If I have a 1kV DC circuit, does the ground wire need to be rated for the full 1000V?
The circuit is pretty simple, two wires from a power supply to a resistive load.
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You have your 1kV power supply, with an impressively insulated wire going out to a resistor. Coming back, you have a return wire with light insulation because, hey, it's grounded!
Now the ground wire breaks at the power supply.
Your "safe" ground wire is now at 1kV. Is that light insulation sufficient?
If the 1kV is AC, the peak voltage may be even 1,4 KV. If the power supply ground is on the same reliable ground as the resistor which is grounded directly at one end (f.e. one common metal casing for power supply and resistor), the ground wire does not need any isolation at all resp. the ground "wire" is the metal casing that does not have an insulation. The high voltage in a microwave oven of 2000 resp. 4000V is grounded at one side at the metal casing which is connected to PE. Only the insulation of the other "hot" wire coming from the high voltage transformer is rated for that high voltage.
But - as mentioned in the first answer - if the ground could be interrupted, both wires must be insulated for the high voltage. For a metal casing of a MW, it is highly unlikely - if not impossible - that the metal casing will be separated by fault in a way to endanger the user with high voltage. There would be leaking MW radiation as well.