surge protector application problem

[actnup]

I have a 480V, 3Ph, 60Hz motor starter control box.  I have a secondary surge arrestor (650V Phase-to-Ground maximum) at the incoming terminals to the control box.  It is installed in a plant that has had some power surge problems with their supply power to the plant and although the surge arrestor has protected the motor and hardwired circuits it has not protected a servo-amp module inside the control box.  The servo-amp module is located on the secondary side of the control power transformer in the control box. It is used in the remote control of the motor and is always in the circuit as such.  My question is should I change to a different surge arrester at the incoming or should I find protection for the servo module itself?  Any help is appreciated.