Minimizing Magnetic Fields

Minimizing Magnetic Fields

A power upgrade presents both pitfalls and opportunities for EMF mitigation measures. The plan was simple: Upgrade the electrical power delivery system of an aging 20-floor commercial office building located in a prime downtown Los Angeles location in hopes of attracting high-tech tenants particularly telecommunications companies (Photo). However, the engineers were forced to confront a problem that

A power upgrade presents both pitfalls and opportunities for EMF mitigation measures.

The plan was simple: Upgrade the electrical power delivery system of an aging 20-floor commercial office building located in a prime downtown Los Angeles location in hopes of attracting high-tech tenants — particularly telecommunications companies (Photo). However, the engineers were forced to confront a problem that has become increasingly prevalent on such projects: electromagnetic interference. As offices are flooded with a deluge of computers, faxes, and printers, the location of electromagnetic field (EMF)-producing power equipment, such as transformers, switchboards, and power cables, and their proximity to those electronic devices are design considerations that can no longer be overlooked.

Left unmitigated, power frequency magnetic fields could potentially render several tenant spaces unusable for technology operations. With this in mind, the engineers designing the upgrade at the Los Angeles site employed mitigation techniques throughout the construction process to lower these fields.

The plan.

As part of the power distribution upgrade, the building owners planned to substantially increase the power capacity by installing new 34.5kV utility transformer service equipment to accommodate the increased demand of the telecommunications switching equipment. The existing utility electric transformer and the building's main electrical distribution equipment were located in the basement parking area. Space and access restrictions precluded installation of the new utility transformer and supporting electrical equipment in the remaining area. After an extensive search for adequate space, the owner selected a 2,500-sq-ft, street-level retail space to house the new equipment.

Putting the new electrical equipment in this location, however, meant tenant spaces filled with sensitive electronic equipment like computers and fax machines would be on all sides and above the new equipment room. This condition would have a significant effect on how the project's engineers implemented upgrade plans.

Magnetic field levels emitted by electrical equipment are generally proportionate to the amount of current flowing through the system. At the power frequency level (50 Hz/60 Hz), electric and magnetic fields operate independently of each other, which is not true at higher frequencies. Because common building materials block the electric portion of the fields, the elevated magnetic field levels were the main focus of concern.

Complicating the situation, one of the building's existing tenants, a large environmental law firm located on the second floor immediately above the planned equipment room, also expressed concern about possible health risks to employees due to increased levels of EMF. Although it hasn't been proven that exposure to power frequency magnetic fields can cause health problems, the law firm advised building owners that any increase in magnetic field levels in their second-floor space created by the new electrical facility would be unacceptable.

Field Management Services developed computer projections of magnetic field emission levels from the planned power system. The firm compared those projections to real- world magnetic field measurements of existing power systems in other facilities with similar characteristics and potential loads. The analysis included calculations for the planned bus ducts and major conduit runs as part of the system. Computer models confirmed that without field mitigation measures, significantly elevated power frequency magnetic field levels would exist in adjacent tenant spaces after installation and energizing of the new equipment.

Projections confirmed that substantial magnetic field levels — in tens of milligauss (mG) — would likely exist in tenant areas adjacent to the new power facility, including the second-floor law offices. The projected magnetic field levels would be significantly higher than typical ambient or background magnetic field levels found in commercial buildings, which generally range from 0.5 mG to 3 mG. The assessment study also indicated that magnetic fields would be substantially above the known threshold interference levels for various types of technology equipment. Most computer monitors exhibit interference in the form of “jitter,” or distortion, at a threshold level of about 10 mG, but some are sensitive to magnetic field environments as low as 3 mG to 5 mG.

The firm developed a scheme to implement EMF mitigation measures during the design and construction of the new electrical facility to ensure magnetic fields wouldn't interfere with sensitive equipment in the tenant areas adjacent to the new power facilities.

Much of the EMF mitigation plan was in the form of low-cost engineering modifications. The first measure was to change the planned routing of a high-current bus duct that connected the utility transformer to the building distribution equipment. Originally, the high-current bus duct was scheduled for installation in the ceiling area of the transformer vault, directly beneath the second-floor law firm tenant space. Computer modeling indicated that relocating the bus duct from the ceiling area to a location near the transformer vault floor level would substantially reduce magnetic field levels in the second floor tenant area.

As a second mitigation measure, Field Management Services incorporated a special magnetic field shielding scheme into the new power facility construction sequence, which included the installation of special magnetic field shielding material below the large utility transformer and on portions of the walls and ceiling, plus in areas of the adjacent electrical distribution room, prior to the placement of switching equipment.

Post-construction magnetic field measurements in all adjacent tenant areas confirmed the mitigation measures had successfully reduced magnetic fields emanating from the newly installed electrical equipment. Magnetic field levels in all of the adjacent tenant areas and the second floor law offices ranged from 0.1 mG to 3 mG — a level below the pre-upgrade values. This suggested that the benefit of new shielding installation, in terms of magnetic field reduction in adjacent occupied spaces, was measurably greater than the reduction due to distance from the prior, much smaller, electrical system components.

Measures to reduce magnetic field levels that may exist in a new building construction or as a result of building refurbishment or expansion are more efficient and cost-effective if implemented as an integral part of the construction process. Although the total mitigation plan slightly increased the price of the utility upgrade, it was a fraction of the cost — not counting the tenant dislocations — that would have been required to correct the problem had the owner tried to mitigate the fields after construction.

With the new power system installed and energized, the building could support its new tenants. The foresight of the engineers on this project prevented a potentially expensive and disruptive post-construction problem.

Munderloh is a senior consultant with Field Management Services Corp. in Los Angeles.

Sidebar: Strategies for Dealing with a Magnetic Field Problem Distance. Magnetic fields generally diminish with the square of the distance. Increasing the distance between the source of magnetic fields and the affected area is frequently the easiest and least expensive option.

Source reduction.

In some instances, it's possible to reduce magnetic field emissions by implementing one of several changes in the electrical system. Such changes might involve grouping cables tightly together; if all of the current in a circuit — both the feed and the return current — is in essentially the same area/space, the resultant magnetic fields will cancel out one another naturally. Conversely, to the extent that the current conductors are separated from each other, cancellation is less. By design, bus bars, which are commonly used to carry high- current loads from transformers to the switchgear inside of a building, have relatively large separations between the conductors for maintenance purposes. If you substitute cables for bars, you can group them together in close proximity and get improved cancellation.

Wiring corrections.

The NEC requires all three phases to run in the same wireway because magnetic fields generated by each phase conductor will be 120° out of phase. This produces an effect called “natural cancellation” — if the wires are close enough to each other, the fields generated by each phase conductor add up vectorially to zero. Ensure the wiring meets Art. 300 requirements.


The use of magnetic field shielding is usually the least desirable mitigation option because it's technically challenging and typically expensive. Power frequency magnetic fields typically pass undiminished through most conventional building material, including concrete, earth, wood, and most metals, including lead.

TAGS: Design
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