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Old 02-13-2005, 01:29 PM   #43
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Originally Posted by pattersontoo
A fuse is considered the "weak link" in an electrical circuit. It is designed to protect equipment from damage or personnel from a dangerous electrcial shock. The NFPA and NEC considers anything over 50V, AC or DC, to be a hazardous voltage. I do not see how your system would have hazardous voltage at the levels we are dealing with in a 12VDC system. This rules out using a fuse to protect personnel.
Now, to use a fuse to protect the switch. These are just plain switches. When the insulator, (the part that is hooked to the tether), is pulled out, (when the trailer goes walk-about), then the two conductors touch. Two simple little pieces of metal hooked to two simple little pieces of wire. The alternative path to ground is completed and power is passed from the trailer's battery to the trailer brake circuit. What would you need to protect from damage in this circuit at that time?
Remember, there are only two reasons you use a fuse in a circuit:
1. To protect personnel from injury or death.
2. To protect equipment from damage.
I teach these in the Electrician Apprenticeship program I do for a number of automotive plants.
A fuse or circuit brakers IS a "designed in" weak link, done on purpose to make sure that a circuit fails in a "safe" manner if a short circuit to ground occurs.

At least in automotive (i.e. 12V) applications, a fuse or curcuit breaker is not used to protect personnel which, in any case, are of too high a resistance to trip such a circuit protection device. And in most cases in automotive applications (there may be few exceptions), the sole purpose of the fuse or circuit breaker is to protect the wiring, nothing more, nothing less. A shorted wire that is not protected by a fuse or circuit breaker may overheat with all of the negative side effects that can come from such overheating.

In the case of a short circuit in the wiring the wire resistance which can be much less than one ohm (depending upon the nature of the short) represents the only resistance to current flow. As an example only, let's say the short is 1/10th of an ohm: 12 volts divided by 0.1 ohm = 120 Amps. A typical brake control circuit will not last very long at this current level.

This is my final word on this message string (really). I will no longer point out incorrect theories about the need for (or purposes of) circuit protection devices. Each person that reads this message string can decide which "facts" they choose to beleive and wire their tow vehicle or trailer in a manner consistent with those choices.

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Old 02-13-2005, 04:10 PM   #44
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"All wires have some resistance. The larger the wire, the smaller the resistance.

Additionally, as any wire heats up, it's resistance increases, which will make the initial current drain reduce. Same as a light bulb. As the element becomes hot, it's resistance increases, therefore the current decreases "


you are correct in the strictest sense on this one. it is a series parallel circuit when you consider the resistance of the wires and connections.

that is my lineman training showing through, we are taught to consider only the resistance of devices. and not the wires connected to circuits because of the extremely high voltages i work with AND because the conductors are usually the size of your arm (not kidding!)

however, if you were to consider only the magnets as you suggest in a pure parallel circuit. there would be the same voltage drop across each magnet with only the current varying with the resistance.

the end result is the same, shorted magnet produces no braking. the remaining magnets get "some" current.

obviously the entire circuit would be in a state of overload at this point, AND adding a fuse would be a recipie for disaster if you were screaming up to a crosswalk full of kids!


you call them ferrets, i call them weasels.
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Old 02-13-2005, 06:18 PM   #45
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More or less

Maybe, more or less to this than we are seeing. Obviously the technical issues have been covered for the electrically challenged. Have you ever bought a old boat and had to replace all the wiring because of inline splices and corrosion. Not to speak of the PO thinking that speaker wire makes a good system because that’s all he had at a moments notice. Fishing being the priority LOL.
If I built brake controllers any amateur, was going to install in his vehicle, I would definitely recommend a intermittent breaker. Actually I would recommend they take it to Inland RV if they were in California. Smooth HUH Andy.
I have seen some horrible installation jobs locally, for this reason I decided to do it myself. Also breakers do fail, a friend of mine just had this happen.
Did I use a intermittent, the answer is NO. I encased the wiring from the battery to the trailer in flexible conduit. I made sure that this wiring would not see daylight in my time, nor would it see any sharp metal object on my Dodge truck.
After 32 years in aviation I have seen many a burnt wire that never gave a indication by “popping” a circuit breaker. I take that back, smoke in the cockpit or cabin is always a good indicator. Protect the wires properly.
I don’t enter this issue lightly, there is a time and place for “fused” circuits and if you doubt your, or your manufactures installation, I think you should have a intermittent breaker. If you trust the installation then I would rather see smoke, than see my family in the hospital or at the grave yard.

Regards Greg
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