~ National Lightning Safety Institute ~

Section 5.5.4

1. HISTORY AND DEFINITIONS

Benj. Franklin perceived that lightning striking an elevated grounded rod would follow a "path of least resistance" to earth. His invention advanced fire safety considerably in those days and still holds true today. The deployment of (a) rods (now called air terminals since they are a termination point for lightning in the air) and (b) a series of horizontal and vertical straps or wires to (c) ground rods (so called because they are a termination point for lightning in the ground) make up the classis definition of a Lightning Protection System (LPS). A LPS can be defined as a "preferential path for lightning from intercept location to ground destination." Lightning can be defined as "arbitrary, capricious, random and unpredictable." When combining the aforementioned definitions, we should not be surprised to achieve less than 100% efficiency in our attempts to control lightning’s agenda.

2. CHANGES IN BUILDINGS

Today’s buildings typically are not simple wooden or masonry structures as in the 18^th Century. Modern buildings may contain: structural steel; steel rebar in concrete; metal clad siding and/or metal roofs; cast iron waste water pipes; copper water pipes in walls; cable TV wiring; telephone wiring; electrical wiring; metal wall studs; metal door and window frames, etc.

3. LIGHTNING BEHAVIOR AND EFFECTS

If lightning strikes a LPS on a structure, it is possible that transfer impedance (see IEEE 1100-1999 Powering and Grounding Electronic Equipment, pp. 61-78) may energize unintended conductors listed above. The magnetic effects of lightning also may couple to these unintended conductors. Casualties to people inside modern structures typically include: while on telephone; while in bath or shower; while washing at kitchen sink; while touching electrical appliance; while touching or near metal door or window. (Benj. Franklin suggested safety while inside could be obtained by lying in a silk hammock slung in the middle of the house.) According to an insurance study (Gugenbauer A., Linz Fire Protection Authority for Upper Austria) indirect vs direct lightning damage is about 100:1. This means that the LPS is of no merit in cases of indirect lightning strikes nearby to but not on the structure. NLSI suggests that detailed attention to bonding, grounding, and surge protection ranks higher in priority than the LPS.

4. OPTIONS FOR CONSIDERATION

According to local situations, the following alternatives to ordinary lightning rods may be useful:

(a) The electric power industry has adopted designs of tall masts or overhead grounded shield wires located above the structure to be protected. This can be observed on most high voltage power lines and on most distribution and substations. Here, the idea is to collect the lightning ABOVE the structure, not directly on it.

(b) Following is from US Air Force AFI 32-1065, section 14.5, p. 10 "Explosives Facilities …that do not use structural steel as air terminals must use either a mast system or an overhead wire system. Since these systems provide better protection, and maintenance is easier, consider using this type of protection for other kinds of facilities."

(c) Sometimes unconventional intercept design approaches might be considered. Perhaps rods installed in tall trees or on a tall pole adjacent to the house? An overhead shield wire installed atop a mountain cabin or a golf course shelter? Attention to detail with good grounding and installing high quality surge suppressors? Since lightning safety is a very site-specific subject, only you can make the best decision to reduce the risk to acceptable levels.

5. WISHFUL THINKING

Air terminals claiming enhanced performance fall into two categories.

Radioactive/Early Streamer Emitter (ESE) and multi-point discharge air terminals have been researched and dismissed as ineffective (see R.H. Golde, Lightning, pp. 566-572. See other information is this site for more recent investigations).

Vendor claims should be challenged by requesting proof of performance from independent third party peer-reviewed sources.

If it sounds too good to be true – guess what – it is !