Joe of NJ, if you are referring to a service entrance TVSS on a grounded system (even ungrounded), common mode is a waist of material, time, money, and serves no purpose.
The common mode is already provided in the "normal mode" via the MBJ on grounded systems. You cannot get any more protection than a dead short piece of copper. Only downstream of the entrance should common modes be used. As stated earlier ground (earth) has no function or purpose with TVSS devices.
