“Electromagnetic fields are a complicated quantity to understand because they are invisible,” says Louis Vitale, president and chief engineer of VitaTech Engineering LLC in Fredericksburg, Va. “When you find a problem at a facility, it is important to be able to present a cost-effective solution.”

Addressing potential problems created by EMI and RFI must be done in the initial planning stages for new construction and space conversions in existing buildings. Mitigation measures can be implemented based on site measurements, project drawings, and equipment specifications to predict where the projected interference levels might exceed appropriate thresholds during normal operating circumstances. It is also important to assess the success of the initial mitigation procedures once construction is completed and determine if additional steps must be taken to resolve problems that are likely to impede the efficient operation of research equipment.

“Unfortunately, equipment manufacturers in the United States do not have a standard for measuring electromagnetic interference for scientific tools, although the Europeans have actual standards in terms of what the equipment will be susceptible to,” notes Vitale. “Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) typically require EMI levels of one milligauss (mG) or less in order to function as designed. Many of the next-generation imaging instruments require even lower levels of around 0.1 milligauss, and performance requirements for high-tech instruments will continue to be the leading driver in determining acceptable EMI levels.”

Interference-Causing Culprits

The most imperative issues to address when constructing or renovating a research facility that houses imaging equipment are geomagnetic fields of the earth and terrestrial ferromagnetic influences. Typical static geoelectric fields, which are about 130 volts per meter, occur with the separation of an electric charge between the earth and the conduction atmosphere, but do not impact imaging equipment. However, lightning strikes typically range between 10,000 and 20,000 volts per meter (V/m) with currents exceeding 100,000 amps and can be detected by equipment.

“One of the most important issues is the geomagnetic fields of the earth,” says Vitale. “At the moment, there are huge abnormalities going on in the Pacific Ocean regarding magnetic fields. Fortunately, our civilization is now relying on Global Positioning Systems (GPS) and not geomagnetic fields for navigation.”

The magnetic field at the North and South Poles is 670 mG, and 330 mG at the Equator. The geomagnetic field at the North Pole is currently gravitating toward Siberia and the geomagnetic poles reverse approximately every 250,000 years with the last event occurring 780,000 years ago. Solar flares, or the impact of the sun on transmission lines, generate serious magnetic disturbances in electrical lines and electronic equipment.

Terrestrial ferromagnetic interference from moving masses, such as vehicles, elevators, and doors, can perturb the geomagnetic field and distort the images produced by imaging tools and research instruments.

“The geomagnetic field impacts all imaging areas and must be controlled,” says Vitale. “However, you cannot easily control the earth’s field without using expensive ferromagnetic shielding.”

Examples of Mitigation Measures

Vitale has measured the magnetic fields, gauged the impact on research equipment, and recommended solutions at facilities throughout the United States.

University of Florida Nanoscale Research Facility

He recorded and predicted the Direct Current (DC) magnetic flux density at the new Nanoscale Research Facility being built at the Nanoscience Institute for Medical & Engineering Technology (NIMET) at the University of Florida. When an emanating magnetic field permeates through a cross-sectional area of a medium, it converts to magnetic flux density. In particular, he wanted to assess the potential impact that moving vehicles on the nearby Center Drive, which is 30 meters from the facility, would have on the research instruments being used in the lab. Trucks and buses generally move through a geomagnetic field that is between 500 and 600 mG.

Measurements were taken at varying distances between the street and the facility, starting at six meters and progressing to the facility itself at 30 meters. The permutation of the magnetic field near the street is as high as 10 mG and gradually decreases as measurements are taken further away at 12, 18, 24, and 30 meters. Magnetic field strength diminishes as a function of distance from the electrical source. If possible, magnetic fields should be less than 0.1 mG.

“However, even at 30 meters away, you will see a permutation due to a moving bus going by the road, so this is a serious problem if you have imaging tools,” says Vitale. “The first thing you need to do if you are going to have a nano lab or any research lab that has imaging tools is to have a site survey done to measure the fields as a function of distance. Document the site to show where it is acceptable to locate instruments based upon threshold levels before you begin to lay out your tool rooms or this will come back to haunt you.”

University of California at Berkeley CITRIS Project

The magnetic fields generated by elevators and by vehicles sitting in parking lots were measured at the CITRIS Building Site on the campus of the University of California in Berkeley. Vitale describes the perturbed magnetic field generated by elevators as a pebble creating a ripple effect in a pond. The permutations can be quite serious going out to as much as 50 meters before you get to 0.1 mG.

The assessment work at CITRIS was performed by VitaTech Engineering and the University of Alberta at Edmonton. Engineers at the University spent several months perfecting formulas that can be used to measure the impact of elevators and other types of moving ferromagnetic masses on imaging tools and other nanotech instruments.

UCLA California NanoSystems Institute

The California NanoSystems Institute (CNSI) at UCLA demonstrates how magnets from nuclear magnetic resonance (NMR) imaging systems can create exposure concerns for building occupants, in addition to knocking sensitive equipment off balance. Initial planning called for the UCLA project to have four unshielded research NMRs located in the basement. Unshielded NMRs do not provide adequate attenuation to protect the public and building occupants from excessive exposure.

Unshielded magnets can generate magnetic fields that permeate several floors where it can adversely impact human health and the operating accuracy of research equipment. Five gauss is the posted limit for implantable devices. Venturing inside the 5 gauss line with an implantable defibrillator can result in serious injury or death because the unit will automatically defibrillate at four gauss, incapacitating the victim. Signs must be posted throughout a facility to alert individuals who have defibrillators or pacemakers about the five gauss danger. Shielded systems offer a 5 gauss line within a closer distance to the centerline of the NMR.

“The NMRs should be shielded and performance specifications should be provided with the shields,” he says. “The shields have to function properly and there must be a way to measure the performance.”

Sources of Alternating Current Extremely Low Frequency

Alternating Current (AC) frequency is defined as an extremely low frequency (ELF) electromagnetic field because the fundamental frequency is between three and 3,000 Hz. Vitale explains that this is the bottom of the electromagnetic spectrum near Direct Current and the earth’s geomagnetic field.

“Emanating ELF magnetic fields are proportional to the AC current in wires, ground conductors, metal water pipes, HVAC metal ducts, or any conductive material where a current travels,” he says. “Very high current sources emanate very high magnetic field levels. Unfortunately, magnetic fields are insidious and penetrate through virtually all objects, including people and building materials.”

Potential health risks and perilous EMI can be created by transformers, network protectors, secondary feeders, switchgears, busway risers, electrical panels, distribution panels, motors, and other electrical items. All of these are AC ELF sources located inside buildings and, therefore, work areas and offices should be magnetically surveyed by an engineer who is knowledgeable about ELF electromagnetic fields.

Vitale recommends a maximum long-term human exposure threshold to time-varying AC ELF magnetic fields of 5 mG for children in homes and 10 mG in labs and offices, figures which are well below, by an order of magnitude, common industry standards.

AC ELF magnetic field sources located outside a building can include transmission lines, distribution lines, circuit lines, MAGLEV trains, and other vehicles. For example, the electrical field adjacent to the outside conductor of a 500,000-volt transmission line is 7,000 V/m—enough to make a fluorescent tube glow.

Electromagnetic induction occurs when time-varying AC magnetic fields couple with any conductive object such as wires, electronic equipment, and people, inducing circulating currents and voltages. In unshielded electronic equipment and signal cables, electromagnetic induction generates EMI. Placement of each scientific tool and instrument should be contingent upon the actual AC and DC EMI susceptibility.

Currents as the No. 1 Source of Interference

Ground currents are a collective term for any errant electric currents measured in amperes that result from the natural ground process, including currents on ground wires, ground rods, building steel, conduits, metal HVAC ducts, and metal water pipes, also known as plumbing currents. The ground currents and plumbing currents can be calculated by recording the magnetic flux density at a measured distance from the source. A clamp-on amp meter can also be placed on a grounding conductor or water pipe to obtain an accurate measurement.

Neutral net currents, also described as unbalanced or zero-sequence currents, can be very problematic. Grounded currents are common at commercial facilities with an error rate estimated by Vitale to be about three percent on most buildings.

“If I have a grounded neutral anywhere within hundreds of circuits, that current will threaten all imaging equipment throughout the building,” warns Vitale. “Any ground and net current from anything, whether it is a distribution system, a transmission line, or just the conduits in the building, are an easy threat.”

According to Vitale, ground and net currents are caused by National Electrical Code (NEC) violations such as grounded neutrals and wiring errors in the electrical service, distribution, and grounding systems of a building, as well as utility code violations on distribution and transmission lines. Magnetic fields from ground and net currents decay at a slow, linear rate.

The Birck Nanotechnology Center at Purdue University is an example of a facility located within an electromagnet loop as a result of the distribution lines surrounding the building. However, EMI which would typically affect instruments in the second-floor labs is cancelled by metal pans located on the ceilings of the first and second floors.

“Everything on the second floor is zero mG and they didn’t even know they had a shield,” says Vitale. “You don’t have to worry about ground and net currents inside, but you will have it on-grade because the building is located within the loop.”

It is difficult to shield ground and net current magnetic field emissions using flat or L-shaped ferromagnetic and conductive shields. The most effective shielding method for AC ELF ground/net current emissions requires a six-sided, seam-welded aluminum plate shielding system with a wave guide entrance or a wideband active cancellation system. Low ambient magnetic field levels can be attained inside a research facility and imaging suite by adhering to proper electrical codes and good wiring practices. EMI exposure can be reduced by placing labs on upper floors, and moving the source of interference away from the area where it may cause problems.

By Tracy Carbasho