It’s all relative!
In several posts we have talked about measuring voltage with reference to a fixed point, or relative to a fixed point. This is an essential concept to understand if you are going to use a voltmeter to measure any kind of voltage. I used the analogy of a distance from home. While we have compared voltage in the past with pressure, in this case we compare it to distance from a fixed point. Both comparisons work, but in this case it is easier to understand the distance comparison.
Let’s look at a practical example of this to better understand why we do it. A very common component in an electrical system is a fuse.
Fuses work to protect electrical circuits by “blowing” if they experience an overcurrent situation, meaning more amps flow through the fuse than the fuse is rated for. When this happens, the fuse “opens” and current can no longer flow because of the open circuit. The fuse now has infinite resistance. Our job is to use our multimeter to find the blown fuse. In order to do this the easiest check is with a voltmeter. I have reasons for saying this and we will talk more about this later, for now, just accept this concept too. In order to check for a blown fuse, you should pick a spot that is your “anchor” or starting point. Typically in an RV electrical system, and when checking for a blown fuse in the 12 volt DC system, the anchor point is “ground“. Ground in an RV is usually the frame of the vehicle. This is because the negative post of the RV battery is connected directly to the frame. Connect one of your meter leads to ground. Again, this may be easier said than done because there is a lot more plastic in RVs than there is metal. I usually try to find a big bolt that holds large metal pieces together. After you connect one meter lead to ground you are ready to check voltages. Find a fuse holder with a fuse in it and check the voltage on each pin of the fuse. You should EXPECT to read somewhere around 12 volts DC at each pin. Now remove the fuse from the fuse holder. This simulates a blown fuse. Now when you check both pins of the fuse holder, you will find that one pin will have the expected 12 volts DC, but the other pin will read 0 volts DC. This is exactly the check you will make to find a blown fuse.
Take time to practice this technique and we will talk more about it later. Until then, have fun camping.
Larry