Principles of Grounding of Lightning Protection Systems per NFPA 780
By Richard Kithil, President & CEO, NLSI
Lightning wants to get to ground. It will follow the path(s) of least impedance to do so. NFPA 780 contains much helpful information on the topic.
2. Fundamental Principles
Low impedance paths to ground are preferred by lightning. Dedicated wire down conductors or building steel, or both, must be installed in accordance with NFPA 780 (see section 4.9.9).
- High impedance paths must be avoided since they can create unwanted heat and/or mechanical damage. Adjacent metal bodies, for example, must be bonded to the low impedance path “so as to prevent side flash or spark-over” (see 780, section B.2.2).
Where down conductors are near metal bodies without bonding the two, lightning tends to jump from one to the other tending to create sparks (see section B.3.9).
For more information, see Annex B of NFPA 780.
- There are several types of lightning grounding, used separately or in combination, as selected by the installation contractor. Ground rods, perimeter (ring) bare wire, radials, plates, and concrete (rebar) encased designs all constitute acceptable earth electrodes. The lightning grounding electrode is a dedicated part of the lightning protection system. No other ground shall be used as a lightning ground (see section 188.8.131.52).
- Other (non-lightning) systems requiring grounding shall be integrated (bonded or connected) to the lightning ground to form a common ground potential (see section 4.14). This is to protect electrical equipment from voltage rise mismatches where separate grounds are employed. Under lightning attachment conditions, the lightning ground will function independently of any other grounds. Should electrical system grounds not be connected to the lightning ground, the overall lightning protection system will still function as designed.
As examples of the above, there are many instances of functional lightning grounding on structures that have no electrical systems, such as railroad tracks, military munitions storage depots, and high-voltage power line towers.