Motorhome Electrical Systems continued --
The following are typical symbols for common varieties of switches.
Just as with the symbol for fuses, use whatever symbols work for you, making sure your diagrams can be understood by others, if necessary, and yourself long after you're done working on the circuit. (I can't count the times I've gone back to a diagram -- or piece of software code -- years later and tried to recall what everything meant … or what the **** I was thinking when I designed it.)
Because OFF-ON tends to be more understandable, many vendors use that rather than SPST or DPST. In those cases, you will often see OFF-(ON), with the parenthesis indicating momentary contact. For example, in a boat with both automatic and manually controlled bilge pumps, people often use an ON-OFF-(ON) switch. (For those who also own boats -- don't nitpick, this is a simplified example to demonstrate a possible practical application for a less common type of switch.)
So, OFF-(ON) is a simpler and clearer N.O. momentary.
There is one type of switch that on the surface appears to have no practical application. It's a double pole OFF-OFF-(ON) / OFF-ON-(ON) switch and is actually used as a OFF-RUN-START switch for engines such as in a genset. The first pole -- OFF-OFF-(ON) -- is for energizing the starter and the second pole -- OFF-ON-(ON) -- for energizing the motor's ignition system. In the "middle" position, the ignition system is on and the starter is not. In the "right," momentary position (opposite the OFF position) both the starter and ignition circuits are energized.
Note: We'll get into it later, when we're done with the Coach System and shift to the Motor Vehicle System, but right now it's worth mentioning the switch in the preceding paragraph (above) is essentially the same as the key switch in your motorhome. The difference is your key switch has a fourth position -- ACC -- for energizing some motor vehicle circuits (such as the radio) without energizing the engine's ignition.
It's important to first consider the logic (i.e., what you want the circuit to do and when) before trying to find switch (and relay) configuration that does what you need.
Also, most people tend to assume the +12VDC supply connection should be made to the center terminals on a xPDT switch but it's often valid to do the opposite. The following diagrams demonstrate this.
The top example illustrates using a single source to exclusively power two separate circuits and the bottom example shows exclusively using two separate sources to power a single, combined group of circuits.
