Electrostatic Shielding

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The demonstration is conducted by setting up the TESV instrument on the tripod, turning on and zeroing the instrument, and then bringing the plastic rod, charged up by rubbing, close to the sensing head. As expected, the instrument responds by indicating the presence of charge. The TESV is then covered by one of the antistatic plastic bags and the experiment repeated. Refer to the photograph shown above. Now, the instrument registers little or no charge. Even when a charged conducting object is discharged directly to the bag, the instrument registers little or nothing. The explanation of this phenomenon is that the instrument has been shielded from the electrostatic field of the charged object by the conductive bag.

Note that it is not necessary to ground the bag. If grounding were necessary, antistatic bags would be much less convenient and effective than they are. Grounding is unnecessary here because electric charge abhors the inside surface of any void enclosed by conductive material. For an ungrounded bag, the charge simply stays on the outside where it harmlessly remains. Now consider the problem of removing a sensitive electronic component or board from a charged bag. If the bag is handled by a person, contact with the hand serves to ground the bag and remove the charge. However, if the person wears insulating gloves, then the component may draw a strong electrical spark as it is withdrawn from the bag and may be damaged.

Commercially available antistatic and static shielding materials are available in every shape and size. In addition to thickness, abrasion resistance, and so forth, the technical specifications usually refer to either MIL specs or some measure of the rate of charge dissipation. Definitions for antistatic materials abound and sometimes can be confusing. For a summary of these, please click here.

Refer to the advertisement shown just below. This bag has a metallic layer which provides "Faraday Cage" shielding, in reference to the above cited fact that charge abhors the inside surface of a conductor (as long as there is no charge within the bag).

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Education

Tags:

• Electrostatic
• Shielding