I have a Pi 3 Model B (powered by its ac adapter) sending multiple PWM control signals to up to 5 servos (SG5010). These servos draw usually 300 mA when they move and up to 1 A each when stalled so I plan to use an external power supply for them.
I know that for the servos to work their power and the PWM signal have to share the same ground. What I'm worried about is if it is safe to connect the Pi ground pin directly to the ground from the external power supply (which is powering the servos). I ask this because (as I understand it) servos usually generate noise that can travel through their ground pin and therefore to the Pi and fry it if it's high enough.
I've seen that in other projects this is not a problem but in all of those projects they only use one or two servos or they don't draw as much current as I'm going to do.
Do I need to isolate the Pi ground from the external power ground? If so, what methods are usually used?