8.12.1 Equipment Disturbances.
A reliable indicator of the need for shielding of an equipment is the degree of interference that it experiences
or causes. Recognizing that interference can be the result of one of the four different coupling modes, it must
be determined that coupling will occur through one of the modes which can effectively be combatted by
shielding. For example if the interfering signal is coupled into the equipment or system on a power or signal
line, shielding the equipment may accomplish little. The line picking up the disturbing signal may be made less
susceptible to interfering signals by careful shielding of the line itself. If inductive, capacitive, or radiated
coupling is the cause of the problem, then shielding of the cable either alone or along with the equipment will
If the equipment is going into a new facility and the decision to be made is whether or not shielding is
necessary, the behavior of that equipment in other similar environments should be considered. If the
performance of the specific equipment is not known, the behavior of equipments of similar types or
construction should be studied. The most reliable method of determining shielding requirements is to compare
known susceptibility levels of the equipment or system with known measured power density levels in the area
where the equipment or system is being installed.
8.12.2 Electromagnetic Environmental Survey.
The most effective way of determining the power densities at the location where the equipment or the
structure is to be located is by conducting an electromagnetic environmental survey. This survey is performed
using calibrated antennas with special field strength meters or spectrum analyzers. These instruments permit
the strength of radiated fields to be determined in terms of volts per meter or in power density, i.e., watts per
square centimeter or square meter. For personnel hazard determination, commercially available rf radiation
monitors may be used.
The spectrum survey should attempt to identify the presence of all potentially interfering fields. Of particular
concern is the field strength of the signals emitted by readily identifiable sources such as commercial radio and
television stations, and radar and communications transmitters. Other possible sources of interference include
rf heating units, rf welders, microwave ovens, and, in locations near medical facilities, diathermy and
electrocautery machines. Desk top evaluations can also be employed to calculate power density/signal strength
levels in a given area if all local emitters (including output power, locations, etc) can be identified.
The electrical power system can also be a source of interference. High voltage transmission systems, in
particular, frequently generate noise through corona discharge and arcing across dirty connectors and
insulators. The frequency spectrum of this noise generally extends well into the HF region (3-30 MHz) or above
and can be a cause of severe problems. The routing, either existing or planned, of power lines should be noted
carefully. If long runs of signal and control cables in parallel with power lines, either overhead or underground,
are unavoidable, shielding of the signal and control cables may be necessary.
In addition to the above identifiable sources of energy against which shielding may be required, other less
obvious sources exist. For example, ignition noise from internal combustion engines can be troublesome. Also,
office machines, vending machines, and fluorescent lights have been frequently observed to produce
interference in digital computers, measuring systems and other sensitive equipments.