The National Electrical Code requires that equipment cases and housings be grounded to protect personnel from
hazardous voltages in the event of an electrical fault. Stray currents in the fault protection network can
present an interference threat to any signal system whose operating range extends down into the lower
frequency range and should be eliminated. Where such problems exist, it is advisable to attempt to reduce the
impedance of the reference plane as much as possible. A practical approach is to interconnect equipment
enclosures with the equipotential plane, via building structural steel, cable trays, conduit, heating ducts, piping,
etc., into the earth electrode subsystem to form as many parallel paths as possible. It should be recognized that
because of the inductance and capacitance of the network conductors, such multipoint ground systems offer a
low impedance only to the lower frequency noise currents; however, these currents can be the most troublesome
in many facilities. Higher frequencies find a much lower impedance to ground through the distributed capacity
of the equipotential plane.
5.4.3 Frequency Limits.
The question remaining concerns the frequency below which signals can be considered as lower frequency.
Certainly the dividing line between the lower and higher frequency should be high enough to include all audio
communications signals. Since digital systems employ frequencies which extend from dc up to several hundred
MHz, a decision based on pulsed-signal considerations is more appropriate. To minimize the possibility that the
approximately 21 meters (70 feet) at 300 kHz. Since the grounding buses in medium to large sized facilities
may extend 21 meters (70 feet), 300 kHz appears to be the maximum frequency for which a single-point
grounding system should be used. At frequencies up to 30 kHz, conductor lengths up to 210 meters (700 feet)
can be approached without exceeding the 0.02 wavelength criteria.
MIL-STD-188-124A establishes the lower frequency network range from dc to 30 kHz and in some cases
(depending on the interface frequency) up to 300 kHz. The higher frequency network range extends above 300
kHz and may in some cases be used at sites where the interface frequencies are as low as 30 kHz. The
frequency range from 30 kHz to 300 kHz is a mutual area and may be considered as either higher or lower
depending upon the interface frequency.