Category Archives: Faraday Cage
A Faraday cage or Faraday shield is an enclosure formed by conducting material or by a mesh of such material. Such an enclosure blocks external static and non-static electric fields. Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836. An impressive demonstration of the Faraday cage effect is that of an aircraft being struck by lightning. This happens frequently, but does not harm the plane or passengers. The metal body of the aircraft protects the interior. For the same reason, a car may be a safe place to be in a thunderstorm.
ESD control products that provide a Faraday Cage or shielding include Statshield® Metal-In and Metal-Out Shielding Bags, Protektive Pak™ impregnated corrugated with shielding layer when using a lid, and Conductive Totes. Statshield® ESD Smocks creates a Faraday Cage effect around the torso and arms of the operator and shields charges from the operator’s clothing from damaging ESD sensitive devices. (Technically, suppressing the electrical field from clothing worn underneath).
There are standard tests measuring the energy penetration of electrostatic discharges to the interior. The Shielding test method per Packaging standard ANSI/ESD S541 is ANSI/ESD STM11.31 and the required limit is less than 50 nanoJoules of energy.
ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area. Per Packaging standard ANSI/ESD S541 section 6.2 Outside an EPA “Transportation of sensitive products outside of an EPA shall require packaging that provides:
- Low charge generation.
- Dissipative or conductive materials for intimate contact.
- A structure that provides electrostatic discharge shielding.”
Definitions from the ESD Association Glossary ESD ADV1.0 include: Faraday cage “A conductive enclosure that attenuates a stationary electrostatic field.”
Electrostatic discharge (ESD) shield “A barrier or enclosure that limits the passage of current and attenuates an electromagnetic field resulting from an electrostatic discharge.”
Electrostatic shield “A barrier or enclosure that limits the penetration of an electrostatic field.”
Note: the ESD Association sells most documents, however, the Packaging standard ANSI/ESD S541 and the Glossary ESD ADV1.0 are free downloads from www.ESDA.org (click Standards and then Documents, scroll down to find documents that can be downloaded at no charge).
Outside the ESD protected area (EPA), the Protektive Pak lids need to be in place to provide the electrostatic discharge shielding ESD control property which is required by the Packaging standard ANSI/ESD S541. Per section 6.2 Outside an EPA “Transportation of sensitive products outside of an EPA shall require packaging that provides: 1. Low charge generation. 2. Dissipative or conductive materials for intimate contact. 3. A structure that provides electrostatic discharge shielding.”
Inside the EPA, it would still be a good idea to have the lid in place, but it is not a requirement. The Protektive Pak impregnated corrugated has a buried shielding layer. In shielding, we utilize the fact that electrostatic charges and discharges take the path of least resistance. The charge will be either positive or negative; otherwise the charge would balance out and be no charge.
Like charges repel and so the electrostatic charge will reside on the outer conductive surface.
A Faraday Cage effect can protect ESDS contents in a container with a shielding layer (this is what a shielding bag has). This Faraday Cage effect protects people in real life when a lightning bolt strikes an airplane or automobile with the charge residing on the outer metal fuselage or car body.
The Faraday cage effect causes charges to be conducted around the outside the surface of the conductor. Since like charges repel, charges will rest on the exterior.
To complete the enclosure, make sure to place lids on boxes or containers, and close shielding bags. Packaging with holes, tears, or gaps should not be used as the contents may be able to extend outside the enclosure and lose their shielding as well as mechanical protection.
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