10.4.1.3 Apertures. No unnecessary openings or discontinuities in the shield should be allowed. Those
openings necessary for personnel and equipment loading and for ventilation should be designed to limit
electromagnetic field penetration. Openings that must permit air flow or light passage can be made more
opaque to HEMP waves by covering them with mesh or, preferably, honeycomb waveguide-beyond-cutoff, as
illustrated in Figure 10-10. High traffic entryways can use waveguide-beyond-cutoff tunnels with doors at each
end (without the doors, the highest frequencies in the HEMP spectrum can penetrate through a tunnel large
enough for personnel to walk through). Where possible, discontinuities in the shield should be eliminated by
continuous welding or a similar process. Those that are necessary for equipment installation and maintenance
should be electromagnetically sealed with durable bonding techniques, such as resilient RFI gaskets and small
bolt spacing. Where access is required infrequently, it might be practical to weld the equipment entry door
shut; large cargo doors are large compromises to shielding integrity and difficult to seal effectively and
10.4.2 Allocation of Protection.
10.4.2.1 Amount of Protection Needed.
The amount of protection needed depends to some extent on how failure is defined for the system. For
communications facilities, the threshold for failure, or the minimum acceptable performance, may be defined
by a maximum allowable outage time or error rate. In some cases, the principal requirement is that the system
not damage itself so that it can be restarted and restored to service after an attack involving HEMP. The
definition of system failure, or operating requirements, should be prescribed in the system specification; it will
be determined by many factors in addition to HEMP.
In determining the amount of HEMP protection required, it is important to be able to define a transient
tolerance or susceptibility level for the facility or the equipment in the facility. Since most communications
equipment have no transient "withstand" requirement, except perhaps on the power terminals, we cannot obtain
the required tolerance from the equipment specification. Nevertheless, it is possible to define a transient
stress at or below which the equipment performance will be unaffected. Zero stress certainly satisfies this
definition, but more practical values can be found. For example, the equipment tolerates its operating signal
levels, and it tolerates the peacetime transient stress inside the facility. Neither of these is a trivial value of
stress, and we can be assured that if the HEMP-induced stress is made small compared to either, the presence
of HEMP will not cause the equipment to malfunction. Additional information on transient withstand
requirements may be found in MIL-STD-461C.
The equipment or internal circuit threshold defined in terms of known peacetime tolerances has several
It takes advantage of known equipment "withstand" capability; no more HEMP protection is necessary
than that required to reduce HEMP transients to a safe margin below this known tolerance.
It is not necessary to determine the HEMP response of circuits and structural elements inside the
equipment; this greatly reduces the complication of hardness verification and maintenance.
It is possible to place all HEMP requirements at the facility barrier, so that concern for interior
configuration control and internal states are alleviated.