Field strength and breakdown voltage
30kV/cm is the normal field gradient or strength (at breakdown) in
uniform fields for small gaps on the order of 1cm under normal
conditions of temperature and pressure. For large gaps of several
meters this can reduce down to 6kV/cm (reference J R Lucas).
Paschen's curve I think is where the 30kV/cm is pulled from. Note
that the curve itself is for 1cm spacing at 1 atmosphere and 25 deg C
for conductors of an infinite plane, parallel, and perfectly smooth.