8.12.3 Equipment EMI Properties.
Different equipments will exhibit different emission and susceptibility properties depending upon the job to be
performed, the method of design, the type construction, the type components used, and a variety of other
factors. The best indicator as to how much shielding is going to be required for a given piece of equipment or
for an entire complex is provided by the measured level of emissions or the susceptibility level of the equipment
or system. These properties are determined by operating the equipment in an electromagnetically controlled
environment and by (1) measuring the frequency and amplitude of the signals radiated or produced by the
equipment or (2) irradiating or otherwise subjecting the equipment to a known field or given signal and noting
the minimum level to which the equipment or system responds. Under field conditions, neither of these
procedures should be expected to provide precise detailed data because reradiation and mutual coupling effects
can cause wide variations in the measured results. However, with a reasonable sampling of the fields or with
illuminations provided at various locations and different orientations, an order-of-magnitude estimate of the
relative susceptibility or threat posed by the equipment or system should be possible. If precise data is needed,
test procedures in accordance with accepted standards, such as MIL-STD-461 and MIL-STD-462 should be
performed. Unfortunately because of the expense of performing detailed and accurate emission and
susceptibility tests of equipments (even the ability to perform these tests on large complexes in a meaningful
manner is doubtful), and because a decision is frequently required on structural shielding before the specific
equipment population is known, it is generally necessary to direct attention only to the most critical equipments
or systems expected to be installed in the facility. Shielding requirements can also be determined by comparing
the susceptibility levels (MIL-STD-461) of the equipment being installed with the measured or calculated power
density levels in the area where the equipment is being installed.
If it is simply not possible to anticipate or project the shielding requirements, then the resultant
electromagnetic environment in which equipment will be required to perform must be measured or calculated
and the information provided to the equipment supplier so that appropriate steps can be taken to assure that the
equipment or system will function in that environment.
8.13 SYSTEM DESIGN CONSIDERATIONS.
The total area or volume of a facility to be shielded and the physical configuration of the shield is a function of:
the size of the equipment or system requiring shields;
the physical layout including orientation between sources and receptors;
the amplitude and frequency of the interfering signals, and
the cost of materials.
These factors typically interact and, although in a given situation one will predominate, all must be considered.