One of the major causes of power line-initiated wildfires is trees or tree parts falling into electrical conductors. What can a utility reasonably do to reduce this risk?
First, any tree that on failure could contact the electrical equipment is deemed a "risk tree." If you are located in North America this is covered in ANSI A300 Part 9. Internationally Risk Management is covered by ISO 31000:2009. As a utility you should be aware of and responsive to these standards. Should you ever have the misfortune of being the defendant in a hundreds of millions of dollars power line-initiated wildfire case, you can expect the plaintiff lawyers will rigorously measure you against these standards.
So let’s go back to that risk tree. Because every tree will fail given enough stress loading, any tree that could contact the electrical equipment is defined as a risk tree. There you have a can of worms. The plaintiff lawyers will want to know the risk assessment made on the tree that initiated the incident. We don’t have a conveniently low statistical probability that you can assign to a tree that appears healthy and structurally sound. In fact, we don’t have probabilistic failure data for trees in general.
When the courts are evaluating a claim for damages, the key question is what a reasonable person or entity would have done to avert the damages. The first step in this, which was covered in the last issue, is recognizing the risk so that it is covered in policy, procedures, and dedication of resources.
The question then emerges: Would a reasonable person examine every risk tree, to what extent and how often? Conversely, we might ask, is it a reasonable expectation that every risk tree be evaluated and to what extent, how often?
International Society of Arboriculture’s Tree Risk Assessment Best Management Practices sets out three levels of tree risk assessment.
- Level 1: Limited visual
- Level 2: Basic
- Level 3: Advanced
Typical utility procedure is to conduct a slow drive by or aerial inspection. These fall under Level 1. This inspection may identify some trees for a more detailed ground inspection. In this case the tree or trees would be viewed from all sides, noting lean, root support, structure and any indications of the tree being affected by pathogens. This would be classified as a Level 2 assessment.
In the aftermath of a power line-initiated wildfire there’s a good chance it will be implied that had you made it a policy to undertake a Level 2 tree risk assessment on all risk trees that the fire could have been avoided. You may find yourself in the uncomfortable position of having to concede that may be true.
Your defense to this argument is that it is not reasonable to undertake a Level 2 tree risk assessment on all risk trees. You can make this argument after the fact but I believe you would be much better served by having the justification for your VM practices documented in advance of any legal proceedings.
What should you document? You should know the percent of your system that has treed edge. You should undertake some studies to determine the total number of risk trees on your system. I’m not talking about an order of magnitude guess like we have millions of trees. It will have far more credibility if you can say we 4.3 million ± 5% risk trees at a 95% confidence level.
Once you know your total tree exposure, which you will find to be stunningly large, you will need to determine the operational and cost implications. That will require a two-week trial having two people doing a walking Level 1 tree risk assessment of all risk trees and conducting a Level 2 assessment where and when necessary. From this trial you should be able to determine the productivity. Let’s optimistically assume that the test reveals they can do 80 trees per hour. Assuming, for example, the previously stated risk tree exposure of 4.3 million trees yields 53,750 crew hours or 107,500 man hours to cover the system once. If you were to do annual inspections, which you should given the magnitude of the risk, this would necessitate 28 crews. You can also then derive the projected annual inspection cost. If we assume a crew cost of $130/hr you will need $6.99 million to cover this program. Keep in mind I have only stated the tree risk assessment costs. Obviously, such an intense program will lead to a higher number of removals and this increased rate you will also determine and cost from the two week trial. The next step is to apply to the regulator to fund this tree risk assessment process and the resulting actual tree work. If you gain approval your program will be one of the most advanced and therefore, difficult to fault. If you do not get approval then the regulator has effectively determined that drive by or aerial inspections as you have been doing are adequate but more importantly to you, as the regulator acts on behalf of the ratepayer, that such Level 1 inspections are what is considered reasonable to the public.
The process I’ve outlined should materially decrease the financial risk associated with power line initiated wildfires. In doing so, it will simultaneously help utilities with the cost and ability to maintain insurance coverage.