The explanation by Dr. Mousa is excellent and I am in total agreement with him as far as it goes. Now, I would like to suggest a more complete scenario for all to consider.

According to the National Electrical Code (NFPA 70), the Lightning Protection Institute (LPI 175) requirements, and the Lightning Protection Code (NFPA 780), the lightning protection system, where designed for the structure of interest, must be the highest conductive points on said structure. These interconnected air terminals are then connected to ground rods driven outside the building envelope all around its perimeter and at prescribed regular spacing intervals via down wires run on the outside of the structure that avoid such things as window frames and fire escapes by minimum prescribed distances. These lightning protection down wires are the only very low impedance interconnections allowed with the air terminal interconnecting grid(s) on the top of the building. The NEC requires that the lightning protection grid be "bonded" to the Main Service Grounding Electrode system, but only as a "supplementary" grounding connection to prevent differing potentials between two non-current carrying grounded systems on the same structure. And, if the earth impedance at the point of any driven rod is insufficient to make this the lowest impedance point in the system (even lower than the Main Service Entrance grounding electrode connection point), either multiple ground rods must be driven and/or chemically enhanced ground rods must be installed to bring the lightning protection system earth impedance to the lowest point in the system.

The issue with using the building steel as the sole lightning protection down conductor is twofold: (1) the huge lightning arc current energy present in a lightning strike will certainly seek both the structural steel as described above, and the interconnected reinforcing steel bars embedded in the concrete, which is both good and bad --- good because it finds "earth", and bad because it will invariably spawl the concrete so aggressively due to the immediate and violent expansion of the concrete internal moisture that becomes high pressure steam, and, therefore causes the concrete spawl fragments to become shrapnel and missile-like projectiles that are obviously quite dangerous as well as structurally compromising in the worst case, and, (2) the strike will energize the interconnected steel throughout the structure to levels that will become extremely dangerous at all levels from the roof to the ground level. When all of this occurs within the envelope of the structure, the "step-and-touch potentials" throughout the structure will be all over the potential difference (voltage) spectrum, at the very least in the danger zones for humans. Since the natural instinct for us humans is to run in such circumstances, the long strides of such activity are immediately conducive to higher than normal step-and-touch potentials between the feet (and, hence, through the most sensitive parts of the anatomy).

Thus, the prudent, safe design would be to keep the lightning protection system both above and outside the total building envelope to present a virtual "shielded envelope" around the structure to keep personnel and structural entities safe in the event of a direct lightning strike. A further note would be to make certain that the building Owner, Facility Manager, and/or Facility Management company has explicit instructions to have the lightning protection system inspected and re-certified each time there is an event large enough to be considered a direct or near-direct strike, with repairs and/or modifications as appropriate, depending on the levels of any damage discovered.

Hi Mike,

Every lightning strike that I've seen has created enough heat to expand moisture within the structure so that; wooden beams are split, grout and stone explode into living spaces, etc. With this in mind, wouldn't it be better to have lightning protection conductors with less impedence than the reinforcing steel to shunt currents developed by the lightning strike, reducing the amount of current / heat in the structure?

Ken

Dear Mike,

This is an excellent article by expert Abdul Mousa, on the expalanation of lightning protection, and particularly on the roof. MANY WOULD BE BENEFITTED BY THIS.

I would suggest to provide more such articles to benefit so many readers. Regards gk

Dear Dan,

The explanation given by you is excelent and very convincing, however as I understood from this, the concept of using reinforced steel bars of the building as down conductors seems to be harmful and not very convincing. Is that right? gk

Very well said! Thank You!

My other concern is that the amateur radio community has found that if the rebars in a tower foundation are all welded to each other and the tower bolts and bonded to a genuine ground rod outside of the concrete lightning will not cause concrete to explode.

Even NEC articles 250 and 680 allow the use of the usual steel tie wires for rebars that are used for power frequency grounding, I would rather weld rebars that might be asked to carry lightning current.

I saw somewhere that steel building columns usually present a surge impedance of around 50 Ohms to lightning and audio frequency current, but using a parallel copper conductor is still the best policy.

This has been a frequently overlooked issue in southern Missouri and I'm glad to have read all this even though my tendancy was to not read this particular newsletter. It incourages me to speak up and try to ruffle some feathers.

Gentlemen: Just a couple of points that occasionally get overlooked:

1-Lightning, like all electrical currents, follows all available parallel paths in inverse proportion to the impedance of that path. That is, current will flow in all available paths, with greater amounts flowing through the lowest impedance path. (impedance = ohmic resistance plus reactive components of capacitance and inductance) Frequently writers give the impression that only the lowest (most direct, with least resistance) impedance path will be followed. If this were true, component damage along other, higher impedance, less traveled paths, would not occur.

2-The energy expended within any area along a conductive path is proportional to the I squared R losses within that area. The heating effects for that energy dissipation includes the element of time. Joules (watt-seconds) give a measure of heating (work) produced by expended energy. Correctly designed and installed Ufer grounding conductors have been shown not to fracture or explode the concrete encasing them, even when having handled multiple lightning strikes. This is due to the fact that the energy is dissipated along the entire length of the conductor and concrete encasing for a relatively short time period. The resulting heating (also spread along the length) is less than enough to cause internal steam generation, and concrete destruction.

While it is true that structural damage can and does result from lightning striking concrete structures, the presence of large quantities of structural steel reinforcement greatly reduces this possibility. Concrete damage is associated with areas where arcing and ionized conduction occurs where there are no sufficiently low impedance paths available.. Were this not so, we would experience much greater incidences of damaged structures than is observed in actuality. A building of sufficient strength to be a multiple floor parking garage must contain considerable structural steel reinforcement, which if properly bonded, should perform well with the protection measures required in the building codes cited above [by others].

Some pronouncements come close to “boogey-man” stories that don’t happen in the real world when proper design, including bonding of structural members is utilized.

The two parking structures I worked on here were required to have no more that 2 ohms grounding from light poles Steel Bonded support beams. The light poles had dedicated equal length bonding runs.. Metal hand rails also had dedicated bonding runs and all bonding runs were run in IMC and allowed to drain into the next lower drains.

The top of the structure was put on hold until we tied every rebar to every other rebar and the Bonding support beams. All such rebar bonds were required to be exothermic and coated with concrete proof sealant prior to the pour. 7328 joints on the last one.

Here in the High Desert we get seasonal and unseasonal Monsoon like rain storms with an average of 57 strikes per 3 hours of storm here in Albuquerque. I unfortunately was struck by lightning walking hand in hand with my girlfriend and now I duck and run at the first crash of thunder even miles away.

The roof top of any structure is a dangerous place during an electrical storm. Lightning protection systems can only improve the statistical chance of survival. I just wish we could put a stop to this practice of roof top parking.

Lightning strokes are short duration high current level with steep slopes giving rise to high frequency currents. Even short straight length grounding conductors will offer relative high impedance to these high frequency components making it difficult to totally eliminate high voltage potentials with down conductors alone. Lightning protection systems do protect people and property, but they are not 100% offective. My own opinion is that no one should be walking around on top of any structure during and electrical storm. I have been there and it is a frightening experience. I just wish we would put a stop to roof top parking. The cost of a roof is cheap compared to the value of a single human life.

There have been news articles that NASA engineers can accurately predict the chance of an electrical storm from atmospheric measurements and this information is used in making a decision on whether or not to launch.