EMC Question of the Week: November 4, 2019
At 1 MHz, the plane-wave shielding effectiveness of a 1-mm thick sheet of lead is,
- is close to 0 dB
- is approximately 20 dB
- is more than 100 dB
- could be less than 0 dB
Answer
The correct answer is "c". While lead (Pb) is often the material of choice for shielding against ionizing radiation; it is not the material one would normally select for electromagnetic shielding. Lead is heavy, toxic and has an electrical conductivity that is an order of magnitude lower than more common shielding materials such as copper and aluminum. Nevertheless, with an electrical conductivity of about 5x106 S/m, it is still a pretty good electric field shielding material. The shielding effectiveness calculator on the Clemson Vehicular Electronics Laboratory website can be used to show that a 1-mm thick sheet of lead has a shielding effectiveness of over 100 dB over a wide band of frequencies..
Note: The shielding effectiveness of an enclosure made from lead (or any metallic material) would not necessarily be more than 100 dB. In fact, it could be less than 0 dB. Resonances set up inside an enclosure can create strong internal fields. When some of the energy in those fields leaks out through seams and apertures in the enclosure, the resulting radiation can be higher than the radiation from the same sources without the enclosure.
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