If I'm working on a battery powered device that has no connection to ground, is it still necessary to guard against voltage spikes?
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\$\begingroup\$ If it has sensitive inputs where static electricity can get in, yes. \$\endgroup\$– rdtscCommented Jun 24, 2020 at 14:14
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\$\begingroup\$ ESD flows to GND through the free space capacitance of the object (which is how much charge the object can store on its surface). Think about how you can charge a steel sphere by touching it to another sphere that is charged, even if both aren't connected to GND. \$\endgroup\$– DKNguyenCommented Jun 24, 2020 at 14:26
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\$\begingroup\$ @DKNguyen I apologize if this is a stupid question. But if electricity can flow if two objects aren't at the same potential, why do birds/line-workers not get electrocuted if they touch a power line when they're not grounded? \$\endgroup\$– IzzoCommented Jun 24, 2020 at 17:03
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\$\begingroup\$ @Izzo That's a good question. Equalizing charge with the free space capacitanc is a momentary, not continuous charge. Like when you feel a static shock versus stick your finger into a socket. It isn't true to say line workers do not get electrocuted if they touch a line when not grounded. They are insulated (and apparently I just read of one method where the worker is isolated from the surroundings and and they themselves are slowly brought up to the potential of the line). \$\endgroup\$– DKNguyenCommented Jun 24, 2020 at 18:31
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\$\begingroup\$ I think birds do feel a static shock from the charge redistribution whenever they land on a particular high voltage line. I don't see how they couldn't. (EDIT: I looked it up and they do. They won't perch on lines that exceed a certain voltage...about 15kV apparently which is, probably not so coincidentally, about the same as a strong static shock you might encounter. Apparently for really, really high voltage lines the spark will jump out and zap the bird before it can land). \$\endgroup\$– DKNguyenCommented Jun 24, 2020 at 18:37
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If I'm working on a battery powered device that has no connection to ground, is it still necessary to guard against voltage spikes?
Any spike or surge does not respect a non-galvanically earthed system. That system (be it battery powered or electrically isolated) will have capacitance to ground and a surge current can flow via this route.
If the circuit is small and has an earth plane and the inputs are only slightly robust you might get away with it but, the devil is in the detail of the layout and where the device is placed or how it is held.
answered Jun 24, 2020 at 14:23