Can the GND pin handle 5V?

Question

In many tutorials and guides they use the 5V pin, the GND pin and a GPIO pin to control a servo with a JR-plug. In these guides, they didn't use any other components (I.e. resistors) in between. Now I am wondering if this is safe to use for my pi, because I've read on many websites that the GPIO pins can handle a maximum of 3.3V.

Does this maximum of 3.3V apply to the GND pins? Is the GPIO pin in danger to be fried because if the voltage? (-> What can I do to make that circuit safe?)

P.S.: I've tried to measure the voltage that goes through the cables when the servo is attached in its original position, an RC-car, the results were: about 5V in + and -, about 300mV in the signal cable. These results may be completely wrong as I am new to this topic.

Technical details (may be translated inappropriate, please look at the measuring units): • operating voltage: 4,8-6 V • Rotation Speed: 0,2 sec/60° at 6 V • Rotation force at 6 V: 60 Ncm
• JR-plug

Answer 1

Ground in a DC circuit refers to either an actual earth ground (i.e., 0 volts vs. anything) or a common ground for different components that may be working at pretty much any voltage relative to it.

I think what you would really want to be concerned with on the Pi is the amperage, if the "common ground" to the Pi is the primary ground for whatever you are connecting -- but it is hard to imagine how an arrangement like that is possible without intending to do so and jumping through hoops to accomplish it. My point here is if you wanted to fry the Pi that way you probably could, but only if you actually wanted to (by shorting a high amperage supply to ground through it).

Anyway, a common ground between between circuits running at different voltages is fine (and necessary, if there is any other connection between them). This doesn't automatically make every circuit using it safe, it just means that using a common ground doesn't by definition make it unsafe.