Underground Cable-in-Duct Installation: Key Considerations for Enhanced Operational Lifetime
Installing electrical cables underground into ducts helps to protect them from environmental factors such as weather, but also introduces unique challenges that can negatively impact the operational lifetime of the cable.
The operational lifetime of a cable is affected by many factors, including the quality of materials, the manufacturing process, the cable design, the installation method, and the installation and operation environment. However, when cables are installed underground in ducts, there are several additional factors that must be taken into consideration. The following topics provide some insight into how to maximize the operational lifetime of electrical cable during the installation process.
How does cable tension and bending affect the operational lifetime of a cable?
When electrical cables are installed in underground ducts, they are subjected to the highest mechanical forces of their lifetime such as tension and sidewall bearing pressures. These forces can cause the cable to stretch and bend, damaging the cable’s insulation and conductor. This damage can reduce the cable’s operational lifetime. The use of high-performance cable lubricants, designed specifically for duct installed cable, reduces friction between the cable and the conduit to lower the pulling tension during installation. Use of generic, industrial lubricants can have a detrimental effect on the cable’s insulation and jacketing, leading to premature failure.
Some generic industrial lubricants may contain ingredients (such as white mineral oil, paraffin waxes and hydrocarbon waxes) that are incompatible with the cable insulation and jacketing materials and can weaken the cable’s integrity. It is essential to use specialty cable pulling lubricants that are compatible with cable materials to prevent damage.
One could assume that all manufacturers—of cables and lubricants alike—regularly test for compatibility. However, the truth is, some more than others. Like in all things, it is buyer beware. Only use cable pulling lubricants from manufacturers that provide cable compatibility data.
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What measures can be taken to ensure cable lubricants are compatible with cable jacket materials?
Preventing a pulling compound from ruining your cable is a two-step process. One, work with reputable cable and lubricant manufacturers to ensure materials are compatible. Two, specify a compatible lubricant to prevent the choice being left to installers who may have no inkling of the issue.
IEEE 1210, Standard Tests for Determining Compatibility of Cable-Pulling Lubricants with Wire and Cable contain criteria and test methods for determining the compatibility of cable lubricants. Tests look at lubricant effects on the physical properties of the cable jacket and volume resistivity of the semiconducting components.
Non-qualified and generic industrial lubricants that have not been tested for cable jacket compatibility such as white mineral oil, paraffin waxes, and hydrocarbon waxes, often have a high percentage of solids and do NOT evaporate over time. In fact, because of the higher temperatures in the duct, once the cable is energized, these kinds of lubricants will most likely flow to the lowest point of the duct. The chemical exposure of this incompatible compound to the cable jacket is constant and for decades of its lifespan. The waxy residues tend to cement the cables in place over time which will make it very difficult to remove the cable from the duct in the future.
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How can the cable be protected after installation to maximize its operational life?
Depending on the underground environment in which the cable is installed, water and moisture can come into contact with the cable. This can wreak havoc on a system, potentially impacting the operational lifetime of the cable, and lead to costly repairs in the system. Industry codes and standards, like NEC, require the sealing of openings and raceways into underground structures. Unfortunately, the codes and standards do not specify what type of material is best to use for sealing openings. So, some installers default to any low-cost duct putty or hardware store foam, unless a material or product is specified by the network owner or planning engineers. These low-cost options will initially create a seal in the opening but are not designed to withstand underground conditions and fail in a very short period of time.
Typically, two-part expanding foam conduit sealants or mechanical seals are used to seal around cables and conduits. They are rated to withstand high waterhead pressures and also seal against other types of intrusions like insects, rodents, and snakes. These types of sealants not only safeguard the cable they also create safer environments for utility workers who may need to enter an underground structure for maintenance.
Sealing around newly installed cables with the appropriately rated conduit sealant ensures that cable systems are protected from environmental elements and perform throughout the intended lifecycle.
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Conclusion
The operational lifetime of electrical cables installed in underground ducts can be negatively impacted by several factors, including improper installation practices, using unsuitable (cable pulling) lubricants, and not using the appropriately rated sealants to secure the cable system. To ensure the longevity of cable performance, proper installation practices, including the use of compatible and tested lubricants and sealants, are crucial. Simple missteps or corner-cutting measures in the installation process can have enormous impact on the operational lifetime of a cable. This should concern not only the installer but every network owner and operator.
The cost implications of a shortened lifespan can be significant, with the cost of replacing a single cable running into millions of dollars in some instances. The cost difference between a fully tested and approved cable pulling lubricant and a non-specific industrial (wax or oil-based) lubricant is small and can be ignored when compared to the overall cable and installation costs. Therefore, it is essential to plan ahead. Research, identify, and select only tested and approved lubricants and sealants to prevent premature cable failure and extend the operational lifetime of electrical cables.