By Charlie Nowlin
Give a man a fish, and he eats for a day...TEACH him to fish, and he eats for a lifetime!
The switch can be either a mechanical on - off , or a relay.
A break anywhere in the circuit is an "OPEN "circuit
It is the Device, (or "load"), that adds to the circuit, a quantity of resistance, needed to
absorb the energy of the current. Each device's load can differ. The load, uses the energy, and converts it into heat.
This is why a fuse doesn't blow the instant the switch is turned on, which it would as you will see shortly.
In this case, the current has found a path to the source, before the fuse. Since the current in this case doesn't have to flow through the fuse, the fuse does not blow, as it is effectively no longer in the circuit, which shown here, is from the Pos battery terminal to the Neg terminal only.
NOTE: Automobiles, use the chassis, as the common, or return wire, as this is less expensive than running a return wire for each device. The proper term for an electrical wire, is "CONDUCTOR". ......So, the "chassis" is also a conductor, because it acts as the return wire.
"SHORTS", are caused by:
A) Wiring connected to an incorrect terminal, that is common to the chassis,
B) Bare , current carrying conductor contacting a "common" or "return"
These are the ONLY cases for a "short circuit".
In the above case, a malfunction inside the device, allows current to travel to the common, or return. See B) above. When doing so, the instant the switch is turned on, the fuse blows.
Reason?? No load in the closed circuit which equals very low resistance = short circuit
Here I have added a symbol, for the chassis common, three horiz lines, spaced closely, with the shape of an arrowhead. This means, that All the places with this symbol, are electrically the same point. It is used, to help make the schematic clearer, and less confusing. You can imagine, what a schematic would look like with hundreds of lines which could be eliminated with a symbol.
Add fuses, and more devices and switches , and you'll get what starts to look like, the complicated wiring schematic of the Automobile. but once you know the basics, you can move to the circuit in question, and easily troubleshoot it.
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