Long term magnetic field monitoring near power lines

As part of regulatory requirements, a US electric utility is carrying out a long-term magnetic field monitoring program before and after a substation expansion. The program began with the drafting of a strict protocol to ensure that data collection over time by different personnel will follow exactly the same procedures. As other utilities may be involved in similar regulatory processes, this paper presents a long-term monitoring protocol, and the results of about 91 site-months of data from six monitoring sites before energization of the new facilities. Operating experience shows that the protocol was well conceived and can be executed with little supervision. Measurement results show daily, weekly and seasonal cycles of magnetic field levels near power lines. Normalized percentiles and means show that these cycles were similar at all six sites, and there was an upper bound for field levels at each site. Throughout the monitoring period, which exceeded 18 months at some sites, maximum magnetic field levels with durations longer than 15 minutes were about 270% or less of the medians.