Install Fuse: Positive or Negative?

[fayan77]

hello, I am trying to wire up an LED light bar on my car, but I'm having trouble understanding why the fuse and on/off switch "MUST" go on the positive terminal. To start off a fuse is technically is a switch protecting our loads and will open the circuit (burn) when there is an overload. Current (electrons) moves from lower to higher potential, hence from the negative side of battery to positive. That being said, my logic is that a fuse can be put on either terminal of the circuit, same with on/off switch since it will open the circuit.

 

[davenn]

To start off a fuse is technically is a switch protecting our loads and will open the circuit (burn) when there is an overload.

yes

Current (electrons) moves from lower to higher potential, hence from the negative side of battery to positive

yes

That being said, my logic is that a fuse can be put on either terminal of the circuit, same with on/off switch since it will open the circuit.

yes they can, it's just convention to put them in the positive line, possibly partly due to the fact that the negative can often be connected to the chassis of a device or a car/other vehicle ( there is only a couple of makes/models that used positive earth/chassis. so putting them in the chassis connected negative rail is impractical as it is the multiple positive leads that go throughout a vehicle to the different places needed and it's those individual positive leads that need their specifically rated fusesDave

 

[anorlunda]

@davenn gave you the right answer, but let me add a bit about consequences.

The purpose of a fuse is to protect your circuit from damaging short circuits. In a car, short circuits happen most often when the insulation on a wire is worn away and the wire touches something else at a different voltage. The most likely thing to touch is the chassis.

The minus wire is already at chassis voltage, so not much would happen if it does short to chassis. But the plus wire would cause battery voltage to be shorted to the chassis, melting the wires.

That is why the plus needs fuse protection and the minus doesn't.

 

[fayan77]

Ok i follow, now i guess my question is, why does minus side of battery need to be grounded? I understand its a pathway for electron to reach ground instead of somewhere else but why does it need to be grounded, a regular 1.5 v battery has plus and minus side, that battery doesn't seem to be grounded when put into use.

 

[anorlunda]

Ok i follow, now i guess my question is, why does minus side of battery need to be grounded? I understand its a pathway for electron to reach ground instead of somewhere else but why does it need to be grounded, a regular 1.5 v battery has plus and minus side, that battery doesn't seem to be grounded when put into use.

If everything was in perfect condition, and if there were no secondary effects like induced voltages, you could make the car's electric system float isolated with respect to the chassis. But there are realities and secondary considerations.

Also some parts like the starter motor, are designed to return their current via the engine block and chassis rather than having a return wire.

Some day we may have composite bodies and engines, with no metal parts. Then we'll have to rethink how the electrical system is designed.

 

[davenn]

Ok i follow, now i guess my question is, why does minus side of battery need to be grounded? I understand its a pathway for electron to reach ground instead of somewhere else but why does it need to be grounded, a regular 1.5 v battery has plus and minus side, that battery doesn't seem to be grounded when put into use.

Don't get too hung up on the word grounded as the car chassis obviously ground ... because of the rubber tyres
It's just a "lazy" use of the word ground. It would correct just to say that this vehicle has a negative chassis
Remember up the page I said some cars/vehicles have a positive chassis ... the old VW Beetle was one from memory

Also some parts like the starter motor, are designed to return their current via the engine block and chassis rather than having a return wire.

this comment from Anorlunda is spot on ! it means only one cable carrying the large 100A or so currents is needed, not two, this also cuts down on voltage drop during those times of high current drain.

Also having the chassis as one side of the power distribution system halves the amount of cables needed to be run throughout the vehicle as you can pic up the negative side anywhere in the vehicle wherever you have access to the chassis/body of the car
Dave

 

[rbelli1]

Also some parts like the starter motor, are designed to return their current via the engine block and chassis rather than having a return wire.

Most of the devices in your car use the free conductor formed by the steel chassis and body for their return path. This is done for economic reasons as well as safety reasons. If every load had a copper ground return you could end up with very high current (accidental or otherwise) ground loops through the small conductors from small loads.

BoB