Most lightning damage that occurs, enters the controller through the telephone line. A lightning strike in the general area of the repeater site is capable of inducing a destructive high voltage spike on the telephone line. The Controller's telephone input is protected by Sidactor VRl and the two 10 ohm resistors Rl and R2. This circuit provides secondary lightning protection.

To truly protect the controller, some form of primary lightning protection is required. Primary protection blocks the high voltage spike from reaching the printed circuit board. The high voltage surge must be shunted to ground as it enters the building or as a last resort, as it enters the repeater's cabinet. This is known a primary lightning protection.

Construct the circuit on a metal bracket using insulated stand-offs. Securely connect the center lead of the Sidactor and the junction of the capacitors to the bracket. Provide a good ground for the bracket on the outside of the repeater's cabinet. The controller board should always be installed in a metal cabinet and mounted on metal stand-offs. It is very important to provide the high voltage surge with a easy path to ground.