20.1 Isolation. To protect equipment, it is necessary to establish a
barrier which is impervious to the EMP wave or greatly attenuates it (see
figure C-3). A peripheral shield encapsulating the system can provide such
protection. For any shield to be effective, it must be totally closed.
However, because of penetrations for signal cables, power, heating,
ventilating, and air-conditioning equipment, personnel entrances, etc., a
totally closed shield is not attainable (see figures C-4 and C-5).
Therefore, additional protection is provided by placing protective devices at
the shield to electrically close any penetrations, treatment of apertures to
prevent entry of EM radiation, and concentration of conducting penetrators in
a single area to reduce current flow in the shield.
20.2 Shielding. An unbroken shield is quite effective against the EMP EM
field. Less than a millimeter of copper, aluminum, or steel will reduce the
field strength to near-ambient level. A two-level barrier approach is often
used. A facility shield is installed to reduce the incident transient to
less than that usually experienced by the contained equipment during normal
operation. The equipment cabinets, cases, and racks then reduce the level of
transients experienced on a routine basis to a level tolerable by the
internal circuitry. This approach has the attractive feature that the
internal barrier is continuously tested simply by operating the equipment in
its normal environment of power surges, switching transients, and signal
crosstalk. Shields installed and tested using MIL-STD-461 and MIL-STD-462
can reasonably suppress the magnitude of the radiated EMP.
20.3 Apertures. The effectiveness of any shield is degraded by any
apertures. Because of the broadband nature of the EMP, the number and size
of allowable apertures is quite small. A single opening in a welded seam,
for example, should not exceed 0.5 inch in its largest dimension. Multiple
openings in the same area must be much smaller. Certain large apertures,
like doors and windows, cannot be eliminated and must be treated in some way
to strongly attenuate incident EM fields. Most apertures which cannot be
eliminated can be treated with waveguide-beyond-cutoff techniques. A
waveguide with a length five times its cross-sectional width provides
approximately 100.dB attenuation of radiated EMP at frequencies below cutoff.
The cutoff frequencies have wavelengths equal to twice the longest
cross-sectional width. This treatment is well suited to personnel entries
and conductor penetrations which cannot be filtered or otherwise treated.
Apertures such as ventilating ducts can be effectively closed using honeycomb
panels also conforming to the 5:1 length/width ratio. Windows may be treated
with wire mesh embedded within the glass.
20.4 Penetrations. Conductors which penetrate the shield are far more
serious than arc apertures in the shield. because the conductors carry the
huge induced transients into the shielded facility. These conductors include
not only signal and power cables and their shields, but also water and gas
pipes, waveguides, etc. One general rule is that all conductors which can be
grounded should be bonded circumferentially to the facility shield so that
any induced current is shunted to earth by the shield. Another principle is
that all penetrators, groundable or not, enter the facility through one
localized region of the shield and are equipped with protective devices.
Cable shields fall into the category of groundable conductors, but the
enclosed conductors do not, and must be relieved of the EMP-induced
transients before entering the facility. The recommended technique is to run
all nongroundable conductors (power and signal) through the entry plate into
an EMP vault. An EMP vault is a shielded enclosure within the facility
shield, having the entry plate in common with the shield. Within the EMP
vault, each conductor goes through a terminal protection device (spark gap,
metal oxide varister, etc.) followed by a capacitor-input filter (lowpass or