 'burn' open.....literally.....
So.....every wire.....whatever size it is....that is run in your camper should have a fuse of a size that will fail before the wire in the circuit does....while carrying the load.....to self check.........do you have such a fuse between every appliance in your camper? or is the only fuse a 200 amp fuse? Does that make it simpler to understand?
None of what I have written is based on code....based on engineering....based on anything you should take to the bank....I would double check it all if it interests you.....this is what I do.....
Craig