Progress Energy is pursuing a balanced solution strategy to ensure an adequate supply of clean, reliable, affordable power for our customers today and tomorrow. To ensure the dependable delivery of this power, our efforts to improve service reliability are a focus every year. The power lines that deliver electricity are affected by many things, including inclement weather, animals, trees, people and equipment failures.
Over the last several years, we have (1) increased our preventive maintenance, (2) installed equipment to reduce the number of customers affected by outages, (3) worked to reduce momentary interruptions by installing guards and signals to deter animals from accessing equipment and by implementing a weekly report to identify problem feeders and address issues early on, (4) replaced more than 840 miles of underground cable, (5) replaced thousands of power poles and transformers, (6) renovated substations and (7) installed specially insulated wire in targeted areas. The insulated wire installation, along with our continued maintenance of thousands of miles of rights of way, has helped reduce the impact of trees on reliability.
In 2011, the company continued its underground cable diagnostics program that helps us identify which cables need to be replaced. We have now replaced more than 70 miles of cable while confirming through testing that more than 80 percent of the underground wire was operating properly. This program continues to save our customers money and helps reduce power outages related to cable failures.
We continually monitor a variety of indices to ensure a superior level of reliability for customers, including the length of time that an average customer on the system is without power during the year (excluding hurricanes or other major storms), or the number of momentary (or very short-duration) power interruptions. This monitoring allows us to identify where we need to act to solve a problem, often before it affects customers.
One of the ways we work to provide reliable power is to prepare for storms and to make effective repairs, restoring power safely and quickly after Mother Nature causes damage.
On April 16, 2011, a large outbreak of tornadoes struck the Carolinas, causing significant damage, affecting three of our four service regions in the Carolinas and leaving more than 240,000 customers without power at the peak of the event. Despite the challenges crews faced in the wake of the storms, we safely restored service to more than 80 percent of customers within about 24 hours, to 96 percent by April 19 and to all customers capable of receiving service by April 20.
On March 30, a cold front moved across our Florida service area from west to east and created hurricane-strength wind gusts, heavy rain, hail and lightning, and spun off several tornadoes. More than 330,000 customers lost power. All were restored within four days.
On Aug. 27, the first hurricane to make landfall in the U.S. in three years left widespread damage across the North Carolina coast and coastal plain. More than 400,000 customers were left without power due to Hurricane Irene. Despite extensive flooding, thousands of fallen trees and other hazards, crews were able to safely achieve 100 percent restoration by Sept. 1. In advance of the storm, we mobilized more than 2,400 personnel into various restoration and storm support roles, helping to accelerate the restoration after the storm passed. Transmission crews restored 17 high-voltage lines, 28 substations and 18 wholesale customer points of delivery. Distribution crews replaced more than 90 miles of conductor, nearly 1,000 poles and more than 700 overhead transformers.
During 2011, we also provided assistance to peer utilities following major storms, as part of the mutual assistance agreement we maintain. We deployed approximately 200 crewmembers to assist with power restorations following a historic Northeast snowstorm in October. We also supplied crews to utilities in Alabama and Georgia following deadly tornadoes in April.
Reliability performance for both Progress Energy Carolinas and Progress Energy Florida is typically in the top quartile for Southeastern utilities. Progress Energy maintains a rigorous inspection process for its delivery system, and works actively to address reliability trends that fall short of our expectations. Significant projected growth in our service territory over the long term will require investing millions of dollars in building and upgrading miles of electric transmission and distribution facilities to ensure that our customers can continue to enjoy a reliable supply of electric service to their homes and businesses.