Water pipes or conduit should be connected to the earth electrode subsystem to prevent ground currents from
entering the structure. Further, at the first service disconnect the ac neutral should be grounded at only one
point (to EES) to prevent the possibility of damage to transformers from circulating currents. This does not
negate the NEC requirement to ground the neutral at the transformer.
1.10.2 EMP Shield Applications.
Whenever feasible, shielding of the overall building should be done in preference to room or area
Individual room or area shields should only be utilized to provide additional protection of critical
when normal protective methods will not reduce EMP to an acceptable level, or where, in
an existing structure, the cost of protecting the entire building is excessive.
Commercial enclosures may be used for small rooms and bolted construction is acceptable. For
large room construction, however, continuously welded steel is preferred.
Electrical wiring and components should be protected from EMP fields by a shield such as ferrous
conduit, RF shielded raceway, or cable armor, that completely surrounds the items to be protected. Electronic
components may be shielded with sheet metal housings.
All metallic penetrations of the facility shielding should enter at a common location as illustrated in
Figure l-94. All shielded cables, conduits, and pipes should be bonded to an entry plate as shown in
Figure l-95. This plate should be large enough so that no penetrations will occur within 1 foot of the nearest
edge. The entrance plate should be continuously welded, around its perimeter, to the building shield. The
conduit should be of steel with threaded or welded couplings. Conduit runs should be as short as practical with
joints held to a minimum. Transient protection for cables entering a building at points away from the building
entry plate is provided by following procedures outlined in Section 22.214.171.124.26.
1.10.3 EMP Bonding Practices.
Homogeneous welds should be used whenever possible because they offer the best protection against
penetration of the EMP signal.
When bolts are used as fasteners, the body of the bolt should not be welded or brazed. The nut and
washers should be located inside the shield region where they will not be exposed to the incident field. Nuts
should be checked for tightness periodically during EMP hardness assurance test cycles.
Pipes, conduit, and connector shells should be welded or brazed to the shield completely around their
perimeter at the point of penetration of the shielded region. Conductors used for grounding purposes shall not
penetrate any metallic barrier designated as an EMP shield ,i.e., shielded enclosure, EMP vault etc., but shall
rather be bonded to a welded stud on the barrier.
minimize the inductance of the path for the EMP-induced current.