FMS Publications

Assessment and Mitigation of Magnetic Fields
in a Commercial Building Adjacent to Transmission Lines

During final construction of a five-story commercial office building in St. Louis, Missouri, the developer's leasing agent expressed concern that magnetic fields from high voltage transmission lines close to the building, could cause interference problems with office equipment and make portions of the building difficult to rent.

Field Management Services (FMS) was retained by the building's developer to assess magnetic field conditions in the new building. FMS documented magnetic field conditions in portions of the building sited adjacent (within 75 feet) to a double circuit 345 kV high voltage transmission line. Using the magnetic field measurement data, FMS developed and implemented mitigation recommendations.

Magnetic Fields and Buildings

The primary concern with such transmission lines or any source of electricity is the fact that electric and magnetic fields (EMF) are naturally emitted as a consequence of the use and distribution of electricity. At the Extremely Low Frequency (ELF), also called the Power Frequency, the electric and magnetic fields operate independently of each other. While the electric fields are relatively easy to stop, the magnetic fields are difficult to block and can cause interference with sensitive electronic equipment, including computer monitors.

Since EMF is present wherever electricity is used, there is an inherent or background level of EMF in any home or commercial building. There are multiple potential sources of EMF within any commercial building that form background levels. These sources include the structure's electrical distribution system, lights, transformers, electric fans, copiers, etc. As such, any measurements of EMF contain some level of background or ambient level of EMF. Background levels of magnetic fields in commercial buildings usually range from 0.2 to 4 milliGauss (mG).

EMF or Magnetic fields from high voltage transmission lines located within 75 feet of a new commercial building cause interference with computer monitors and other sensitive equipment.

Magnetic Field Conditions in the Building

Magnetic fields that naturally emanate from sources such as the subject 345 kV transmission lines at the St. Louis site are directly proportionate to the amount of current flowing on the transmission line at any given time. A higher loading condition such as may be present in hot weather summer months, will result in increased magnetic field levels. It was therefore prudent to evaluate EMF conditions in the building under maximum loading conditions.	3D plot of magnetic field levels in building interior before mitigation measure
Utilizing historical peak loading data and power line circuit loading conditions, FMS was able to "scale" magnetic field measurements taken during winter months (low loading conditions) to EMF levels which are representative of peak loading conditions which normally occur on peak load days during summer months. Magnetic field conditions in the southeast area of the first floor ranged from 2 to 8 mG in mild weather months and were likely to range from 6 to 24 mG during peak loading conditions. Magnetic fields in the southeast portion of the building on floors 2 through 5 generally averaged considerably less (under 2 mG) and were considered ambient.	3D plot of magnetic field levels in building interior after re-phasing of transmission lines
However, somewhat elevated magnetic fields on floors 2 through 5, were present within four to eight feet of the exterior glass windows along the south and east faces of the building and within ten to fifteen feet of the glass in the extreme southeast corner of the building. EMF values in these areas immediately adjacent to the exterior window generally averaged approximately 3 mG and could increase to 10 mG or more during peaking loading conditions. Therefore, computer monitors and other sensitive equipment located within ten feet of windows on the southeast portion of the building were at some risk for experiencing interference.	3D plot of magnetic fields in building interior after re-phasing of transmission lines plus installation of special magnetic field shielding material

Mitigation of Magnetic Fields

The majority of the building was found to have acceptable or ambient EMF conditions present even when peak loading conditions are present on the passing transmission lines. However, as previously mentioned, somewhat elevated magnetic field conditions were documented in the southeast corner of the of the building on Floors 1 though 5. It initially appeared that to lower magnetic fields in the southeast corner of the building, it would be necessary to permanently block exterior windows on all floors in the southeast corner of the building to allow for the installation of magnetic field shielding material in the area of the windows. The building's leasing agent advised that loss of exterior vision glass may seriously effect leasing rates for tenants in the subject area of the building.

FMS developed and implemented an alternate multi-phase mitigation plan to reduce EMF levels in the affected southeast portion of the building without requiring loss of the exterior window vision glass. As a first measure, the local utility agreed to "re-phase" one of the 345 kV power lines. FMS computer simulation studies and prior experience indicated that EMF levels could be reduced in the order of 40% by re-phasing the transmission line conductors in order to improve natural cancellation of magnetic fields between opposing conductors. This was accomplished by changing the phasing configuration of the conductors where they connect at a terminal point. Magnetic field measurements were taken by FMS in the southeast portion of the building after completion of the re-phasing project. The measurements confirmed that re-phasing of the transmission lines successfully reduced magnetic field conditions throughout the building by 35 to 40%.

The first floor of the building is a concrete slab on grade and does not have the galvanized steel decking present on Floors 2 through 5, which benefit from the shielding effect provided by the composite concrete/galvanized steel floor deck. FMS recommended that steel plate shielding of similar characteristics as the steel decking on the other floors, be installed on the first floor slab, throughout the southeast corner of the building to lower EMF to similar levels present on the 2nd through 5th floors. Special steel shielding plates were subsequently installed at the request of the building's developer on the floor slab in the southeast portion of the first floor. Magnetic field measurements taken by FMS confirmed that installation of shielding material on the first floor slab successfully reduced magnetic field conditions in the southeast corner of the first floor to levels comparable on the 2nd through 5th floors.

As a final mitigation measure, special conductive magnetic field shielding material was installed on floors 1 through 5 in the extreme southeast corner of the building. The shielding material was installed on the floor slab in a "zone" of approximately 10 feet in front of the windows in the extreme southeast corner of all floors. Shielding material was also installed on short wall surfaces beneath the windowsills in the southeast corner of each floor. Magnetic field measurements taken by FMS after installation of the shielding, confirmed that modestly elevated magnetic field conditions close to the window locations were successfully reduced to levels comfortably below 5 mG in all areas of the southeast corner of floors 1 through 5 as a result of the shielding installation.

Installation of special magnetic field shielding material on portions of the floor
and beneath windows effectively reduced magnetic field levels.

Summary

Elevated magnetic field conditions in the five story commercial building were successfully reduced by the implementation of a multi-phase mitigation plan that did not require loss of exterior window locations to magnetic field shielding installation. Mitigation measures included re-phasing of the passing 345 kV transmission line conductors and with supplemental installation of special magnetic field shielding material in the extreme southeast corner of the building on floors 1 through 5. Magnetic field levels in all areas of the building should remain comfortably below 5 mG under normal and peak loading conditions present on the passing transmission lines. As such, all areas of the commercial building should be free from any interference issues from the passing high voltage transmission lines.