SteveP75904
join:2008-03-29
Lufkin, TX
 ·Suddenlink
1 edit | this article explains whats happening.
The DC resistance and AC impedance of the grounding system (ground vertical(s), connection points, grounding rod(s), and ground
resistance) is also of major concern in cable systems for proper dissipation of voltage transients with resulting overcurrent
conditions.
The DC resistance of the grounding system relates to the systems ability to properly dissipate overcurrent conditions caused by
longitudinal sheath currents, and elimination of potential differences between grounding or neutral conductors in the utility plant,
poles or pedestals [non-transient conditions]. The condition of connectors used at strand and ground electrodes, and attaining and
maintaining a good ground are the primary determinants in success.
The AC impedance of the system, which exists under voltage transient conditions, is determined by the following:
• The inductance of the ground wire (vertical), it does not change significantly with wire size.
• The length of the vertical run [determines overall inductance].
• How the vertical system is positioned; proper placement involves vertical and horizontal runs only with smooth bends. An
esthetically pleasing system (looks good to the eye) many times employs poor practices in attaining low AC impedances.
• Using more than one grounding conductor and electrode, which places two grounding electrode systems in parallel and
therefore halves the impedance of the system.
Summarizing, size of the ground conductor is not as important (NESC requires #8 at minimum) as is the overall length. The length
should be kept to an absolute minimum given site conditions. This applies for both utility pole and service line (drop)
bonding/grounding.
Investigation into longitudinal sheath current phenomena in CATV systems also indicates the following:
• The overall importance of frequent and quality (low impedance/resistance) grounds [not bonds].
• Proper location of bonds between CATV plant and power company verticals. Improperly placed or too many bonds may in
fact increase the percentage of load imbalance current carried, the major contributor to long term longitudinal sheath current
conditions in CATV systems.
In the final analysis, grounding and bonding issues can and should be treated separately - to assist in simplifying the myriad of
issues involved. Bonding serves the primary purpose of meeting NESC code requirements, which in general has personnel safety
as the main emphasis. Cable bonding procedures should be designed to meet NESC requirements only. Grounding serves to
properly dissipate voltage transients in the CATV plant; through draining of the transient to ground, and enabling proper operation of
transient shunting devices such as crowbar circuits which should be the main methodology in transient protection for the cable
system.
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