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Environmental Health Investigations Branch

General

Site Planning
  • Place vacant or low-occupancy areas near magnetic field sources
  • Verify building/utility placement with the project electrical engineer
  • "Underground" on-campus distribution lines if possible
  • Consult with electric utility about predicted fields from power lines at proposed building locations
  • Explore possibility of optimizing conductor configuration, phase arrangement, and split phases of any new distribution lines
If the best site has transmission lines or substations nearby or if the distribution lines create above-average fields on parts of the campus, use that part of the campus for functions where few people spend much time, such as parking lots, storage areas, etc.

Reduce EMF Created by Distribution Lines

The figure below illustrates the impact of three strategies to reduce magnetic fields created by electrical distribution lines. This example is based on one set of assumptions. Relationships could vary with different load balances and circuit configurations.

The "existing configuration" has one 10-foot crossarm 40 ft above ground. The field drops below 2 mG at a distance of 83 ft and below 1 mG at 123 ft from the center of the line. The first mitigation example is to change to an "increased pole height configuration" by using 20-foot taller poles. The field drops below 2 mG at 71 ft and below 1 mG at 115 ft from the center of the line. The second mitigation example is to change to a "split phase configuration" consisting of two 10-foot crossarms, one 40 ft and one 35 ft above ground. The field drops below 2 mG at zero feet and below 1 mG at 27 ft from the center of the line. The third mitigation example is to change to an "underground configuration" consisting of three cables in one duct which is buried 3 ft below the surface. The field drops below 2 mG at a distance of 8 ft and below 1 mG at a distance of 13 ft from the center of the line. (Citizens Concerned about EMFs, San Ramon, California, 1995. Field profiles calculated using Southern California Edison Co.ís FIELDS program.)
mitigation options

Note in the figure that undergrounding provides the lowest magnetic fields ten feet or more beyond the center line of the buried cable. The close grouping of cables and subsequent cancellation of magnetic fields produces this effect. However, within ten feet of the buried lines the underground lines' proximity to the surface produces fields higher than the aboveground split-phase option. The practical implication is that undergrounding does not always produce the lowest fields.

  
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Building Design

  • maximize distance between magnetic field sources such as electrical service panels, transformers, mechanical equipment, raceways, etc., and occupied areas
  • disburse electric power via low-occupancy areas
At the design stage it is often easy to separate high EMF-source spaces from heavily-occupied spaces. Separating these two types of spaces becomes more and more difficult as the project progresses until, in the occupied building, it may not be possible to move a person or function without a great deal of dislocation costing time and money.

Sources of Elevated EMF

Potential sources of elevated magnetic fields may include such building elements as the electrical service connection, transformers, panels and large electric motors used for elevators, air conditioners, and the like.  Low-occupancy spaces which could be located next to the EMF-source areas include: storage, mechanical rooms and duct spaces, janitors' closets, and stair and elevator shafts. In some instances large field sources can be located remote from occupied spaces by selecting equipment such as remote chillers.

Motors, transformers, subpanels and wiring have typical drop-offs in magnetic fields. You may gain a better understanding of the range of magnetic fields from school electrical equipment and wiring by reading this chart.

  
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Construction Documents

  • follow the National Electrical Code (NEC)
  • ground electrical system correctly per code
  • wire three-way switches correctly per code, with three wires rather than two
  • minimize distance between conductors in a circuit
  • recheck subpanel locations and consider reducing their loads
  • assess whether electronic ballasts for fluorescent lights are a no or low cost option
Verify Adjacencies, Wiring and Energy Use

At the design development phase and during the production of the construction documents there should be reviews of the drawings and specifications in order to verify that the proper adjacencies still exist, that wiring and grounding are called out properly, and that the design is energy-efficient to reduce current loads. Where drawings do not spell out specific wiring routes and locations, a performance specification based upon this checklist may be utilized which reflects the EMF policies of the school district and local governing bodies.

Fluorescent Lights

Overhead fluorescent lights can be a dominant source of fields in classrooms, particularly in multistory buildings where lights can produce fields at the floor in the level above. Electronic ballasts will minimize EMF from fluorescent light fixtures.

  
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Construction

  • review EMF Checklist with contractor (see our summary sheet)
  • verify that work is in conformance with contract documents
  • a gaussmeter may be used to detect deviations from the National Electric Codes (NEC)

Minimize Distance between Conductors


There are two factors which determine the strength of the magnetic fields from properly wired circuits in buildings: the amount of current and the distance between the conductors in the circuits. The strength of the magnetic field is in direct proportion to the flow of electric current and the distance between the "hot" and "neutral" wires; double either the current or the distance between the wires and the fields will correspondingly double. In three- and four-wire circuits, distance between phase conductors plays a similar role. If neutrals are not connected according to code situations can be created in which the neutral current does not return next to the corresponding hot wire and therefore does not produce optimal cancelation. Adhering to the code also prevents fire, shock and damage to electrical equipment.

Locating all the conductors for any individual circuit in as compact a cross section as possible will provide the greatest degree of cancellation among the conductors. Since even fractions of an inch have an effect, laying conductors loosely in wide trays or large conduits can increase field levels. Care should be taken to keep conductors together in order to minimize EMF. Hot and neutral conductors should be carried in a single tray.

  
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Occupancy

  • use "low-field" EMF criteria in equipment selection
  • consider field exposure within the room and in adjacent rooms when placing appliances
  • optimize equipment layout
  • monitor building use
All electrical end-use devices will create electric and magnetic fields when in use. The selection and location of these devices should be made with the occupancy patterns of the buildings in mind. So-called "smart machines" turn themselves off or go into standby mode when not in use.

Consider EMF in Placing Appliances

desk setup The location of some devices may affect the exposure they create for nearby users. For example, since computer monitors typically produce higher fields at the rear and sides than at the front of the unit, large, circular group layouts result in reduced fields for users compared to arrangements in rows.

desk setup







Layouts for other electrical devices should respond to the field characteristics of each device and the usual position of the operator.
  

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