What are all the wire shields grounded to on a system that has no contact with earth, like an airplane? I found this document that says all shields and chassis and other grounds should be connected at a single point next to the main power source ground. What if the power source is a battery and the lightning strikes with a lot of voltage and current, won't that blow up the battery? My guess there should be some big resistor or something that will quickly and orderly convert all that electricity into heat, but that is just my guess. Surely I can't just connect it to the negative on the battery, or can I?
Electronic – How are airplanes grounded
grounding
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It is typical for the Ethernet Center Taps to be connected through a 75 ohm resistor (one per center tap) and then all of them connected through a 0.01 to 0.1 uF 2-4KV cap to chassis gnd. I am not 100% sure, but it might even be part of the Ethernet spec to do it that way.
The other caps and bead between chassis GND and signal GND is done that way more out of ignorance or CYA than anything else "real". It is common to see people put those components in the schematics and then figure out how to best populate them during EMI testing. But, simply put, these components are not there because of Ethernet.
What you have here is this:
simulate this circuit – Schematic created using CircuitLab
It may be most advantageous to tie the shield to the case, the goal of a shielded cable is to take electric fields that might capacitively couple into the signals you wish to shield and shunt them someplace else. Since the isolated supply is most likely capacitively coupled (isolated) to the box tying the shield to the isolated supply is not going to provide a good pathway to shunt currents away from your signal, it could also create common mode problems if a large current is moving down the shield. If you tie the shield to the chassis ground, the currents that impinge across the will flow down to ground away from the signals you want to protect.
It really depends on which is the lesser of two evils, there are circumstances where one would be better than the other, I'm talking about the general case. Cable shielding is about controlling currents on the shield. Typically in my experience tying the shield to the case is best, but I've also seen a few cases where it isn't.
If your requirement is also having the shield isolated then tie it to the powersupply ground.
A second thing to note is what is tied on the other end of the shield, because if the shield is tied on both ends, there can be currents that flow from ground loops and if you get that, the current can inductively couple and create noise in your signals inside of the cable. In a general sense, tying the shield on both ends to ground is bad for this reason.
Best Answer
I think you need to know the difference between "ground" and "earth".
Ground is the term EEs use for a reference point. In principle we're completely free to choose any reference point we like (crazy example: the + terminal of the 9 V battery lying on my desk). Some choices of Ground are more sane than others, my crazy example is a pointless reference but the metal chassis of an amplifier, a car or an airplane is a good one. Also the - pole of a battery might be a good choice.
Earth is usually an actual connection to our planet earth like a metal pin sticking in the earth. This is used to make a "safe to touch" connection in a mains (120 V or 240 V AC) setup like in your house. You would not want the get an electric shock when you touch the metal frame of your washing machine. So we connect it to earth.
At the same time we often also use this earth as the ground (reference) of our circuit so then earth = ground. They're connected and they're the same.
It is difficult to answer what happens when lightning strikes because many different scenarios are possible and it heavily depends on what happened exactly. Also, a direct lightning strike contains a lot of energy, it isn't always possible to make the electronics survive such a hit without damage.