Some background on Ben and installing Lightning Protection systems, attached and below:
Lightning conductors for buildings are common-place. Similar systems for
trees do exist (and many are installed, particularly in the USA) but have
been based on systems originally for use on built structures. Arborbolt
Ltd’s initial concern was to protect important, valuable trees from damage
resulting from lightning strikes, with minimal impact on the tree and its
roots. However it quickly became apparent that equal importance needed
to be given to the protection of nearby structures and buildings that might
be liable to damage as a result of a strike.
Current best practice for the installation of lightning conductors in the
UK is set out in BS6651:1999 but says nothing about arboricultural con-
straints. Working to this standard, the conducting cable and fixing screws
would become encapsulated within the bark, if not removed and replaced,
leading to extensive damage. Such systems are likely to cause some degree
of long-term harm to the living tree while trying to protect it.
Enter the Arborbolt Lightning Protection System for Trees.
Where did this development stem from?
Ben Fuest was, for many years, an established harvesting contractor in the
Marches area, working softwood and hardwood. As with so many similar
businesses, the lack of continuity of work, poor rates (and getting worse),
hard-to-find cutters, and trying to maintain sensible cash-flow, forced
Ben into making the decision to stop contracting and sell the kit while
it still had some value. With this change in his work routine and other
opportunities coming along, the issue of lightning protection emerged.
In conversation with colleague and local arb consultant Jerry Ross, some
research was undertaken and it was found that there were gaps in current
lightning protection systems for trees.
The British Standard for Lightning Protection requires a lifespan of 30
years for a system. The Arborbolt system addresses the issue of mechani-
cal fixing to the tree and support of the air terminal and cable, while
accommodating growth of the tree. Up to now conventional systems
require periodic removal and refitting to avoid being absorbed by the
growing tree. Any such protection system will be invasive to a degree but
using Arborbolt means the disruption to living tissue is kept to a mini-
mum and in fact reduces over time. However, a good lightning protection
system will only work if there is an equally good earth.
The design of each system begins with an investigation of the soil proper-
ties to enable a specification to be drawn up – sufficient to take away the
current, while keeping disturbance to surrounding roots at a minimum.
To improve the earth’s efficiency an inert soil conditioner, Bentonite, is
used to increase conductivity between the earth terminal and soil.
At the other end, the top of the tree, the air terminal is similarly impor-
tant: since a fundamental reason for installing lightning protection is
the aesthetic benefits of particular trees, reduction of the system’s visual
impact must be in sympathy with the tree as a feature in the landscape.
Research by Arborbolt has concluded that a small, discreet air terminal is
not only wholly adequate, but much better than conventional, buildings-
based air terminals which are large, obtrusive and not easily able to allow
for future tree growth.
www.forestmachinejournal.com
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What happens when a tree suffers a lightning strike?
The arc of lightning occurs once the potential difference, measured in
volts, between two points (cloud to ground) becomes so great that the air’s
inherent insulating properties break down – it becomes ionised due to a
very high potential difference – and it conducts electricity. Trees act as a
convenient conductor to ground (especially when wet) and once a strike
occurs a current of over 20,000 amperes can discharge causing instanta-
neous heating of the surrounding area. The sapwood under the bark also
provides a convenient route to ground and when this sap is subjected to
the intense heat produced by the current surge a vertical strip of bark is
‘blown’ off the tree. This happens instantaneously and with huge force. If
a pocket of sap deeper within the tree is turned to steam then the damage
will be more explosive with branches or stems splitting open.
I was fortunate to be on site during a recent Arborbolt installation on 2
Sequioia giganteum for East Sussex CC at one of the primary schools in
Uckfield. The dangers were obvious and should one of these 45m trees
(150ft in old money) be struck the results could be catastrophic for the
nearby buildings and anyone inside.
What struck me most was the calm, methodical manner in which the
whole operation took place. All the design work on the system had been
completed some weeks before, everyone knew what they had to do and got
on with it. On the day, there were changes to the original design as a result
of physically installing the system, but problems were straightforwardly
overcome. I later concluded this was due purely to the vast amount of
research, effort, expense and time that has been put into developing what
is a new discipline within arboriculture – Arborbolt is, as far as anyone is
aware, ‘the first serious attempt to integrate lightning protection into an
arboricultural context’.
Two very capable climbers, Bill Blythe and James Nicholson, who had
been recommended by Treevolution’s Liam McKeown, climbed the tree
(no chainsaws required, only a hand saw). Equipment (cable, tool bag etc.)
was sent up to them once a safe position was established and they began
by fixing the solid copper air terminal to the top of the tree. Meanwhile at
ground level the earth system was being prepared to bring the cable into
open ground away from the main root system (and away from other root
systems). This involved carefully hand digging a 30cm deep trench from
the bottom of the tree out to (if possible) open ground, being careful not
to damage roots on the way - the idea being to have established the earth
system as the copper cable is fixed down from the top of the tree by the
climbers.
At a distance (away from the main root system, outside the drip line) an
earth auger was used to drill a 1m deep hole so that the top of the earth
rod when in place is approximately 1m below the ground level. The soil
conditioner was then mixed and placed in the hole before the first rod
is driven in so as to ensure best conductivity between rod and ground,
assisting to reduce resistance in the system to an acceptable figure – 10
ohms system-to-earth resistance is the final target. Earth rod extensions
are added ideally to a depth of about 3 metres. Once the rod was as deep
as possible (about 2.5 m in this case), the soil resistivity was checked. The
result was too high and required that a second earth rod be sunk, about
2m further out (in a straight line). On re-testing the soil resistivity was
15 ohms. Back-filling the trench (and thereby increasing contact between
cable and soil) accounts for at least 5 ohms and so the desired 10 ohms
was reached. Soil resistivity continues to improve for a time following
installation. With the turfs replaced and the site cleared there was little to
reveal the work that had gone on.
The second system was then installed in the other tree – a similarly tall
Sequoia. No unexpected problems were encountered although trench dig-
ging was more complicated due to the number of different root systems
in the area. A third earth was required on this system.
Many questions remain to be answered. Figures for the number of light-
ning strikes per annum in the UK stand at 550,000 – that’s 1500 per day.
What is the likely effect of global warming? More storms are predicted
for the UK as a whole. Arborbolt have found that a whole host of other
factors need further investigation: Altitude of the storm, its intensity, the
amount of rainfall, topography, soils, tree species, and the nature of previ-
ous arboricultural operations, are all largely unknowns. There is clearly
much more development to be done.
Thanks are due to Ben Fuest and Jerry Ross for inviting me to this
installation, and helping with these words. It was a pleasure to work
alongside both them and the 2 excellent climbers in Bill Blythe and James
Nicholson. When Ben asked me in passing whether 2 climbers recom-
mended by Treevolution would be any good I was sure they would be that
and more. These 2 installations will not be the last they undertake.
For more information please contact Ben Fuest on 01989 740608.
Ian Sheffner
