FMS Articles
As seen in 
by Jon W. Munderloh
Magnetic fields produced by electrical distribution systems can cause a variety of interference problems with sensitive electronic instruments. Such Magnetic Field Interference (MFI) problems are on the rise and there appears to be nothing in current trends which will lessen MFI problems in the foreseeable future. As a result, Power Quality Engineers are increasingly called upon to analyze and recommend cures for a problem not usually associated with the quality of delivered power. MFI and the magnetic fields which cause it are a natural consequence of the use and distribution of electricity. Although engineering solutions to MFI problems are sometimes available, often the problem cannot be easily resolved and can involve controversial issues such as concern about possible health effects, which are unrelated to the technical problem at hand. In these instances, the engineer on the scene should proceed with caution.
MFI differs from its better-known cousin RFI, in that it is caused by power frequency sources. It is in many ways more prevalent than RFI, and certainly more insidious, as magnetic fields which cause interference are not materially affected or reduced by all but the most specialized materials. Indeed, magnetic fields at power frequencies pass undiminished through most building materials including cement and lead.
The cause of the increasing MFI problem is simple: more people, using sensitive electronic instruments, forced into smaller spaces by the economics of the workplace. Additionally electronic instruments are not being designed to resist MFI. And to make matters worse, the most common instrument affected, the computer monitor, is becoming more sensitive, not less, to interference from magnetic fields.
The Common and (sometimes) Cheap Fix: Move
Since the strength of magnetic fields reduces quickly with distance, it is often possible to simply move the instrument (and its associated person) a short distance to eliminate interference. The textbook example of a very common MFI problem is a computer monitor work station recently located next to a wall which separates the work space from the building's electrical distribution equipment. Moving the computer work station a few feet can often be enough to eliminate interference patterns on the monitor which are caused by the adjacent electrical equipment.
At times, even such small movements are not feasible. It may be that use of an alternative space is simply not available or that the company has already invested substantially in a current workspace configuration and that the cost and disruption to reconfigure is not acceptable. In these cases, the instrument (the computer monitor, for example) can be shielded from the interference with an FMS JitterBox. This device which is adjustable according to monitor size, protects the monitor by attracting and absorbing the external magnetic fields and is often the most efficient solution.
The cheap fix, that of creating distance, however, is sometimes not so cheap or simply not an option. In Figure 1, note that a relatively large first floor area of an older commercial building is located directly above the building's electrical equipment (transformers, distribution equipment and bus ducts). As magnetic fields strong enough to cause MFI are present in several of the first floor offices above the equipment, the tenant cannot use the space for computer use and asserts that the lease gives clear assurances for such use. Furthermore, the building owner recognizes that the highly desirable first floor space could not be rented at a premium rate. Everyone needs to use computers these days so this situation becomes an environmental issue that translates into a smaller market and lower rents for the space. After discussions with all concerned, FMS developed a mitigation plan which included the installation of custom magnetic field shielding in the area of the offices as shown in Figure 2. The plan provided for a reduction to a target average field strength range between 3 and 5 mG. As shown on the Figure 3 "before" and the Figure 4 "after" graphs, the magnetic field shield design and installation achieved the target level. An important element for the owner was a guaranteed performance level so that the space would continue to be available, even after substantial increases in future electric load.
Medical Equipment Failures Have Unusual Risks
Two representative examples illustrate the complexity of the issues involved in medical equipment failure from MFI. Recently, a prominent hospital noticed that certain patient EEG traces were sometimes, but not consistently, distorted. The potential consequences of such erroneous readings caused the administration to authorize an engineering solution to resolve the problem. After a frustrating series of attempts to rectify the problem through equipment replacement, external MFI interference was suspected and the hospital turned to the local utility for assistance. Measurements confirmed the presence of elevated magnetic field levels in the EEG suite. A power quality analysis revealed that the source of the MFI was likely from newly installed electrical secondary feed cables for a wing of the hospital which traveled under the EEG test suite as illustrated in Figure 6. Normally, such secondary feed conductors would not give rise to elevated fields of the levels noted in the EEG suite as the conductors were located two floors distant from the EEG suite. Further measurements and investigation revealed that the hospital wing electrical riser distribution system connected to the secondary feed conductors allowed for alternative return paths to the transformer for circuits connected to the riser system, causing a "net current" condition to exist as shown on the Figure 6 diagram. This condition created elevated magnetic field levels in areas adjacent to the cables, including the EEG test suite. The field strengths were relatively modest, in the range of 15 mG, but some of the bioelectric sensors utilized with the EEG equipment were found to be sensitive to anything above 5 mG. Two alternatives to resolve the problem were proposed by FMS. The first solution included the quick, but relatively costly electrical isolation of the secondary feed conductors by the installation of an isolation transformer at a location beyond the location of the EEG suite. The second recommendation included the potentially more time-consuming and technically challenging identification and elimination of the sources of the parallel return paths in order to eliminate or significantly reduce, the presence of the "net-current" condition with a proportionate reduction of magnetic fields. The hospital administration opted for the solution which would reduce the "net-current" condition present on the secondary feed conductors as the presence of this condition also was indicative of building wiring errors which could present potential shock hazards and violations of the National Electrical Code. Figure 7 illustrates the successful reduction of both "net current" conditions and magnetic field levels to an acceptable level in the EEG suite.
In another example of MFI medical equipment failure, an employee recently asserted that elevated magnetic fields present in the workspace caused a personal, automatic insulin delivery device to suddenly malfunction. The potential consequences of such a condition motivated the employer to move quickly to resolve the issue. Attempts by the building electrical engineering contractor to determine the source of elevated magnetic field levels in the employee's work area were unsuccessful. Ultimately, the local utility suggested consulting with FMS, which has a reputation of resolving difficult magnetic field problems. A magnetic field survey of the employee's work area revealed the presence of a "net current" magnetic field source due to an apparent wiring error in the modular office system. Within a few hours the wiring error was located and corrected. The magnetic field strengths were thereby reduced to ambient levels, the potential medical problem was resolved and a concurrent and long standing interference problem with several high resolution CAD terminals was also eliminated.
Summary
As can be seen from the examples cited above, MFI can cause costly and even potentially dangerous equipment failure. MFI problems can be very complex and costly to identify and resolve. Many professionals in the power quality field, while generally familiar with the subject of MFI, lack the specialized knowledge and hands-on experience to isolate and quickly resolve MFI problems. As work space becomes increasingly valuable, building managers and owners are increasingly concerned that environments are free from MFI. Power quality specialists should endeavor to recognize the presence of MFI and be prepared to recommend resources to provide MFI solutions.
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