It is a fact that shielding effectiveness varies with frequency, shield geometry,
the positioning of a victim, component or device within the shield, the type of field
being attenuated, the direction of incidence and polarization. Moreover, the definition
of shielding effectiveness for electric and magnetic fields will be identical if the
fields concerned are uniform plane waves and the media on each side of the barrier
are identical. For example, for 20 mil copper (1/2 mm), the combined attenuation is
140 dB (107:1) and for 20 mil steel, the combined attenuation is 180 dB (109:1)
for a signal of 25 kHz (typical of a first stroke); these attenuation values are higher
for 1 MHz signals (associated with subsequent strokes) [26]. Cases outlined in IEC
62305-4 [18] are only valid for gridlike spatial shields; however, they provide the
basis for an analysis of the effectiveness of the shielding of metallic framed structures
like the OOI itself, and others. For example, for a 1-mm width copper/aluminium
gridlike spatial shield, the magnetic attenuation would be 80 dB for 25 kHz and
for 1 MHz, but for 5-m width, the magnetic attenuation would only be 5 dB.
The above analysis indicates that perfectly welded sealed rooms are ideal for producing
effective lightning shielding. However, all openings, such as doors, windows,
side panels, I/O panels, and ventilation ports and cable exits that ingress/egress can
endanger the integrity of the shielding, especially through the production of conducted
signals. Thus, all openings should correctly be treated so that the effiency of shielding
is not undermined.