FMS Publications

Application Of Laboratory Developed ELF Magnetic Field Shielding Schemes To Commercial Power Panels

INTRODUCTION

Considerable laboratory research has been completed and is continuing on the ability of various materials to shield low frequency magnetic fields. The fundamental equations governing these interactions are well-known and are being applied to test materials in laboratories throughout the world.

Although this research is helpful in the design of a shielding scheme for an actual source, the real world of commercial buildings often presents complications not anticipated in the laboratory. Electric service entrance and switching panels, conduits and bus bar distribution paths are often significant sources of elevated magnetic field levels within commercial, industrial and institutional buildings.

The purpose of this study was to (1) apply some of the shielding material research developed in the laboratory to a typical electric service entrance panel as a well-known commercial source of magnetic fields, (2) develop a set of shielding schemes in a laboratory environment and (3) test the assumption that general, empirical shielding results from the laboratory can be used to predict results achieved in actual commercial sites.

OBJECTIVE:

Industrial single and three-phase service entrance panels can produce high levels of ELF magnetic fields in adjacent areas. Service entrance panels produce highly complex magnetic fields due to the geometry of internal bus bars and conductors which travel asymmetrically in all three axes within a panel's interior. Although service entrance panels are often similar to each other, they can vary greatly in design and construction by manufacturer, load capacity and date of manufacture. The characteristics of magnetic fields produced by service panels therefore vary significantly from panel to panel. Unfortunately, due to these differences, a field management plan utilizing ferromagnetic and/or conductive shielding material developed for a specific service panel may yield uncertain results when applied to a similar but different service panel.

The objective of this study was to develop a variety of simple magnetic field shielding schemes for a typical three-phase service panel in a laboratory environment and to subsequently install the laboratory-developed shielding schemes in several commercial building sites with similar service panel configurations to determine if shielding results could be generally estimated despite differences in the individual service panels.

METHOD:

In a laboratory with a large vertical and horizontal open area and minimal ambient fields, a three-phase service panel was positioned parallel to one side of a 16 ft x 20 ft measurement grid to simulate an office located adjacent to an electric equipment room.

The test service panel, rated at 800 amps, was connected to a three-phase power network wherein three single-phase variable transformers were fed by a 4-wire, three-phase 208 volt source. The transformers were capable of supplying variable voltages from 0 to 140 volts to a set of three loading transformers which were configured for 120 volt input to 5 volt output. The output from each loading transformer was connected to the test service panel. The four conductor paths within the panel were connected together beyond the panel's breaker to create three-phase current circulation. This arrangement allowed for three-phase current settings of 100, 400 and 800 amps to be circulated through the panel generating three separate levels of magnetic fields in the area of the measurement grid.

A variety of ferromagnetic and conductive shielding materials were positioned between the service panel and the measurement grid, and field reduction data was recorded at the 99 data points. This test data was utilized to develop optimum shielding schemes and an average shielding factor (SF = field strength after shielding divided by field strength before shielding) computed for each plan.

After completion of the laboratory testing and development of shielding schemes, two commercial offices and a school site with electric service panels similar to the laboratory study test panel were located to serve as study sites for installation of appropriate magnetic field shielding schemes. Extensive ELF magnetic field measurement surveys were conducted at each of the study sites prior to installation of shielding. At each study site, measurements confirmed that elevated magnetic field strengths in rooms adjacent to the panels were generally comparable in magnitude to those noted from the laboratory service panel.

ELF magnetic field shielding schemes developed with the laboratory service panel were installed in each of the three study sites. Magnetic field measurements were taken before and after installation of shielding, and average shielding factors were computed for each site.

STUDY SITE #1

Elevated levels (700 mG at the wall) of ELF magnetic fields present in two executive offices adjacent to a commercial office building's 1600 amp three-phase service entrance panel rendered the offices useless for computer operation due to extreme monitor interference. The owner had converted these offices to other office support functions.

To facilitate implementation of an ELF magnetic field shielding scheme requiring installation of a continuous layer of 1010 steel on the entire surface of the wall separating the offices from the Utility Room, special interlocking modules containing the steel were manufactured and installed at the site. After installation of the shielding, a new layer of drywall was attached to the shielding modules to restore the original finish.

Based on laboratory data, it was estimated that the average B-field, after the installation of shielding, would be reduced to less than 8 mG.

STUDY SITE #2

ELF magnetic fields at elevated levels in the front portion of this elementary school classroom were emanating from the school's main 800 amp three-phase service panel located in a utility area immediately behind the front wall of the classroom.

The ELF magnetic shielding scheme consisted of the installation of a layer of aluminum on the front wall. The existing drywall surface was removed to allow attachment of the shielding material directly to the wall's wooden framing members. After installation of the shielding material, a new layer of drywall was attached to the shielding material to restore the classroom finish.

Based on laboratory data, it was estimated that the average B-field, after the installation of shielding, would be reduced to less than 2 mG.

STUDY SITE #3

ELF magnetic fields at elevated levels sufficient to distort the image on a color computer monitor were present in an executive office which were emanating from a 800 amp three-phase service panel located immediately behind a wall dividing the executive office from a utility room.

Due to a built-in rosewood bookcase in the executive office and a space restricted electrical room, a special two-layer Aluminum magnetic shield was fabricated and inserted directly behind the service panel via a small cut in the adjoining open office wall.

Based on laboratory data, it was estimated that the average B- field, after installation of shielding, would be reduced to less than 2 mG.

RESULTS:

Magnetic field measurements taken prior to and after installation of shielding schemes at each of the three study sites, confirmed that field strength levels and computed shielding factors were generally of the same magnitude as noted in the laboratory setting. A comparison of shielding results with laboratory projections is presented in the following table.

As expected, the magnetic field strength measurements in each of the study sites displayed significant variation in resultant strength at comparable distances from the wall separating the area of study from the electric service room. Differences in configuration and loading were principle contributors. Similarly, panel load and configuration differences caused the relative locations of maximum and minimum field values to vary with respect to the center of the service panel.

In two of the study sites, conduit paths associated with the service panel were found to be significant contributors to the magnetic field in areas which were to be shielded; the result of vagabond currents in the electric distribution systems.

At the functional level, there was sufficient similarity between results achieved in the laboratory using a particular shielding scheme (a layer of 1010 steel for example) and the performance of the same shielding scheme when applied to an electric service panel of similar configuration and loading in a commercial application to predict the resulting shielding factor within a reasonable order of magnitude level.

CONCLUSION:

A field management plan developed for a specific electric service panel will provide generally equivalent results when used with a similar service panel provided that the unshielded ELF magnetic field strengths and characteristics in the affected space are similar. The confounding and potentially misleading influence of associated factors at actual sites such as conduit or bus bars entering or exiting the panel, switching and circuit panels which are fed from the service panel, and the presence of other magnetic field sources including circuits with high imbalances or net current conditions, must be individually considered for an overall field management solution.