1.8 SHIELDING. The shielding provided in a given facility should be adequate to provide the needed equipment
and personnel protection; however, it need not go beyond what is required for that particular facility. To
determine the shielding required at a facility, the electromagnetic environment at the planned location should
first be surveyed; then this threat environment should be compared with the response properties or
susceptibilities of the equipment to be located in that environment. If a need for shielding is indicated, then it
should be provided either as a part of the facility or the equipment shielding should be upgraded. The final
decision will be based on a trade off between the known (or estimated) shielding requirements and the relative
cost to provide this shielding. Conducted as well as radiated susceptibility and emission requirements of C-E
equipments should meet the specifications of MIL-STD-461.
1.8.1 Establishing Requirements.
Tailor the shielding of the facility according to the needs of the equipments or systems to be located
(1) Conducting an electromagnetic survey at the facility location (see Volume I, Section 8.12.2)
(The performance of these surveys requires specialized instrumentation, careful equipment calibration
procedures, and calibrated antennas. Have this survey performed by an experienced team.),
(2) examining the history of performance of the similar equipments at other sites with comparable
electromagnetic environments, and
considering the measured EMI characteristics of the equipments (if available).
If measured susceptibility data (the incident field levels which cause equipment interference) are
available, determine the amount of additional shielding necessary by subtracting the equipment susceptibility
level (in dB above a microvolt per meter, dB V/m) from the field strength (as measured in dB V/m) of the
incident signals. If the measured signal strength is greater than the susceptible level, arrange to provide the
extra shielding necessary either as part of the structure or building or require that the equipment's shielding be
upgraded (see Section 3.4). If susceptibility data is not available, make a best estimate of the amount of
required shielding from the historical performance of the equipments (or similar types) at other sites.
Before deciding what type or how much supplemental shielding material is necessary, estimate the
amount of shielding inherently provided by conventional building materials and techniques. For example:
(1) Use Figures 1-81 and 1-82 to estimate the shielding provided by normal construction
techniques (steel skeleton with brick or concrete block exterior with standard wood, gypsum board, or concrete,
block interior walls).
(2) Reinforced concrete offers additional shielding because of the presence of the rebar.
Estimate the shielding effectiveness of single course rebar to low frequency magnetic fields from the curves
shown in Figure 1-83. (Use Table 1-28 to obtain attenuation correction factors to apply to Figure 1-83 for
other size rebar and other spacings.)