Cable supports are used to organize and support medium voltage power distribution cables in underground manholes, vaults, and tunnels. Cable supports are also used to organize and support underground low voltage power cables and control cables, high voltage power transmission cables, and communication cables. Cable supports may also be used above ground and in areas other than underground manholes, vaults and tunnels.
These cables for electric power, control and communication lines are run underground in order to protect them from above-ground elements and from the interference and damage they would suffer when installed above the ground or on poles or structures. The underground environment may be less hostile in some ways, but the history of underground cables suggests that the underground environment is not benign. The environment in underground power and communications manholes is indeed harsh.
While there may be fewer ultraviolet rays and less severe weather underground, and the temperature is more constant, moisture and humidity are always present. There are other considerations, such as the constant and higher danger from flooding, and underground pests that consider electrical insulation, and even steel, a tasty treat. Manholes may fill with water that is often contaminated with sewage, fertilizer runoff, tree roots, and chemicals, including caustic materials. Very harsh sea or salt water sometimes fills manholes. Many manholes are completely or partially filled with such contaminated water all of the time, except when pumped out for maintenance. Others fill periodically but are hot and have extremely high humidity, while still others fill and empty with ocean tides.
As noted, most power and communications manholes are partially or completely full of water some of the time or all of the time. The amount of water in a given manhole is influenced by location, surrounding conditions, drainage, and weather. Manholes located at higher grades generally will be filled with less water for a shorter period of time than those located at lower grades. Manholes located where the surrounding area has a high ground water level and/or a high amount of rain generally are filled with water to a higher level and more of the time than those located in areas that have a low surrounding ground water level and/or a low amount of rain. The water level in manholes located close to the ocean often changes with the tide, and the constantly-changing interface only increases the likelihood for corrosion. The condition of water in underground power and communications manholes occasionally is fresh and clean but most often is contaminated, as noted above, or is salt water, both of which can be very corrosive and also conductive.
Communication and power cables should be kept off surfaces, such as a floor or the ground, and should be organized and protected to the greatest extent possible. Cables are thus typically supported underground by racks that elevate cabling and keep the cabling off the ground, thus shielding the cables from at least some of the worst underground dangers. Racks for supporting cables must be able to withstand both heat and cold, all conceivable temperatures and humidities in every combination. In addition, the racks must be able to support very heavy loads from power and communication cables. The racks themselves are preferably supported, e.g., attached to a wall, rather than free-standing structures. Thus, the racks will have penetrations, or stress concentrators, to deal with, in these hot, humid, and stressful environments, along with the high loads expected from supporting cabling. The walls themselves may have penetrations for supporting bolts, pins or other fasteners used to secure the racks in place. The walls, such as concrete walls or other structures, will also be in intimate contact with the racks, adding their chemical potential for corrosion to the racks.
All these stresses combine to make the underground a challenging environment for cable racks. For the most part, existing cable supports used in underground manholes, vaults, and tunnels are manufactured using steel stampings, steel forms, or steel weldments. They may also be ductile iron castings. After the supports are stamped, formed, welded, or cast, they are hot dip galvanized in an effort to prevent corrosive deterioration. The steel arms and posts are bonded together and grounded in an attempt to prevent corrosion. Eventually, the galvanized coating is consumed and the steel racks may oxidize or corrode away, leaving the power and communications cables without support.
Two phenomena, galvanic corrosion and stray current corrosion, occur in flooded underground manholes to cause this deterioration. Galvanized steel cable supports are very vulnerable to both galvanic and stray current corrosion and often become severely corroded to a point that they will no longer support the cables in a very short period of time.
Galvanic corrosion is an electrochemical process in which one metal, the anode, corrodes preferentially when in electrical contact with a different type of metal, the cathode, and both metals are immersed in an electrolyte. In flooded underground power and communications manholes the galvanized steel cable supports are the anodic sites of the galvanic corrosion reaction. Cathodic parts in the manhole, parts made from more noble metals such as stainless steel, may be damaged in the galvanic corrosion process due to generation of electrolytic hydrogen on their surfaces causing hydrogen embrittlement. Stray current corrosion of underground power and communication cable supports is usually caused by power and communications manholes being located in the vicinity of electric rail tracks, pipe lines that are cathodicly protected or the like.
Underground galvanized steel cable supports that are severely corroded and can no longer support the cables result in power and communications interruptions and a safety hazard to technicians who enter the manhole. Another safety issue is that galvanized steel cable supports are conductive. If a power cable's insulation is compromised and the electrified conductor contacts a galvanized steel cable support, the cable support is energized. If a technician inadvertently touches the energized cable support he may be electrocuted.
What is needed are safer cable racks better able to withstand the environment and better able to tailor themselves to a greater variety of situations, for fewer stresses, and for longer service.