Surge coupling to AC/DC mains does not require a reference plane.  Applying surges into shielded cables may require a ground reference plane as a low impedance return path of the surge.  If a plane complying with 61000-4-2 ESD or 61000-4-4 EFT/B requirements is installed, the same will be all right for surge testing, too.  For more detail, refer to the IEC61000-4-5 standard Chapter 7, Sections 6.