Ohm's law dictates that a voltage potential will result when current of any magnitude or frequency flows through a grounding impedance. Power substation GPR is the result of a 60Hz earth return fault current flowing through a substation ground grid impedance. There are recorded cases where 60Hz GPR lasted more than 15 minutes, causing millions of dollars in damage. These strikes cause most of the electronic circuit damage on wire-line facilities at radio tower sites. They may not destroy every circuit immediately, but may weaken individual components that will fail later on.
Lightning causes much latent damage to electrical equipment.
These documents have existed for years, but most field engineers and technicians seem unaware of the valuable information provided. Although IEEE 487-1992 addresses protection from GPR due to 60Hz fault currents, GPR from lightning strike energy may also be applied. Both currents generate a GPR and have potential to harm personnel and damage or destroy communication facilities. These devices normally are placed at each end of a cable communication facility and are designed to direct foreign voltage impulses into a grounding system. During a GPR, these devices merely offer an additional path to remote ground reference and actually provide a path for current to flow in the reverse direction from which they were intended to operate.