Lightning Protection And Trees

Lightning Protection and TreesBy Ben FuestINTRODUCTIONThis research was established for the purpose of developing a betterunderstanding of lightning protection systems specifically designed to be fitted totrees. Coming from a background in silviculture my initial concern was to enableimportant and intrinsically valuable trees to be protected from damage resultingfrom lightning strikes. However it quickly became apparent that the protectionof nearby structures and buildings that might be liable to collateral damage inthe event of strike was of equal significance.Research showed that on those occasions where lightning protection had beeninstalled in trees, the system employed had been based upon designs originallyintended for use on buildings and other essentially non-dynamic man madestructures. The particular problems of installing the necessary hardware intoliving and growing trees did not appear to have been adequately addressed. Thusthe direct effect on the tree of the installation of the required hardware is notfactored in, with the result that trees are likely to be caused some degree of longterm harm in the very process of attempting to protect -------THE APROACHThis document will refer to lightning protection for trees in the UK. The type oflightning I will be referring to is cloud to ground and not inter-cloud as this typeis not at present considered to be a threat. In the UK we can expectapproximately 450,000 – 500,000 strokes per year. Of this approximately 40% iscloud to ground. (EA Technologies cloud to ground data bass) This is quite lowcompared to some countries, but is still considered significant enough in someinstances to warrant lightning protection. I will only be referring to passivelightning protection. Active systems are not considered, as there is insufficientevidence to support their performance claims. (The University of ManchesterInstitute of Science and Technology Test report No: 43427): The need forlightning protection is evaluated either to conserve specimen trees or due to aconcern about collateral damage to surrounding people and property.We can ascertain the extent of lightning activity in the immediate area aroundthe subject tree by performing a cloud-to-ground density analysis. Cloud-toground lightning produces extra long wave radiation (low electromagneticfrequency). Low frequency emissions have very good transmissioncharacteristics in the atmosphere and potentially can travel for hundreds ofkilometres before decaying to an undetectable level. This frequency is at 1070Hzand transmission is further enhanced by the entrapment of the ionosphere; thisis referred to as wave-guide effect, a useful tool in the evaluation process. Itshould be noted that this is a two-dimensional representation of a threedimensional phenomenon. As such it will be necessary to look at the topographyof the site taking into account other tall structures, dynamic and non-dynamic. Alightning protection system is comprised of three main components: air terminal,down conductor and earth (ground) termination.Copyright Ben Fuest

The earth (ground) termination is where we will be attempting to direct the hugecurrent in a manner that minimizes risk to property and people. In thehorticultural context we look at soil as a medium for growing. In the context oflightning protection we look at it as a medium for conducting electricity. TheANSI A300 Part 4 ( American National Standards Institute, Standard forLightning protection Systems For Trees ) recommends designing the earth(ground) termination based on a visual inspection of the soil and its moisturecontent. This is not possible as water is an insulator not a conductor; it is thedissolved salts in the water that give it its conductive properties. These salts arenot detectable with the human eye. We can ascertain the electrical value of thesoil to a given depth using a system sometimes referred to as the four-point testor the Wenner method. This was first introduced in a paper published inOctober 1915, A method for testing soil resistivity, by F. Wenner. This testenables us to prescribe the number of earth (ground) electrodes to employ for agiven ohms resistance on completion of the earth (ground) termination. This isimportant if we are to achieve an effective earth (ground) without over specifyingthe number of electrodes causing unnecessary expense and disruption, or too fewresulting in an inadequate earth (ground). It has been my experience thatmultiple electrodes are required in almost all installations.Electrodes come in different sizes. The ones I shall refer to are 1.2m (4 feet) inlength and 12.5 – 13 mm diameter, and are made from mild steel with a twomicron copper bond coat. The ends are threaded so that with the aid of a couplerthey may be joined together. The minimum requirement for length is 2.4m (8feet). If the space we are working in is limited then it is sometimes best to addone on top of another, 2.4 – 3.6 – 4.8. If there is room to work in, it is possible toinstall individual electrodes in parallel. Tip depth is an important factor to beconsidered when deciding configuration. Tip depth determines the distancebetween electrodes. If we install them and they are too close they will interactand we lose the performance benefits. If they are too far apart we are wastingtime, materials and causing unnecessary disruption. (BS 6651 1999)When working with soils of poor conductivity it is possible to introduce soilconditioning agents. Bentonite is de-composing volcanic ash; it has hygroscopicproperties that enable us to enlarge the diameter of the earth (ground) electrodewithout the cost implication. Instead of driving the electrode into the soil we candrill a hole approximately 25cm (10 inches) wide and as deep as the intendedelectrode then back fill with the Bentonite mix. The electrode is then insertedinto this. It should be noted that Bentonite is an excellent conductor while wetbut works just as well as an insulator when dry. If we are employing thistechnique, it is useful to remove turf around the hole and put it back oncompletion resulting in a more professional finish.There are alternative ways to overcome poor conductivity. In the past it has beenrecommended that we install our earth (ground) electrodes just outside the dripline. Observations show that if the soil around the tree has undergonedecompaction with the addition of organic mulch, it is possible to obtain a muchimproved earth (ground) working just inside the drip line. With regard to thearea affected by the dissipation of the current around the electrode, it has beenobserved that this will be contained within a radius of approximately 35-50cmCopyright Ben Fuest

(15-20 inches). There will be occasions when driving the electrode into theground it abuts an impenetrable layer or obstacle. The temptation to cut it willbe strong. I would not be in agreement with this practice. I would suggestremoving the electrode and trying again only this time at an angle of anything upto 30 degrees from the vertical; this way we retain the integrity of the electrodelength without the requirement for depth. We can test the value (ohmicresistance) of the earth (ground) during the installation and on completion; Ihave found the 62% method to be the most reliable and industry standardaround the world. In the UK the target figure is 10 ohms or less. (BS6651 1999)Having established the need for lightning protection and designed the earth(ground) termination we can look at the aerial aspect of the system. The ANSIA300 Part 4 recommends a maximum distance between air terminals of 35 feet. Iam in agreement with this but, as with all things organic, there will be exceptionsand we should look closely at the tree and see how it lends itself to the formulaand what we think will work best for the circumstances that prevail in thecanopy.The conductor should be firmly secured to the fabric of the tree. In the UK wefix at 1 m (3 feet) intervals; in the US they are specified at a maximum of 1.8 m (6feet). This difference is not of any real concern, as I believe we are all of theopinion that the distance will vary with the form of the tree. It is however ofgreater importance to consider the type of fastener we employ. The ANSI A300Part 4 recommends a type of fastener that is attached to the tree in a manner notdissimilar 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 growthreaches the pinch portion of the fastener ANSI recommends removing theconductor from it and installing a new fastener 30 cm (1 foot) above or below theold one, leaving the old one in the tree. This is a contradiction within thestandard and ISA Best Management Practices (BMP) for Tree LightningProtection Systems, creating other metallic conductive objects in the vicinity ofthe conductor that are not bonded. The reasons for bonding metallic conductiveobjects 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 fastenerswill not be subject to potential difference.I believe these components will be subject to potential difference and an arcingspark is definitely possible. There exists a common misconception that lightningalways takes the path of least resistance. It would be more accurate to say thatalthough lightning has a preferred path, it takes all available pathssimultaneously to a lesser or greater extent. Where we have two parallel pathsfor the current to flow, the total current will divide in inverse proportion to theresistances (impedance). The conductor would be the preferred path to ground(primary) but the higher the resistance of the earth (ground) termination and thehigher the impedance in the conductor, the greater the current in the alternativepath (flashover). (Dr V. A. Rakov Per com 2007) Having seen the conductors inthe ANSI system and the recommendations for earth (ground) termination, Ibelieve the systems will be of high impedance.Copyright Ben Fuest