I'm building the circuit of this article:
C7 is rated 1nF 2kV. Is this right, does this capacitor need to be able to handle 2kiloVolt? Why is that? Is ethernet dangerous?
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Yes, it needs to be high voltage, although it does little useful considering where it is connected. Ethernet is transformer-isolated, and specifies a fairly high isolation voltage. I don't remember what the spec says exactly. 2 kV is likely the max plus some margin. I know that the isolation spec is high enough to be able to connect between ground-based equipment and other equipment riding on a 250 V AC power line.
That said, the capacitor in that location doesn't make a lot of sense. About all it does is filter common mode voltage a bit. I suppose the point is to slow down common mode spikes to the point that whatever stray common to differential coupling there might be accross the transformer won't corrupt the signal.
Whenever I've put capacitors on the network side of ethernet it's been to reduce emissions coming from the board. For that purpose, it is better to use a transformer with common mode chokes built in (I often use the Pulse H2019), then put small caps (like 47 pF or less) to ground on each of the lines. The apparent differential capacitance will be half that, but the full capacitance will attenuate RF that got coupled onto the line from the board.