Question E4E14:
What is the purpose of a single point ground panel?  

Choices:  
A: Remove AC power in case of a short-circuit  
B: Prevent common-mode transients in multi-wire systems  
C: Eliminate air gaps between protected and non-protected circuits  
D: Ensure all lightning protectors activate at the same time    

Correct Answer is  D: Ensure all lightning protectors activate at the same time  

To coordinate surge protection at the station entrance, all coax, control, and rotor lines are bonded to a single metal panel that is tied to the station’s ground electrode system with a short, wide strap. This creates one equipotential reference so all protectors “see” the same voltage at the same time.

Why D is correct (Ensure all lightning protectors activate at the same time):
- A single point ground panel puts every surge protector (coaxial, control line, etc.) on the same reference node.  
- During a lightning surge, equal potentials at that node cause all protectors to reach their clamp voltage simultaneously, so the surge is shunted to ground without rushing through inter-equipment paths.

Why the other choices are incorrect:
- A: Remove AC power in case of a short-circuit  
  That is the job of fuses/breakers, not a ground panel.
- B: Prevent common-mode transients in multi-wire systems  
  A single-point ground can help reduce ground loops, but its primary purpose here is lightning protector coordination, not general common-mode suppression.
- C: Eliminate air gaps between protected and non-protected circuits  
  Ground panels do not remove “air gaps”; they provide bonding. Air-gap arresters are a different device.

Useful formulas and rules of thumb:
- Fast surge drops across inductive ground leads can unbalance protectors:
  $$V = L \frac{di}{dt}$$
  Example: about $200\,\text{nH}$ per foot of wire and a lightning edge of $5\,\text{kA}/\mu\text{s}$ gives
  $$V \approx 200\times10^{-9}\,\text{H/ft} \times 5\times10^{9}\,\text{A/s} = 1000\,\text{V per foot}$$
  Even a foot or two of skinny, coiled wire can add kilovolts—causing uneven firing.
- Rules of thumb:
  - Use a single-point “star” panel at the entry. Do not daisy-chain grounds.  
  - Bond the panel to the building’s grounding electrode system with one short, straight, wide copper strap (minimize $L$).  
  - Keep protector leads equal and as short as possible to promote simultaneous clamping.  
  - Bond all grounds together (RF, AC safety, tower) at the single point.