~ National Lightning Safety Institute ~

Section 5.5.6

1. Overview

A recent look under "lightning damage at dispatch coummunications centers" in the Google search engine showed 15,600 hits. Is this indicative of a problem here? NLSI research into lightning costs and losses in the USA show amounts of $4 to $5 billion annually. This database does not include dispatch communications centers for ambulance, fire, and police.

NLSI has been retained by VFIS/Glatfelter Insurance Group to conduct selected site assessments at client locations around the country. The study is in process as this article is being prepared. Conclusions should result in a "Lessons Learned" matrix which can be helpful to dispatch communications centers in strengthening their communications facilities from lightning insults.

Lightning protection in an absolute sense is impossible because of the arbitrary, capricious, and stochastic nature of lightning strikes. However, mitigation of the hazard can be achieved through a deployment of inexpensive and low-technology subsystems. The purpose of this article is to lay out those subsystems in general terms. They are described in total as a lightning protection system (LPS).

2. Air Terminals

Lightning usually terminates on grounded objects sticking up in the air. Franklin rods are air terminals. Steel towers are air terminals. Old Ben’s designs developed in 1752 carried lightning from rods in the air via conductors to rods in the ground. This lightning protection system was, and is, based upon the "path of least resistance" laws of physics. Nowadays, some vendors are promoting unconventional air terminal designs seeking to gain advantage over competitors. Caveat emptor. Of course, should lightning strike across the street from a dispatch communications center and couple into sensitive electronics via underground wiring, then an air terminal subsystem has not performed and has no value.

3. Bonding

Without proper bonding, all other elements of the LPs are useless. Bonding of all metallic conductors in a dispatch communications facility assures everything is at equal potential. When lightning strikes, all grounded equipment will rise and fall equipotentially. This eliminates the unequal voltages in separate sensitive signal and data systems. Bonding should connect all conductors to the same "Mother Earth." A partial listing includes the following: tower legs, adjacent fences, ice bridges, incoming coaxial cables, cable trays, cabinets and racks, signal reference grids, halo grounds, the proliferation of conduits carrying various AC power and low-voltage DC current conductors. Not convinced bonding is important? Check out NEC 250.90 through 250.106 for more details.

4. Grounding

Low-resistance grounding provides an efficient destination for the "lightning beast." If your site soils are composed of sand or rock, they are resistive, not conductive. If your surrounding soils are of clay or dirt, they may be conductive. "Good grounds" are achieved by volumetric efficiencies. We recommend buried bare 4/0 copper wire – the so called ring electrode or ring ground. Cadwelding© security fences, tower legs, and other adjacent metallics to the buried ring will improve grounding. NEC 250 describes other grounding designs such as rods, plates, water pipes (beware of plastic pipes underground), metal frame of buildings, and concrete-encased electrodes. Choose your grounding design based upon localized conditions and the amount of available real estate at your location. NEC 250.56 suggests a target earth resistivity number of 25 ohms. Lower is better.

5. Surge Suppression

Surge suppression devices (SPDs) all function either by absorbing the transient as heat or crowbaring the transient to ground (or some combination thereof). They should be installed at main panel entries, critical branch or secondary panels, and plug-in outlets where low voltage transformers convert AC power to DC current and voltage. SPDs also should be installed at signal and data line entry points to electronic equipment. Telephone punch blocks should be SPD-protected. Beware of the junk SPDs that proliferate in the marketplace. Beware of counterfeit or false UL and IEEE labeling. Beware of the "sounds too good to be true" marketing hype employed by vendors. Insist on Certified Test Results to substantiate performance claims by manufacturers. Consider SPDs that have capabilities to remotely signal their operational performance. If you write the SPD specifications, you won't have to accept the low bid. SPDs rank right behind bonding in the hierarchy of important steps to mitigate the lightning hazard.

6. Codes and Standards

There aren't any. It is important that the dispatch communications community consider adopting contemporary lightning protection guidelines. Taking a few items from Motorola R56, adding some from IEEE 1100, borrowing from FAA-STD-019d, copying some language from NASA-KSC-E0012E, and mixing in some NEC 250 information would result in a very good document to satisfy the needs of the dispatch community for a pragmatic lightning protection standard. Some leadership is needed here. Please step forward.