Information condensed & edited from website of VitaTech (this page for internal personnel reference only)

Whenever electricity (i.e., batteries-DC or electric power-AC) is in use, electric and magnetic fields are produced. If it is alternating current (AC) electric power, the fields fluctuate (expand and collapse) 120 times every second as the 60 cycles-per-second (known as Hertz) alternating current (AC) changes polarity. Since the AC frequency is 60 Hertz it emanates extremely low frequency (ELF) electromagnetic fields (EMF) - hence the term ELF EMF.

Electric fields emanate from unshielded power lines, wires, equipment, and appliances. The electric field strength is measured in volts per meter (V/m). So under high voltage transmission lines (230 - 500 kV) there are very high electric fields -- people can usually hear the crackle and feel the presence on their skin. Fortunately, electric fields can be shielded by grounded objects and materials including metal conduits, trees, and buildings.

Magnetic fields also emanate from unshielded power lines, wires, equipment, and appliances. The magnetic field strength is measured in amperes per meter (A/m) and is proportional to the load current. Therefore, high current loads such as transmission and distribution lines, transformers, feeders, switchgears, and heaters produce high magnetic field levels. Unfortunately, people are not able to sense the presence of high magnetic fields which are extremely difficult to shield -- permeating people, buildings, and most metals.

Magnetic field exposure is measured as magnetic flux density with a gaussmeter in units of milligauss (mG), which is one-thousandth of a Gauss (G). In scientific terms Gauss (G) is the standard U.S. unit of magnetic flux density -- the area permeated by magnetic fields. Therefore, in the United States human exposure to magnetic fields is normally measured and published in milligauss (mG). It should be noted that in Europe microtesla (µT) is the preferred unit - where 1 mG equals 0.1 µT.

Geomagnetic & DC - 0 Hz geological, medical & industrial processing

Sub-Extremely Low Frequency (SELF):
0 HZ- 3 Hz MRIs, PETs, DC Subways & motors, trains & MAGLEV

Extremely Low Frequency (ELF):
3 Hz - 3000 Hz AC Power Sources, CRTs, trains, motors & audio

Very Low Frequency (VLF): 3 - 30 kHZ Harmonics & transients, CRTs & audio

Low Frequency (LF): 30 - 300 kHz
AC Harmonics & transients, CRTs
& medical

EMF Questions & Answers

Q. What are electromagnetic fields (EMFs) and how will I know if they are present in my building or facility?

A. Electromagnetic fields are invisible electric and magnetic fields created as a by-product wherever electricity (AC and DC) is utilized. Magnetic fields can penetrate buildings and people and can be detected when computer monitors jiggle or lose color, noise in audio-visual equipment or data errors and loss in magnetic media.

Q. What are the most common sources of electromagnetic fields?

A. AC sources include transformer vaults, network protectors, secondary feeders, switch gears, busway risers, electric panels and transmission lines. DC sources include subways, electrified trains and MRI units found in hospitals.

Q. Are there other electromagnetic field sources that are important to know?

A. Yes. National Electric Code (NEC) wiring violations and shorts can inject ground currents onto metal conduits, water pipes, building steel and HVAC ducts, which can cause electromagnetic interference (EMI) throughout a building.

Q. Are there other electromagnetic field sources outside a building that can cause EMI?

A. Yes. On many roofs and upper floors, antenna farms and microwave dishes can generate RFI (radio frequency interference) and it could exceed minimum acceptable human exposure standards.

Q. What are the most common solutions to electromagnetic field problems?

A. National Electric Code violations must be found and eliminated (an EMF survey instrument used by a qualified engineer is the quickest way to find shorts). The electromagnetic fields generated by shorts cannot be shielded. As for electromagnetic fields created by plumbing currents, a dielectric coupler will generally solve the problem. If the electromagnetic field source or the affected people and equipment can be moved easily (known as "prudent avoidance"), the creation of empty space (EMFs diminish quickly over distance) could solve the problem. It is very unlikely that productive space will be vacated permanently to solve electromagnetic field problems. Generally, shielding is the most common solution to electromagnetic field problems and shielding the source or shielding the room(s) and people are the two choices. If the electromagnetic field sources are not accessible, then shielding the room(s) or the area in which people work with EMF sensitive equipment is selected -- the solution most often used.

Q. What criteria should I use to evaluate an electromagnetic field problem-solving solution?

A. Any electromagnetic field solution should include a written guarantee of performance, i.e. the affected area(s) will not exceed 10 milligauss (the measurement for detecting EMF) at one meter above floor level (average height of computer desks) over 95 percent of the affected area. Generally, 10 mG is the threshold for computer interference. The guarantee should include price, shield design, installation time, final verification report of performance and references. No one shield design cures all electromagnetic field problems. Each shield must be designed to mitigate the particular EMF source(s) and levels present -- one shield does not fit all. For example, a steel plate shield will not solve most electromagnetic field problems.

Q. If an electromagnetic field problem is discovered at a commercial, institutional or industrial site, can I expect financial help from other sources?

A. Unless you have specific EMF coverage in your casualty and property insurance, it is not likely you will get financial help from those sources. If you rent or lease space, you can ask the owner to solve the problem or move out. Historically, the power companies have taken no responsibility for electromagnetic field problems. The best course of action is to have a full spectrum EMF survey conducted before you build, buy, rent, lease or renovate any building, area or space. If electromagnetic field problems are discovered, make corrective action mandatory before building or moving in. Prudent casualty and property insurance companies as well as mortgage bankers should survey for electromagnetic fields before offering their services.

EMF Shielding

People are typically exposed to very high 60-Hz magnetic field levels ranging between 10-1,000 mG (milligauss) when their offices and apartments are next to, over or under transformer vaults, network protectors, secondary feeders, switchgears, distribution busways and electrical rooms. Usually employees and tenants are not aware of this potential hazard unless the magnetic field source compromises audio/video equipment, electronic instruments, magnetic storage media, VDT's, computers, and networks. Once detected it ultimately becomes the responsibility of the building owner/manager to remedy, otherwise the employee and/or tenant may seek legal action. Unfortunately, there are only three practical solutions to mitigate magnetic field exposure produced from electrical systems within buildings: move the victims (people and equipment) away from the source, shield the source or shield the victims from the source.

It is usually not desirable, especially if office or living space is limited, to evacuate an entire room or several rooms exposed to very high magnetic field levels. So, when space is at a premium the only other alternative is magnetic shielding. To shield or not to shield the source? That is the question! Generally, when physically practical, source shielding is the most effective and least expensive alternative. However, if there are multiple magnetic field sources (i.e., parallel transformer vaults, network protectors, secondary feeders, etc.) it may not be economically feasible to separately shield each source. In that case shielding the room, and consequently the victims, is the preferred solution.

There are two basic types of 60-Hz magnetic shields: flux-entrapment shields and lossy shields. A flux-entrapment shield is constructed with ferromagnetic, highly permeable (µ-mu), 80% nickel-20% iron alloy (i.e., Hipernom Alloy, CO-NETIC AA, Aumetal, AD-MU-80, etc.) which either surrounds (cylinder or rectangular box) or separates ("U" shaped or flat-plate) the victims from the magnetic source. Ideally, magnetic flux lines incident upon the flux entrapment shield prefers to enter the highly permeable (µ-mu) material, traveling inside the material via the path of least magnetic reluctance (R), rather than passing into the protected (shielded) space.

Lossy shielding depends on the eddy-current losses that occur within highly conductive materials (i.e., copper, aluminum, iron, steel, silicon-iron, etc.). When a conductive material is subjected to a time-varying (60 hertz) magnetic field, currents are induced within the material that flow in closed circular paths - perpendicular to the inducing field. According to Lenz's Law, these eddy-currents oppose the changes in the inducing field, so the magnetic fields produced by the circulating eddy- currents attempt to cancel the larger external inducing magnetic fields near the conductive surface, thereby generating a shielding effect.

Shielding factor (SF) is the ratio between the unperturbed magnetic field Bo and the shielded magnetic field Bi as expressed in: SF = Bi/Bo The final shielding design depends on several critical factors: maximum predicted worst-case 60-Hz magnetic field intensity (magnitude and polarization) and the geomagnetic (DC static) field at that location- whichever is greater; shield geometry and volumetric area; type of materials, permeability, induction & saturation; and, number of shield layers.

Small fully-enclosed shields (conduits, video display terminals, etc.) follow simple formulas that guide the design engineer through the process to a functional, but not necessarily optimal design. After assembling a prototype, the design engineer measures the shielding factor (SF) and modifies the design (adds materials, additional layers, anneals bends, etc.) to achieve the optional shielding requirements. This is a very interactive design process, from concept to final product. Unfortunately, magnetic shielding is more of an art than a science, especially when shielding very large areas from multiple, high level, magnetic field sources. At this time there are no reliable design formulas or current EMF simulation programs that offer design engineers practical guidelines for shielding large exposed areas from multiple, high level, magnetic field sources.

Shielding Transformers & Switchgears In Commercial Buildings

People are typically exposed to very high 60-Hz magnetic field levels ranging between 10-1,000 mG (milligauss) when their offices and work areas are next to, over or under transformer vaults, network protectors, secondary feeders, switchgears, distribution busways and electrical rooms. Usually the occupants are not aware of this potential hazard unless the magnetic field source compromises VDT's, computers, networks, electronic instruments, audio/video equipment, and magnetic media. Once detected it is the responsibility of the building owner or manager to remedy.
 

Information edited and condensed from the pages of VitaTech (this page for personnel reference only)

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