Hi,
we don´t know your exact situation.
Thus my thoughts (they may be fit to your application or not)
let´s consider a situation in a house.
You have a device with plastics case, no metal case, no EARTH.
It is isolated for standard situations.
But when a lighning happens it might be too much for the device´s isolation and an arc can happen:
Signal flow: lightning --> power lines --> device --> arc (across case) --> human (standing on concrete floor) --> earth.
Here a surge suppression against EARTH will limit the voltage at the device, prevent form arcing, saves life.
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In your case, when a lightning happens, then install the "surge suppression to EARTH" on the surface = wave generaor side.
Klaus