Question E4E13:
Where should a station AC surge protector be installed?  

Choices:  
A: At the AC service panel  
B: At an AC outlet  
C: On the single point ground panel  
D: On a ground rod outside the station    

Correct Answer is  C: On the single point ground panel  

Correct answer: On the single point ground panel (SPGP)

1) Why this is correct
- The SPGP is the common bonding point where all conductors entering the station (AC mains, coax, control/telephone, etc.) are tied together to the same ground reference.
- Mounting the AC surge protector on the SPGP ensures surge energy is shunted to the same reference as the coax and control-line protectors, minimizing dangerous voltage differences between cables during a surge event (e.g., lightning).

2) Why the other choices are incorrect
- A: At the AC service panel  
  - A service-entrance (whole-house) SPD is good practice, but it does not protect against differential voltages that can develop between the AC line and other cables at the station because of long ground/bonding runs. You still need the station’s AC SPD at the SPGP to keep all entries at the same potential.
- B: At an AC outlet  
  - A plug-in protector at the outlet is not bonded to the SPGP and often has long, inductive leads. This can leave large potential differences between the outlet ground and the SPGP during a surge, encouraging current through your equipment.
- D: On a ground rod outside the station  
  - A protector must be installed in the conductor path and bonded to the station ground system. Simply putting it “on a ground rod” doesn’t place it in series with the AC feed, and a separate, unbonded rod can worsen surge currents and ground loops.

3) Useful formulas and rules of thumb
- Inductive surge voltage on bonding leads:  
  $$V = L \frac{dI}{dt}$$  
  Keeping all surge bonds short and wide reduces $L$ and thus the surge voltage.
- Inductive reactance perspective:  
  $$X_L = 2\pi f L$$  
  Lightning has high-frequency components; even small $L$ creates large impedance at those frequencies.
- Rules of thumb:
  - Bring all cables through one entry point and bond them on the SPGP.
  - Mount all surge protectors (AC, coax, control) directly to the SPGP with the shortest, straightest leads possible.
  - Bond the SPGP to the building grounding electrode system with a short, low-inductance strap.
  - Use a service-entrance SPD at the main panel plus a station AC SPD at the SPGP for best protection.