Component case size and spacing for ESD protection circuit
The "30kV" refers to a ESD test voltage with a specified circuit as in EN/IEC 61000-4-2 standard. At no time will 30kV appear across the device if it is within the specified limits and working- it's intended to clamp the voltage to a relatively safe level assuming a limited current. 

Here is the circuit used in the standard (which is not free), from this application note: 


As you can see, at a 30kV test voltage, the current through the device will be approximately 30kV/300\$\Omega\$= 91A, which typically corresponds to Ipp (peak pulse current) in the datasheet. For example a 3.0SMCJ20AQ with a standoff voltage of 20V will clamp an Ipp of 92.6A to less than 32.4V. So, for example, a voltage regulator with an absolute maximum input voltage of 35V should be protected in the test situation.