What are electromagnetic fields and what efforts should be made to minimize them in designing electrical systems today?
While it may be years before a clear consensus is established on the health risks of electromagnetic fields (EMF) exposure, the topic remains highly controversial. In the meantime, electrical systems can be designed and installed to minimize the extent of the magnetic field component of EMF, often at little or no added cost. Before discussing the steps that can be taken during the design stages of electrical distribution systems, let's find out what EMF is.
What is EMF?
EMF at 60Hz is really made up of two separate entities: electric fields and magnetic fields. An electric field, which exists when voltage is present and which is easily blocked by metal, can cause currents to flow on the surface of the human body. Electric fields are not generally considered to be a biological hazard.
A magnetic field, which exists when current flows and which is not appreciably blocked by common materials, can cause current to flow through the human body. Magnetic fields at 60 Hz are considered a possible biological hazard.
A magnetic field decreases as the distance from the source increases. However, the configuration of the source actually determines how quickly a field diminishes. Multiple conductors with current flowing in opposite directions or 3-phase circuits have magnetic fields that are inversely proportional to the distance squared. Appliances and transformers are point sources and their fields drops off inversely proportional to the distance cubed.
Exposure factors
Without consensus in the scientific community that a hazard actually exists, economics play a big factor in setting limits. We don't know for sure whether long-term, low-level exposure is worse than short-term, high-level exposure.
Some say that switched fields are more dangerous than steady-state fields. Individual characteristics of the person being exposed may need to be considered; age, health, and even fertility and pregnancy may be factors. Obviously, more research is needed. But in the meantime, it may be practical to make some educated guesses, set some interim exposure limits, and consider some changes in design.
Practical design suggestions
Services. For an overhead MV service lateral, consider selecting spacer cable in lieu of cross-arm construction. Utilities are becoming more sensitive to the magnetic field issue and may welcome your suggestions. Another choice for a service lateral, of course, is an underground service.
Preferably, pass a service lateral under a storage room rather than an office that is occupied for long time periods. If this cannot be done, then the service lateral should be enclosed in a metallic raceway. The magnetic field will induce a counter EMF in the metallic raceway, which will help reduce the field.
Locate pad-mounted transformers at least 20 ft from buildings. Another consideration is to encircle a pad-mounted transformer with a fence, located at a 4- to 6-ft distance from the edge of the pad.
Switchboards and panels. Locate distribution equipment on exterior walls or walls adjacent to storage areas or corridors. Where possible, locate free-standing switchboards to allow 3 ft of working space on all sides. Use walls abutting occupied spaces for telephone or fire alarm equipment. Avoid locating an electrical room directly below or above an occupied space.
Indoor transformers. Although a typical dry-type, step-down power transformer creates a large magnetic field, this field's strength dissipates very quickly. However, it would still be good practice to locate such a unit in an electrical room remote from occupied spaces.
Bus duct. For vertical distribution of power in a tall building, follow a procedure similar to an indoor transformer siting. Where possible, run a vertical bus duct on a wall common to the elevator shaft, janitor closet, or corridor. In a factory, a bus duct run should be located away from operator positions.
Underfloor ducts. Individual conductors are usually installed in an underfloor duct system, and it is possible for the phase and neutral conductors of a 2-wire branch circuit to be from 6 to 23 in. apart (depending on the cross section of the cell). This configuration would create significant magnetic fields. Recommended practice would be to twist the individual conductors of a branch circuit together in pairs. For a large new construction project, twisted cable assemblies can be purchased from specialty cable companies with only a slight increase in cost. At an existing installation, all the wires in the duct could be tie-wrapped at each outlet in the duct system.
What to watch out for
Wiring errors. When conductors are installed in such a way that circuit conductors are not in the same cable or raceway, substantial magnetic fields are created by the separated supply and return currents. The most common error is violating Sec. 250-23(a) by making a connection between a neutral and an equipment grounding conductor on the load side of the service disconnect; this usually happens at sub-panels. In this instance, neutral currents will flow on both the neutral and the equipment grounding conductor.
Other errors include incorrect wiring of 3-way switching circuits and the connection of neutrals from two different branch circuits at some point other than panel neutral busses, such as at junction boxes, switches, receptacles, etc.
Water pipe problems. Three specific areas can be addressed here.
* Neutral grounds to a water pipe on the load side of the service can cause problems similar to the case mentioned above, where Sec. 250-23(a) is violated by making a connection between a neutral and an equipment grounding conductor, on the load side of the service disconnect. If any electrical equipment, such as a hot water heater, is also connected to a metal water pipe, the water pipe will probably become a parallel path for current that should be flowing over the neutral.
* Physical damage or corrosion can cause the neutral conductor (often uninsulated) on an overhead service drop to open up, or sever. With this condition and with water supply laterals connected to conductive water mains, the water system can function as a parallel neutral. When this happens, unbalanced neutral currents seeking to return to the utility source will pass out of the building over the water lateral. They will return to the source after passing through the water main, a neighboring water lateral, and then through the neighboring service disconnect and out over the neighboring neutral.
* Currents can originate outside a building from the power company distribution system (typically, a looped primary circuit) or from neighboring buildings via the water service. An insulating coupling, or fining, can be installed in the water service. However, it must be located outside of the premises, at least 10 ft from the building wall, per NEC Sec. 250-81(a).
Note that installing an insulating fitting in a water line to prevent the entry of currents has the disadvantage of decreasing ground conductivity, which under certain fault conditions may cause excess voltage to appear on plumbing fixtures and appliance enclosures. Thus, an insulating fitting can cause both a shock hazard and damage to an appliance that would not have occurred if there had been a connection to the water system.
However, a device called an automatic ground connector, which reconnects the ground in the event of a severe fault, is available. This device is similar in principle to a conventional surge arrester, but operates on a different voltage range. Because this is a new device that has not yet been tested by a recognized testing authority, approval from the local authority having jurisdiction should be obtained.
When an insulating fitting is installed, a ground wire should be clamped beyond the fitting and extended back into the building. This allows the option of installing an automatic ground connector or reestablishing the ground in the future.
The importance of testing
Conducting a magnetic field survey upon completion of an installation in new construction is recommended. Such a survey can be done also at an existing location to detect long standing wiring errors or other abnormal conditions.
Two different types of meters are used. A single coil, or single axis, meter (about $235) determines the direction of the field and is useful for finding the field source. This type of meter can provide an approximate rms reading, but a calculation is required.
A 3-coil or 3-axis meter (about $1400) gives a true rms reading of fields from all directions and is not dependent on the orientation of the meter. Capable of making readings every 4 sec over a 24-hr period, this type of meter can be left at a site, or a person can carry it along a predetermined path. Accumulated data can be down-loaded into a computer.
John D. Gaskell, P.E. is President of Gaskell Associates, Ltd., a consulting engineering firm in Warwick, R.I., and a member of the National Electromagnetic Field Testing Association.