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These erosion control and vegetation establishment practices are regularly used in construction projects and are referenced in the construction permit plans. Long-term impacts are minimized by using these construction techniques. 5.1.6 Maintenance Procedures Transmission lines and substations are designed to operate for decades and require only moderate maintenance,particularly in the first few years of operation. The estimated service life of a transmission line for accounting purposes is approximately 40 years. ' However, practically speaking, transmission lines are seldom completely retired. Transmission infrastructure has very few mechanical elements and is built to withstand weather extremes that are normally encountered. With the exception of severe weather such as tornadoes and heavy ice storms,transmission lines rarely fail. Transmission lines are automatically taken out of service by the operation of protective relaying equipment when a fault is sensed on the system. Such interruptions are usually only momentary. Scheduled maintenance outages are also infrequent. As a result, the Iaverage annual availability of transmission infrastructure exceeds 90 percent. The principal operating and maintenance cost for transmission facilities is the cost of inspections, ' usually done monthly by air. Annual operating and maintenance costs for transmission lines in Minnesota and the surrounding states vary. For transmission lines with voltages ranging from 69 kV through 345 kV, experience shows that the annual maintenance cost is approximately $300 to $500 ' per mile. Actual line-specific maintenance costs depend on the setting, the amount of vegetation management necessary, storm damage occurrences, structure types, materials used, and the age of the line. ' Substations require a certain amount of maintenance to keep them functioning in accordance with accepted operating parameters and the NESC and NERC requirements. Transformers, circuit breakers, batteries, protective relays, and other equipment need to be serviced periodically in accordance with the manufacturer's recommendation. The site itself must be kept free of vegetation and drainage maintained. 5.2 Electric and Magnetic Fields The term electromagnetic fields ("EMF") refer to electric and magnetic fields that are coupled ' together,such as in high frequency radiating fields. For the lower frequencies associated with power lines (referred to as "extremely low frequencies" [`ELF"J), EMF should be separated into electric fields ("EFs") and magnetic fields ("MFs'), measured in kilovolts per meter ("kV/m") and ' milliGauss ("mG'), respectively. These fields are dependent on the voltage of a transmission line (EFs) and current carried by a transmission line (lb's). The intensity of the electric field is proportional to the voltage of the line, and the intensity of the magnetic field is proportional to the current flow through the conductors. Transmission lines operate at a power frequency of 60 hertz (cycles per second). 5.2.1 Electric Fields There is no federal standard for transmission line electric fields. The Commission, however, has imposed a maximum electric field limit of 8 kV/m measured at one meter above the ground in its previously issued permits, including the recent order In the Matter of the Route Permit Application for a ' Orono Project June 7, 2011 MPUC Docket No. E002/TL-11-223 31