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This article explains research and recommendations on how to protect trees from lightning strikes.This article series describes common lightning protection systems, certification, installation, and lightning protection system inspection. We provide information about lightning strikes, lightning hazards, related equipment, sources of lightning protection system installers, and lightning strike risk assessment.
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Introduction - Lightning Protection Systems Fitted to Trees
[The research and recommendations for protecting trees from lightning strikes, and the concomitant protection afforded to nearby buildings discussed in this article focus on conditions in the U.K. However the principles of tree protection from lightning discussed here are generally applicable to other countries as well, though the statistics on the frequency of lightning strike and other local conditions vary. -- Editor]
This research on lightning protection systems for trees was established for the purpose of developing a better understanding of lightning protection systems specifically designed to be fitted to trees. Coming from a background in sylviculture my initial concern was to enable important and intrinsically valuable trees to be protected from damage resulting from lightning strikes. However it quickly became apparent that the protection of nearby structures and buildings that might be liable to collateral damage in the event of strike was of equal significance.
Research showed that on those occasions where lightning protection had been installed in trees, the system employed had been based upon designs originally intended for use on buildings and other essentially non-dynamic man made structures. The particular problems of installing the necessary hardware into living and growing trees did not appear to have been adequately addressed. Thus the direct effect on the tree of the installation of the required hardware is not factored in, with the result that trees are likely to be caused some degree of long-term harm in the very process of attempting to protect them.
The Approach to Lightning Protection for Trees, Passive Cloud-to-Ground Lightning Protection
This document refers to lightning protection for trees in the UK. The type of lightning considered is cloud to ground and not inter-cloud as this type is not at present considered to be a threat. In the UK, we can expect approximately 450,000 - 500,000 strokes per year. Of this approximately 40% is cloud to ground. (EA Technologies cloud to ground database) This is quite low compared to some countries, but is still considered significant enough in some instances to warrant lightning protection. I will only be referring to passive lightning protection.
Active systems are not considered, as there is insufficient evidence to support their performance claims. (The University of Manchester Institute of Science and Technology Test report No: 43427): The need for lightning protection is evaluated either to conserve specimen trees or due to a concern about collateral damage to surrounding people and property.
We can ascertain the extent of lightning activity in the immediate area around the subject tree by performing a cloud-to-ground density analysis. Cloud-to-ground lightning produces extra long wave radiation (low electromagnetic frequency). Low frequency emissions have very good transmission characteristics in the atmosphere and potentially can travel for hundreds of kilometers before decaying to an undetectable level.
This frequency is at 1070Hz and transmission is further enhanced by the entrapment of the ionosphere; this is referred to as wave-guide effect, a useful tool in the evaluation process. It should be noted that this is a two-dimensional representation of a three-dimensional phenomenon. As such it will be necessary to look at the topography of the site taking into account other tall structures, dynamic and non-dynamic.
A lightning protection system is comprised of three main components: air terminal, down conductor and earth (ground) termination.
The earth (ground) termination is where we will be attempting to direct the huge current in a manner that minimizes risk to property and people. In the horticultural context we look at soil as a medium for growing. In the context of lightning protection we look at it as a medium for conducting electricity.
The ANSI A300 Part 4 recommends designing the earth (ground) termination based on a visual inspection of the soil and its moisture content. This is not possible as water is an insulator not a conductor; it is the dissolved salts in the water that give it its conductive properties.
These salts are not detectable with the human eye. We can ascertain the electrical value of the soil to a given depth using a system sometimes referred to as the four-point test or the Wenner method. This was first introduced in a paper published in October 1915, A method for testing soil resistivity, by F. Wenner.
This test enables us to prescribe the number of earth (ground) electrodes to employ for a given ohms resistance on completion of the earth (ground) termination. This is important if we are to achieve an effective earth (ground) without over specifying the number of electrodes causing unnecessary expense and disruption, or too few resulting in an inadequate earth (ground). It has been my experience that multiple electrodes are required in almost all installations.
Lightning Protection System Electrodes
Lightning Protection System Conductors on Trees
The conductor should be firmly secured to the fabric of the tree. In the UK we fix at 1 meter (3 feet) intervals; in the US they are specified at a maximum of 1.8 am (6 feet). This difference is not of any real concern, as I believe we are all of the opinion that the distance will vary with the form of the tree.
It is however of greater importance to consider the type of fastener we employ. The ANSI A300 Part 4 recommends a type of fastener that is attached to the tree in a manner not dissimilar to a nail. The conductor is secured at the outer end in a pinch portion. As the tree grows it will envelop the fastener.
When the incremental growth reaches the pinch portion of the fastener ANSI recommends removing the conductor from it and installing a new fastener 30 cm (1 foot) above or below the old one, leaving the old one in the tree. This is a contradiction within the standard and ISA Best Management Practices (BMP) for Tree Lightning Protection Systems, creating other metallic conductive objects in the vicinity of the conductor that are not bonded.
The reasons for bonding metallic conductive objects to a component of a lightning protection system are well established (ANSI A300 Part 4, 46.1.7). The BMP is of the opinion that these old fasteners will not be subject to potential difference.
Lightning Arcing & Flashover Considerations
Effects of Lightning Conductor Connectors on Transfer of Lightning Energy to Tree Interior
The higher the resistance of the tissue to the flow of current then the greater is the potential for transformation of electrical energy to thermal energy. So it is not that the flow of current is seeking to earth (ground) through the tissue of the tree but to convert from electrical energy to thermal energy as the easier of the two options.
This creates the possibility for the energy to convert again only this time from thermal energy to kinetic energy. (BMP While a trees inner bark and cambium are its most conductive areas, the heartwood is also conductive and, when lightning is conducted to the heartwood, the tree is often shattered). (Isolated metal bolts, nails and lightning fixtures can change lightning paths from exterior to interior: Dr Kim Coder. Spark of death. Arborist News June 2004)
Use of the Arborbolt Lightning Protection Connector on Trees
Final Tree Protection Lightning System Details
Once the system has been installed, all the components should be mapped; this would include any bonding to aerial braces and cables as well as underground utilities. A lightning protection system fitted to a tree should have a minimum functional life expectancy of at least 30 years, and a regular inspection and earth (ground) re-test is required. This is best done at 11or 13 month intervals and the results recorded.
The reason for eleven or thirteen month period is so that after twelve years we have tested throughout the seasons (BS 6651). In my experience the ohms resistance in the earth (ground) termination can change by an ohm or two but not much more. If Bentonite was incorporated and there is a significant rise in resistance then irrigation should be considered. Also it is observed that at near zero temperatures the resistance can rise significantly and, although lightning activity is unusual at these temperatures, it is possible.
Conclusions on Lightning Protection for Trees
To conclude, it is suggested that there are very few definitive solutions to lightning issues. All that can be done is to work with the technology available and importantly record all we do. This will then help gather information from practical; experience in order to develop a better understanding of the relationship between trees and lightning activity.
It could be said that the approach to lightning protection and trees described here is over engineered but, unlike a non-dynamic structure, we cannot re-point or replace a lightning-damaged tree. Lightning is a very powerful natural phenomenon and as such is capable of overwhelming our defence mechanisms. The better our systems are engineered, the better they may control this powerful force of nature.
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References for Lightning Protection Systems & Protecting Trees from Lightning Strikes