BMP - Utility Tree Risk Assessment - 39
fall characteristics, fall distance, and any protective factors that may reduce
the force of impact, and the type and robustness of the utility target. Consequences can also include considerations of tree-conductor contacts that
don't cause mechanical damage but that may result in an electrical fault,
interruption, and service outage to customers.
There are two categories of consequence considered in UTRA: (1) direct
and (2) indirect. Direct consequences describe physical damage to overhead
transmission and distribution lines and equipment caused by the tree strike.
Indirect consequences consider outcomes that may occur as a result of damage to the energy delivery system and may consider the cost of repairing
damage, impact to electric service reliability, public safety, and other concerns, such as the risk of wildfire ignition.
Direct Consequences
In estimating how much damage could occur from a tree failure, consider the
relative amount of force from its impact. A falling tree or branch will gain
speed as it accelerates toward the ground. So, in general, the higher the distance from which a branch falls, or the greater the distance from the tree to
the target, the greater the force that the tree or branch will have at the point
of impact.
Generally, a small branch has less potential to cause damage than a large branch.
Small branch failures occur more frequently and may result in power outages if
they provide a fault pathway between energized conductors. Branches less than
1 inch (2.5 centimeters) in diameter usually are not considered in most tree risk
assessments. The minimum size branch that should be considered by the utility
tree risk assessor may be specified in the scope of work.
If the distance from a tree trunk to a distribution line with relatively largediameter conductors is small, a tree that fails may simply lean against the line,
causing minor physical damage. On the other hand, if the distance is such
that the tree can accelerate significantly before the trunk strikes the line,
damage may be much greater.
Direct consequences of a tree strike to electric utility systems can be categorized using a four-level classification system (severe, significant, minor, and
negligible). Consequences categories should be determined by the utility vegetation manager based on experience and the type of utility infrastructure
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BMP - Utility Tree Risk Assessment
Table of Contents for the Digital Edition of BMP - Utility Tree Risk Assessment
Table of Contents
BMP - Utility Tree Risk Assessment - Cover1
BMP - Utility Tree Risk Assessment - Cover2
BMP - Utility Tree Risk Assessment - i
BMP - Utility Tree Risk Assessment - ii
BMP - Utility Tree Risk Assessment - Table of Contents
BMP - Utility Tree Risk Assessment - iv
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BMP - Utility Tree Risk Assessment - Cover4
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