FMS Project:


Assessment and Mitigation of 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.

Interior photo of part of the affected space adjacent to transmission lines

The building’s leasing agent advised that loss of exterior vision glass may seriously affect leasing rates for tenants in the subject area of the building. To achieve the necessary reduction in fields while preserving the marketability of this property, a multi-phase mitigation plan was required.

3-D Plot of magnetic field levels in portion of building prior to mitigation measures

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.

345 kV double circuit line looking northwest maximum cancellation phasing

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.

3-D plot of magnetic field levels after re-phasing of transmission lines

The measurements confirmed that re-phasing of the transmission lines successfully reduced magnetic field conditions throughout the building by 35 to 40%.

Plan view of shielding plan for first floor

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 provides some shielding benefits. FMS recommended that steel plate shielding 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.

Plan view of shielding plan for all floors in extreme southeast corner

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.

Photo of completed corner shielding

The special 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.

3-D Plot of magnetic field levels after re-phasing of transmission lines and shielding

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.


Over its 20 years, FMS has successfully completed hundreds of EMI projects which included a diverse range of consulting and mitigation services.

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