- California has suffered major devastation from power lines sparking wildfires during high wind conditions. As a resort, Southern California Edison (SCE) is shutting off the electricity to prevent fires caused by downed power lines and high winds during extreme Red Flag Warning periods (National Weather Service alert). Many recent fires are suspected to have been caused by power lines damaged during periods of high wind, including the Woolsey, Camp, Cascade, Thomas, Atlas, Nuns, and Partrick fires.
The issue for many residents is that there little to no notice or public outreach regarding when and where this will happen, leaving people without the opportunity to prepare. SCE will shut off electricity in high risk areas but the public does not know which areas SCE has identified as high risk. This can be an inconvenience at best and a safety concern at worst for residents. Other complaints have been that the power stayed shut off well after the winds had calmed down.
“We understand the impact [that] turning off power could have on customers,” Southern California Edison’s incident commander, Paul Grigaux, said in a statement on Edison’s website, Energized. “We will only shut off the power as a last resort when weather conditions are so dangerous that flying branches, palm fronds and other vegetation pose a threat to power lines and the safety of the community.”
Act of Nature or Faulty Equipment?
How Public Safety Power Shutoff Works:
- SCE officials understand the impact turning off the power could have and will only act when weather conditions combine to pose an extreme wildfire threat in an area.
- The power shutoff will not necessarily affect the whole community, only the areas identified as high fire risk. Based on historical weather records, SCE anticipates conditions that might prompt a power shutoff could possibly happen two to 10 times a year across the company’s entire 50,000-square-mile territory.
- Before a power shutoff happens, SCE meteorologists will begin, up to a week in advance of a predicted severe weather condition, running computerized weather simulations of expected conditions.
- Two days in advance, if the National Weather Service calls a red flag alert, SCE will begin working with local governments and first responders and will notify customers by phone message, text or email, that a power shutoff is possible.
- The power will only be shut off after the weather data, confirmed by SCE personnel in the field, show there is an imminent danger of objects such as tree limbs, palm fronds or other vegetation blowing into power lines. The shutoff will be done in consultation with local officials and emergency response personnel such as local fire departments. Customers will be notified again when the power is shut off.
- It could take some time for power to be restored because SCE must wait until the weather conditions abate and for the circuit to be visually patrolled for possible problems, like downed lines or trees in lines.
How Do Power Lines Cause Wildfires?
Downed lines– Just like homes and office buildings, power distribution systems contain protective devices (e.g. fuses, circuit breakers) that detect short-circuit fault conditions and operate to limit damage to the system. These devices are intended to clear faults quickly, but in as many as 30% of cases in which a single energized line conductor breaks and falls to earth, surface contact resistance causes the resulting fault to draw too little electrical current to blow a fuse or trip a circuit breaker. Such a condition is known as a high-impedance fault, sometimes abbreviated as a HiZ (pronounced “high zee”) fault. A line with a HiZ fault can remain energized on or near the earth for an arbitrarily long period of time, often multiple tens of minutes, during which it produces high-energy, high-temperature arcing. The image below shows a downed-conductor, HiZ arcing fault on a 7,200-volt power line. It is common for a downed line conductor to remain energized and arcing until a customer calls the utility company to report a lights-out condition, which may occur only after several tens of minutes. An arcing downed conductor readily ignites proximate vegetation and other materials, particularly if it occurs in an area of elevated fire risk. Even if conventional protection finally operates, the period of arcing already may have started a fire.
Vegetation Contact – Trees and other vegetation intruding into power lines can cause fires in multiple ways. A tree falling across a line can tear the line down and result in a downed line. A branch spanning two line conductors for a sufficient period of time may ignite the branch and also may produce high-energy, high-temperature Jacob’s Ladder arcs multiple feet in length. If the branch remains in contact and arcing, it can cause progressive damage that eventually breaks the line. The picture below shows a tree branch that contacted a line intermittently, over a 24-hour period, and eventually burned the line down.
Repetitive Faults – Each power line fault creates some risk of fire. Most faults are isolated events (e.g. animal contact, etc.) that do not repeat. Some faults, however, will occur multiple times unless a utility takes corrective action. Repetitive faults can be caused by vegetation, conductor slap, or equipment that is in the process of failing, such as a cracked insulator.
Apparatus Failures – Many power line components (e.g., switches, insulators, transformers, …) provide trouble-free service for decades. A typical circuit may have hundreds or even thousands of components, making it impractical to inspect or test all components on a frequent basis. Components eventually fail. As they do, they often go through an pre-failure period, during which they continue to serve load until progressive deterioration causes complete failure. Pre-failures often involve arcing and sparking at levels too small to be detected by conventional technologies. Over time the arcing and sparking may increase in intensity and, under the right conditions, can ignite proximate combustibles. More commonly these pre-failures cause progressive damage that eventually evolves into high-energy arcing or even burns conductors in two, resulting an energized wire on the ground, which provides a ready source of ignition, as discussed above.
Pros and Cons of Underground Power Lines
- Reduces outages because buried lines are not susceptible to damage from high winds or falling trees
- Looks better aesthetically
- Generates a positive economic impact by reducing outage-caused downtime for local businesses
- Can be coordinated with road repair activity to reduce excavation costs
- Installing underground lines can cost 7-10 times more than overhead lines, a cost that would likely be paid by customers in the form of higher rates
- Buried lines must be protected by conduit, otherwise they are susceptible to shortages from groundwater infiltration
- Buried lines can take longer to repair because the damaged area is usually more difficult to locate
- Undergrounding can be risky due to the presence of underground obstacles or other utility lines like gas, water or telecommunication lines.
- Underground power lines would not prevent outages caused by damage to high-voltage lines or towers