Altivar process product datasheet declares under electromagnetic compatibility that it complies Electrical fast transient/burst immunity test level 4 according to IEC 61000-4-4
In this standard, the peak value of the voltage for level 4 is 4kV, see table in attachment. From this reason Altivar process must withstand impulse voltage of 4kV.
Conditions for repeated impulse withstand test as well as parameter of the impulse are specified in this standard IEC61000-4-4.