Drexel University

Drexel University

The Bossone Center at Drexel University redesigned their first floor to host a JEOL transmission electron microscope (TEM) approximately 60 ft. from an underground Amtrak line, 260 ft. from the SEPTA line on Market St., and 400 ft. from an overpass Amtrak rail on the opposite side of the Bossone building.  Time-varying AC and DC current magnetic fields emanate from the overhead catenary cables and rail supplies as the trains and subways move along the rails traveling to 30^th Street Station.  As a result, the overhead and underground 25kVA AC catenary overpass produces magnetic fields greater than 25 mG and quasi-static DC EMI levels greater than 40 mG peak-to-peak when more than 300 trains pass through these rail lines weekly. (Diagram #1).  A magnetic shielding design was necessary.

Vitatech was hired to provide preliminary 24-hour testing, design, and installation services in which tri-substrate shielding was combined with Active Compensation System (ACS) technology and special low profile floor ACS coils. The passive AC ELF/DC tri-substrate magnetic shielding system reduced the elevated 25 Hz and 60 Hz AC ELF magnetic flux density levels from the Amtrak to less than 0.05 mG p-p with no ACS in all axes. The AC ELF passive tri-substrate shield provided -50 dB to -70 dB of attenuation depending on the frequency and axis.

The quasi-static DC EMI nested tri-substrate shield with ACS provided between -28 dB to over -50 dB of attenuation from multiple train sources surrounding the building. The Stefan Mayer Fast MR-3 ACS can provide between -20 dB up to -60 dB (ideal) depending on the frequency (0.001 Hz to 5000 Hz) and magnitude of EMI source. Vitatech’s magnetic shielding design successfully reduced the quasi-static DC EMI levels from 43 mG peak-to-peak to less than 0.10 mG p-p without using very expensive annealed nickel-iron alloy (otherwise known as mu-metal).

Posted in: ACS Mitigation Scientific
Tags: #electronmicroscope,