FYI your link got corrupted:

https://en.wikipedia.org/wiki/USB_Killer

And interesting, always knew high voltage was a threat, but never thought of capacitor being used, since it wouldn’t require an independent power source.

Yeah but I can drop an ADUM3160 isolator on the port for a few bucks. If they kill that, I replace that for a few bucks more.

In most cases a chain of cheap hubs would work too.

For clarification, all USB ports are 5 volts DC; transformer (or computer) will take care of converting the 120/220 AC current to the necessary 5 volts DC.

Commonly two types of isolation data and power chipsets. For a dead drop, you would want a cheap voltage isolator set to trip/blow a fuse at 5 volts DC — with no data isolator.

What?

So what do you do if someone puts 120 volts on the data lines?

You very much want to isolate both. Which is precisely the function of the chipsets I mentioned.

Data chipset blocks all data flow - whole point of connecting to the deaddrop is to get data; and yes, I agree all lines when capping voltage show be checked, though would not be surprised it’s not common for off the shelf usb power isolators to only check the power lines as defined by USB standards.

No.

No, no, no.

A USB data-line isolator does not block data flow. Its entire purpose is to alow data to be communicated across a galvanically-isolated gap. It uses magnetic (transformer) or optical isolators to do that.

It blocks POWER flow, while passing data. Look up the ADUM3160 datasheet.