Folks have been telling me that transmission lines don’t have as much chance of causing wildfires as distribution lines. Naturally, I was interested in the details of why this is the case. A great big shout-out to BLM NEPA folks who have to examine all these things in detail. I’ll give you some highlights from the Transwest Express EIS (which actually is a generally magnificent EIS with info on everything anyone, or at least I, can possibly imagine). What I like about NEPA docs is that usually the authors don’t have a particular axe to grind, their job is to discuss the pros and cons, hopefully so regular people can understand them.
I’ve organized the risks about wildfire (causing them) and (being harmed by them). If we believe (I don’t, thanks to fire suppression folks) that wildfires will increase by 50 % (or whatever) due to climate change, does it make sense for us to decarbonize with sources that will need thousands of miles of transmission lines through wildfire-prone areas?
Workers Doing Construction and Maintenance
Impacts to ignition points from operation or maintenance activities such as welding, vehicle ignition, blasting, blading, and overland travel would be similar to those described under Section 126.96.36.199, Impacts from Terminal Construction and Operation. In addition, the presence of the energized transmission line could increase the risk of wildland fire ignition in areas of high fire risk and lightning strikes. Lightning protection would be provided by overhead shield wires on the top of the line.
Failure of Transmission Structures
Fires where power lines are located can be started by contact between the conductors and/or insulation and anything flammable or that could create a spark such as vegetation, floating or wind-blown debris, bullets, airplanes or helicopters, or other conductors. Failure of transmission structures could occur as a result of intentional damage (e.g., vandalism, terrorism), natural disasters, vehicle or aircraft collision, or a design or engineering flaw in a system component. However, the conductors and structures for high voltage lines tend to be of sufficient size to be resistant to physical damage. In addition, the transmission line would be protected with power circuit breakers and line relay protection equipment. If a conductor or component failure occurs, power would be automatically removed from the line. All buildings, fences, and other structures with metal surfaces located within 300 feet of the alignment would be grounded to the mutual satisfaction of the parties involved.
High Voltage vs. Lower Voltage and Distribution Lines
Here’s the answer to our question about the different kinds of lines, it’s about height and spacing:
While the risk of wildland fire ignition does increase with power lines, high-voltage power lines are much less likely to cause wildland fires than lower voltage and distribution lines due to their height and spacing, which limits contact with other lines, vegetation, and debris. In addition, the applicant will implement the Vegetation Management Plan described in Section 3.5, Vegetation, and Appendix D, POD, to minimize contact and or arcs with vegetation. The Vegetation Management Plan is designed to maintain trees and shrubs within certain heights to limit direct contact with the line, as well as prevent arcs from the power line to trees. A key component of the Vegetation Management Plan is the identification of hazard trees. Hazard trees are defined as trees located within or adjacent to the 250-foot-wide transmission line ROW that present a hazard to employees, the public, or power system facilities.
Looking for more exact numbers, I found a cite in this article
For example, per mile of power line, distribution lines are three times more likely to cause ignitions compared with transmission lines 
And that cite was to this paper: Pacific Gas and Electric Company, “Pacific Gas and Electric Company Amended 2019 Wildfire Safety Plan,” Tech. Rep., 2019
I don’t know if there is better info out there somewhere, please put in comments if you find any.
Impacts to Fire Suppression Activities (Good and Bad Impacts)
If a wildland fire occurs near the Project, wildland firefighters and fire suppression efforts could be negatively and positively impacted. The ROW and structures could be an obstacle, and another feature requiring fire suppression efforts. The energized line during fires could be a risk to fire fighters on the ground, and could limit the area in which aircraft could assist in fire suppression activities. The Project would alter fire suppression priorities during wildland fire events. In portions of the route, the Project may be the only infrastructure in the area, and as such may be an obstacle to letting a fire burn safely to natural or engineered containment boundaries. The energized line and broken conductors can deliver currents long distance, especially if the line or conductors come in contact with linear features such as fences. Smoke particles can carry electrical charge, and dense smoke can allow arcing from the
conductor to the ground. If the Project is not de-energized during a wildland fire event, buffers would be required around structures and conductors for the safety of fire personnel. Positive impacts from the Project on wildland fire suppression would include the development of a 250-foot-wide transmission line ROW and additional access roads acting as fire breaks, and providing access to fire personnel. In addition, the vegetation management associated with the Project could decrease fuel loads, and fire intensity and severity within the ROW.
The presence of the transmission line does increase risk of ignitions through increased access. However, more importantly, it prevents the use of certain fire and fuels management tools (prescribed fire or wildland fire use) in the vicinity of the transmission line. Additionally, it requires a prioritization to suppress fires in and around the line to protect human lives and infrastructure. This, in turn, can result in fiscal impacts to the agency due to the risk of ignitions to suppress, additional values to protect, and the reduction in areas that wildland fire can be used to meet land management objectives. Additional risks include increased potential for undesirable fire effects and increased risk to fire suppression personnel.
But the Same ROW and Roads That Are Good For Fuel Breaks Are Also Bad For Ignitions
Increased access through the new and upgraded network of access roads and the maintained ROW would increase recreation traffic, and trespassing which would increase the potential for more vehicle and human caused ignitions. However increased access roads would increase fire breaks, and allow easier access for fire suppression activities during wildland fire events.
And Some Places Won’t Be Fuel Breaks
Level 1 and 2 (Wire Zone) vegetation management levels (as described in Section 3.5, Vegetation), would create fuel breaks within forested areas. Fuel breaks can assist in wildland firefighting by slowing down fire growth, reducing fireline intensity, and providing enhanced fire suppression opportunities. Level 2 (Border Zone) and 3 vegetation management levels would not receive intensive vegetation management within the ROW, and may not provide a substantial fuel break should a fire occur near the Project. In sage-grouse habitat, the BLM’s WO-IM 2013-128 (Sage-grouse Conservation in Fire Operations and Fuels Management) includes forming partnerships with linear ROW holders to maintain fuel breaks, which reduce fuel continuity and serve to protect at-risk landscapes. As the majority of sagebrush is under the height limits outlined in the Vegetation Management plan, vegetation clearing in TransWest Express EIS Section 3.21 – Wildland Fire 3.21-23 Final EIS 2015 sagebrush would typically not occur. However, the implementation of fuel breaks of sagebrush habitat
could provide a benefit to sage-grouse management by facilitating fire suppression, reducing the acres of habitat burned, and limiting vegetation clearing in suitable habitat.
I wonder if groups that are against hazard tree removals from roads are also against 250 foot transmission ROW’s? Or in Bob Berwyn’s story
The proposal would allow the Forest Service or utility companies with powerlines on the three forests to fell and remove all hazardous trees within approximately 200 feet from the centerline of transmission lines and within 75 feet of centerline of distribution lines.
And finally, are wildfires bad for transmission lines? The flip side of the other question. I couldn’t find the answer easily in the voluminous BLM EIS (maybe we need machine learning to help find things in lengthy environmental docs) but here is one from Southern Cal.
The potential for wildfires to impact the operation of transmission facilities is a concern which must be considered when siting new transmission lines. This is particularly true for transmission lines passing through the southern portion of San Diego County due to the history of wildfires in this area. SDG&E’s existing 500 kV line, the Southwest Powerlink (SWPL), has experienced a number of outages as a result of wildfires along this transmission corridor. A second 500 kV line, collocated for the entire distance between the Imperial Valley and Miguel substations, would be expected to experience a similar outage frequency. The simultaneous loss of both transmission lines could pose a significant reliability concern for SDG&E.
Then there was the threat to California’s electricity grid from the Bootleg Fire. Now, I think that NEPA wise, solar and wind build-out to feed those transmission lines would be a connected action (?) but I didn’t go there. Finally burying transmission lines is a thing, but apparently too expensive, and not even doable for cross-mountain transmission.