Source:
Lightning Protection and Trees
Ben Fuest
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.

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

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Electrodes [used for lightning strike protection] come in different sizes. The ones referred to here
are 1.2m (4 feet) in length and 12.5 - 13 mm diameter, and are made from mild steel with a two-
micron copper bond coat. The ends are threaded so that with the aid of a coupler they may be
joined together. The minimum requirement for length is 2.4m (8 feet).
If the working space is limited then it is sometimes best to add one on top of another, 2.4 - 3.6 -
4.8. If there is room to work in, it is possible to install individual electrodes in parallel.
Tip depth is an important factor to be considered when deciding configuration. Tip depth
determines the distance between electrodes. If we install them and they are too close they will
interact and we lose the performance benefits. If they are too far apart we are wasting time,
materials and causing unnecessary disruption (BS 6651 1999).
The photograph shown here is an example of how not to protect a tree and a nearby building
from lightning strikes. The owner of this home in New York reported recurrent lightning strikes
on tall trees surrounding the building, but even a casual inspection showed that the lightning
protection system had been badly compromised. It is possible that these conditions actually
increased the risk of damage to this tree and building from lightning.
Soil Conductivity & Lightning Strike Protection
When working with soils of poor conductivity it is possible to introduce soil-conditioning agents.
Bentonite is de-composing volcanic ash; it has hygroscopic properties that enable us to enlarge
the diameter of the earth (ground) electrode without the cost implication. Instead of driving the
electrode into the soil we can drill a hole approximately 25cm (10 inches) wide, and as deep as
the intended electrode, and then back fill with the Bentonite mix.
The lightning protection system electrode is then inserted into this. It should be noted that

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

It is suggested that these components will be subject to potential difference and an arcing spark is
definitely possible. There exists a common misconception that lightning always takes the path of
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least resistance.
It would be more accurate to say that although lightning has a preferred path, it takes all
available paths simultaneously to a lesser or greater extent. Where we have two parallel paths for
the current to flow, the total current will divide in inverse proportion to the resistances
(impedance).
The lightning conductor cable would be the preferred path to ground (primary) but the higher the
resistance of the earth (ground) termination and the higher the impedance in the conductor, the
greater the current in the alternative path (flashover) (Dr V. A. Rakov pers. comm. 2007).
Having seen the conductors in the ANSI system and the recommendations for earth (ground)
termination, it is believed that the systems will be of high impedance.
If we look at the scenario of a system having just undergone an upgrade with new fasteners, the