in 110 VAC, EVERYTHING attached to, or sitting on the ground, is a return path for the voltage. Including you if you’re even slightly grounded. On (almost all) vehicles, the return path is the chassis, so anywhere the positive wire touches is a potential short circuit that should blow a fuse. I always have this explanation of moving heat through refrigeration-less heat means less “vibration” of the cooled area, so heat “flows” to the cooler area, the cold is the absence of vibrating molecules. Kind of like electrons flowing from negative to positive.
Why is the fuse on the positive side?
- Thread starter Elkins45
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Isn't that what I said?
dnewton3
Staff member
KJSmith explained this correctly, above.
But I'll give an actual example such that the clarity of his statement makes sense. Consider the following:
- we are discussing a traditional, 12v, DC, negative ground vehicle (aka "chassis ground" - meaning any conductive part of the chassis is bound to the "-" battery post via one or morel ground straps at various locations
- we are NOT discussing household electrical loads; that's not what the OP was asking about
- you have an electrical load with 15 feet of wiring going from the "+"battery post to the positive side of the load (eg: let's say the "load" is a blower motor for the HVAC)
- that same electrical load has only 2 feet of wiring going to the negative side of the load (because it's a negative ground, it does not need to go all the way back to the battery, it just "grounds" to the firewall or a bulkhead). This is a major cost and weight savings by not having ground wires all run back to the battery; the conductive parts of the chassis are acting as part of the negative wire circuit.
Two conditions could potentially occur to create an electrical problem in regard to vehicle wiring:
1 - breach of the positive side wiring ...
In this example, anywhere along that 15' of "+" wire that would have a bare spot where the positive wire touches any conductive ground spot would cause a short, potentially risking a damaged load device or fire (or parasitic draw for low-load conditions). There exists a myriad of places the wire can be pinched or rub against a sharp spot that would then allow the bare positive wire to "ground out" and cause a problem. In this example, once a positive wire is compromised and connects directly to ground, it no longer needs that 2' of negative wire to complete the circuit. So placing a fuse on the positive side of the circuit stops the flow of electricity if it becomes compromised and blows (melts) the fuse. In fact, the closer you put the fuse to the battery the better, because it protects more of the wiring; if you put the fuse on this side within 2' of the battery, then 13' feet of the remaining "+" wiring are protected, as well as the load itself. Fusing the positive wires as close to the battery as possible protects against the greatest potential for shorts anywhere downstream of the fuse.
2 - breach of the negative side wiring
In this example, none of that 15' of positive wire, nor the load itself, are protected. If you "fuse" the 2' of negative wire, that does not stop an additional short from developing elsewhere along the 15' of positive wire. Fusing the negative side on a negative ground system can stop the flow, but only until a short would develop on the positive side and the circuit would be re-initiated (without any fuse protection).
Hope that helps you understand the "why" a positive side fuse is proper. (The closer to the power source, the better).
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BTW, the concept of breaking the circuit on the supply side of the electricity is the same in household electrical wiring. DC has a positive and negative. But AC has two legs of single phase potential (split by the neutral). Don't think of it as positive or negative; think of it as hot and ground/neutral. With rare exception, you always put the protection device (fuse or breaker) on the supply side ("hot"), as close to the actual supply as possible. In a car, it's in the red wire "hot" side, close to the battery. In a house, it's on the black wire "hot" side in the panel box where the power comes into the structure. This way the supplied power is protected from downstream breach of the wiring.
But I'll give an actual example such that the clarity of his statement makes sense. Consider the following:
- we are discussing a traditional, 12v, DC, negative ground vehicle (aka "chassis ground" - meaning any conductive part of the chassis is bound to the "-" battery post via one or morel ground straps at various locations
- we are NOT discussing household electrical loads; that's not what the OP was asking about
- you have an electrical load with 15 feet of wiring going from the "+"battery post to the positive side of the load (eg: let's say the "load" is a blower motor for the HVAC)
- that same electrical load has only 2 feet of wiring going to the negative side of the load (because it's a negative ground, it does not need to go all the way back to the battery, it just "grounds" to the firewall or a bulkhead). This is a major cost and weight savings by not having ground wires all run back to the battery; the conductive parts of the chassis are acting as part of the negative wire circuit.
Two conditions could potentially occur to create an electrical problem in regard to vehicle wiring:
1 - breach of the positive side wiring ...
In this example, anywhere along that 15' of "+" wire that would have a bare spot where the positive wire touches any conductive ground spot would cause a short, potentially risking a damaged load device or fire (or parasitic draw for low-load conditions). There exists a myriad of places the wire can be pinched or rub against a sharp spot that would then allow the bare positive wire to "ground out" and cause a problem. In this example, once a positive wire is compromised and connects directly to ground, it no longer needs that 2' of negative wire to complete the circuit. So placing a fuse on the positive side of the circuit stops the flow of electricity if it becomes compromised and blows (melts) the fuse. In fact, the closer you put the fuse to the battery the better, because it protects more of the wiring; if you put the fuse on this side within 2' of the battery, then 13' feet of the remaining "+" wiring are protected, as well as the load itself. Fusing the positive wires as close to the battery as possible protects against the greatest potential for shorts anywhere downstream of the fuse.
2 - breach of the negative side wiring
In this example, none of that 15' of positive wire, nor the load itself, are protected. If you "fuse" the 2' of negative wire, that does not stop an additional short from developing elsewhere along the 15' of positive wire. Fusing the negative side on a negative ground system can stop the flow, but only until a short would develop on the positive side and the circuit would be re-initiated (without any fuse protection).
Hope that helps you understand the "why" a positive side fuse is proper. (The closer to the power source, the better).
***************
BTW, the concept of breaking the circuit on the supply side of the electricity is the same in household electrical wiring. DC has a positive and negative. But AC has two legs of single phase potential (split by the neutral). Don't think of it as positive or negative; think of it as hot and ground/neutral. With rare exception, you always put the protection device (fuse or breaker) on the supply side ("hot"), as close to the actual supply as possible. In a car, it's in the red wire "hot" side, close to the battery. In a house, it's on the black wire "hot" side in the panel box where the power comes into the structure. This way the supplied power is protected from downstream breach of the wiring.
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The neutral in a GFCI is actually connected to ground in the breaker panel, but a GFCI can sense ANY leakage to ground, or to neutral. I even run into 3 phase GFCI breakers that fault/trip on unbalanced loads between phases, not to ground.
A GFCI does not use a " third wire " . It monitors the current between hot and neutral . No green wire used nor needed .
Because YOU would become the ground in that circuit if there was an issue.
No. It is close but a misunderstanding of how it works. Close=Dead in the electrical world.
No. It is close but a misunderstanding of how it works. Close=Dead in the electrical world.
A GFCI does not trip if all the current that goes out on the "supply" wire comes back on the "return" wire. Current can flow anywhere downstream of the point the GFCI monitors, including through you, as long as it all flows back to where it is supposed to be downstream of the GFCI.
Correct but a misuse of terms.
I am using the terms supply and return to simplify the description of how a GFCI works. There are many forms of ground fault monitoring that do not necessarily use a "neutral" or a "hot". A GFCI only monitors the current going into a circuit and returning from a circuit. Voltage, phase and number of wires is of no consequence.
you're getting a bit confused between the electron flow from negative to positive outside of the battery.
Most important is that everybody does the same thing, so you don't need to make 2 types of everything. Important also that you fuse the battery side, and not the ground side so that you can prevent that an unfused section of wire can touch ground.
Positive earth was something to do with reducing corrosion at the battery terminals but that just moved the problem to the body. No British cars were made positive earth after something like 1965.
I understood that from the beginning. What I’m saying is that in a negative ground the entire body of the car is connected to the high side of the potential difference, whereas on a positive ground the car body is connected to the return side. The fuses would still be placed as close to the battery as possible, just on the negative side in this case.
But that doesn't matter, high or low. You make it common and everything is relative to that. It's were you zero your scale so to speak. You touch the body so you are at that potential at all times. negative or positive voltage is just as bad for you if you touch the other side (not very if we are talking 12V)
Say you have a bucket that you need to fill, you can set the scale to 0 with the empty bucket on it and fill until you reach the weight you want , or you set it to 0 with a full bucket and have negative numbers when the bucket isn't full.
Many many years ago, I saw a young man trying to test the lights of a trailer, and he was stumped because he could not get any lights to come on. He had ran a long wire from a trucks battery positive and was using that to power pins of the trailers connector. But because the truck was a decent distance away, he tied the metal chassis of the trailer to a nearby metal garbage can with a wire for the "ground". He asked me why what he was doing did not work. I could have explained it to him, but instead wanted him to figure it out on his own. So I said "Airplanes have radios and therefore a battery to power the radio. Where is the ground for the radio on an airplane?"
My brother in law used to take a generator to the beach and ground it to the frame of his truck ....
Conventional Current:
- Direction: Flows from the positive terminal to the negative terminal of a power source.
- Why: This was defined before the discovery of electrons. Early scientists guessed the flow direction of charge, assuming positive charges were moving.
Electron Flow:
- Direction: Flows from the negative terminal to the positive terminal.
- Why: Electrons, which are negatively charged, are the actual moving particles in most circuits. They are repelled by the negative terminal and attracted to the positive terminal.
Key Point:
- Both are valid ways to describe electric current.
- Electrical engineers and circuit diagrams usually use conventional current (positive to negative).
- Physicists, when focusing on the physical movement of electrons, discuss electron flow (negative to positive).
That’s exactly backwards from my experience. I’ve never seen a physics textbook, either HS or college that didn’t use conventional current.
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