10.4 PROTECTION AGAINST HEMP.
There are important considerations in designing this protection that affect the value that can be placed on
HEMP protection. The HEMP protection adds cost to the facility, and the value received for the added cost is
confidence that the facility will survive HEMP. This implies that (1) the protection against HEMP can be
verified, and (2) this protection is retained and can be maintained throughout the life of the facility. The
protection has low value when it is designed in such a way that it is difficult to verify or maintain. The
protection may be difficult to verify when the HEMP-induced stresses inside the facility are large enough to
cause spurious arcing or other insulation breakdown. It may also be difficult to verify when it depends on
unknown or uncontrolled electromagnetic properties of materials used in the facility. Finally, hardness
verification will be difficult if the number of features that must be tested is very large. For example, if the
HEMP-induced stress is large deep inside the facility, the number of system states, modes of excitation, stress
waveforms, etc., that must be evaluated may be enormous.
Since HEMP does not ordinarily occur during peacetime, degradation of the protection is not evident from
peacetime operation of the facility. Therefore, the HEMP protection has greatest value if it is durable. The
protection should not be degraded by normal use and maintenance of the facility. The protection should not
depend on extraordinary configuration control. It must accommodate facility growth and modification.
Components critical to the protection should be few in number, accessible, and testable.
Protecting communication facilities against the HEMP typically consists of developing a closed HEMP barrier
about the facility. The barrier consists of a shield to exclude the incident space waves and various barrier
elements on the essential penetrating conductors and in the apertures required for personnel and equipment.
The number of penetrating wires, apertures, and other features that must be evaluated to verify the HEMP
protection is kept as small as possible. In addition, attention is given to the number of system states or
configurations for which the protection must be determined. Durability and accessibility of the protection
elements are also important.
10.4.1 HEMP Barrier.
10.4.1.1 Shield. The facility-level shield used for protection against HEMP is typically fabricated from
welded sheet steel. The thickness is usually selected for ease of fabrication, but in areas where exceptional
mechanical abuse is likely, mechanical strength, as well as workability, may be a consideration. Shield
assembly is typically accomplished by continuous welding, brazing, hard soldering, or other fused-metal process
to minimize the number of discontinuities in the shield (a weld or other fused-metal joint is considered
10.4.1.2 Penetrating Conductors.
Concepts for penetrating conductor treatment are illustrated in Figure 10-9. Penetrating conductors that can
be grounded, such as plumbing, waveguides, grounding cables, and cable shields, are bonded to the shield wall at
their point of entry by peripherally welding them to the wall or by the use of clamps, collets, etc., that
peripherally bond the penetrating conductor to the shield with little or no discontinuity.
Signal and power wires that need not penetrate the shield should not penetrate the shield. Wires that must
penetrate the shield must be treated with a barrier element, such as a filter or surge arrester, that closes the
barrier above a voltage threshold or outside the passband required for signal or power transmission.