XtheEdgeX, since you will not post anymore, maybe you might listen for minute.
Functionally there is no difference between Gas Tube, MOVs, and SAD's. They are do the exact same thing, they are all shunt devices. Meaning they appear as an open circuit (or load) until a threshold voltage is reached, at which point they become low impedance devices (or short circuit). In other words they are all shunt devices dsigned to be installed in parallel with the load they are protecting.
The only differences between the three is the amount of energy they can dissapate measured in Joules (Gas having the highest, and SAD's the lowest), the response time in which they operate measured in fraction of a second (SAD's being the fastst, and Gas being the slowest), and robustness or durability (Gas is the most rugged, while MOV's & SAD's are prone to failure from operating). Other than that they operate in the same manner, a shunt.
As I stated earlier in a gounded service N and G are the exact point at the disconnect, they are bonded together. Is that clear? No device type (Gas, MOV, or SAD) can match the performance of a bolted fault. Therefore adding any type of device from N-G or L-G at the entrance can add anything of value to the user. It is a waist of material and money. Only modes needed at a grounded service entrance are L-L and L-N, Ground is already provided by the N-G bond.
Once you are downstream from the entrance then L-G and N-G modes become useful. However earth still has nothing to do with the circuit. The impedance of the EGC and the series impedance of the electrode appear as an open circuit to the fast rise time of a transient. What any SPD does do is clamp or limits the voltage between the two points it is connected too to a safe or reasonable limit. So in the case of a terminal strip TVSS you would select the SPD clamp voltage for the N-G mode to be something like 10-volts, and L-N and L-G to 330-volts.
Dereck Campbell, PE
