EMC Question of the Week: August 17, 2026
In an experiment to evaluate the effect of power line fields on mice, the mice are placed in plastic boxes. Some of the boxes are completely covered in aluminum foil, while others have no foil. The boxes are placed 5 cm above a metal plane and 50 cm below a wire. The wire carries 100 amperes of 60-Hz current that returns on the plane. The amplitude of the 60-Hz voltage between the plane and wire is 220 volts. Compared to the mice in the boxes without foil, the mice in the foil-covered boxes are exposed to much weaker
- electric fields
- magnetic fields
- both of the above
- none of the above
Answer
The best answer is “a.” Low-frequency electric-field shielding requires materials that are conductive enough to redistribute their electric charge in response to the applied electric field. Virtually any aluminum foil is capable of effectively shielding a 60-Hz electric field.
On the other hand, low-frequency magnetic field shielding with conductive materials requires materials that support the formation of strong eddy currents. This requires a material thickness on the order of a skin depth or more to provide significant shielding. At 60 Hz, the skin depth in aluminum is about 8.5 mm. Aluminum foil is much too thin to provide effective magnetic-field shielding at 60 Hz.
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