8.1 FUNCTION OF AN ELECTROMAGNETIC SHIELD.
Groups of equipment or subsystems may be made electromagnetically compatible by any combination of three
fundamental approaches: (1) the interfering signal source level may be reduced, (2) the receptor susceptibility
may be reduced, or (3) the attenuation of the path or paths over which interference is transmitted from source
to receptor may be increased. Radiated interference signals generated by electromagnetic fields may be
attenuated effectively by electromagnetic shielding, e i t h e r a t t h e s o u r c e o r a t t h e r e c e p t o r . A n
electromagnetic (EM) shield reduces the strength of electric and/or magnetic fields on the side of the shield
away from an interfering EM source. When a shield encloses an EM source, the field strength outside the shield
will be reduced; when the shield is used to enclose a sensitive (susceptible) assembly located near an external
EM source, the field strength inside the enclosure is substantially reduced. Shielding, when properly designed
and implemented, offers significant wideband protection against EM radiation where source and receptor are
not sufficiently separated for adequate free space radiation attenuation. It is relatively easy to obtain 40 dB of
shielding effectiveness in a frequency range above 100 kHz with a single shield, and values as high as 70 dB are
readily obtained with careful single-shield construction. For higher values of shielding effectiveness, double
shields are normally used, yielding shielding values as high as 120 dB.
Radiated energy may still be coupled into a susceptible device through a shield of inadequate thickness, through
holes provided for ventilation and other purposes, and through imperfectly joined shield sections. Precise
calculation of shielding effectiveness, even for perfectly joined solid shields, depends on the form of the shield
and the type field for which the shielding is to be used. Both electric and magnetic coupling can occur, but
normally it is relatively easy to provide electric shielding. Magnetic shielding, however, is more difficult to
provide, particularly at frequencies below 100 kHz. To avoid uncertainties in critical situations, tests should be
performed to check shielding effectiveness. Such tests require the establishment of a known field and the
measurement of insertion loss introduced by the shielding.
In the construction of a facility, the installation designer should take advantage of all the inherent shielding
which the installation and its individual equipments and terrain have to offer. Items such as building walls,
partitions, towers and other similar structures may be used to advantage. The shielding effectiveness afforded
by these items can be used to isolate EM radiation generating equipment from potentially susceptible devices,
personnel, flammable mixtures, and other items. In addition, equipments used in a console or rack may be
placed to take advantage of the inherent shielding of that rack.
Shielding, although an important technique for reducing EM interference effects, is not the only technique
available for this purpose. Application of shielding techniques should not be made without due regard to the
roles which filtering, grounding, and bonding play in the interference suppression program.