Lightning Arresters or Surge Arresters are always connected 
in Shunt to the equipment to be protected, they provide a 
low impedance path for the surge current to the ground.

Wave trap is a parallel tuned inductor - capacitor 'tank' 
circuit made to be resonant at the desired communication 
frequency.
It is the effort to utilize the same transmission line 
between two substation for the purpose of communications.
At this communication frequencies the tank ckt provides 
high impedance and does not allow to pass through them & 
onto the substation bus & into transformers.