Optimizing Cable Installation: The Importance of COF and Material Testing

Material composition and cable lubricant drive COF. Because of the large variety of cable jacket blends and different conduit choices, the identification of a “good” COF is nuanced. Polywater® has been testing and compiling COF data for decades. Here are some of the things we have learned.

Understanding the Role of Jacket Material in Cable Design

There are several considerations in the choice of cable jacket material, but the primary function is protection. Cable jacket is the outer layer of the cable that acts as a barrier to moisture and chemical ingress. It protects the cable from mechanical wear and tear. It can add strength or provide flexibility. The jacket material may need to withstand extreme environments: high (or low) temperatures, corrosion, and fire exposure. In some cases, the electrical properties are also important.

Table 1: Basic Properties of Common Cable Jacket Materials

Polymeric cable jacket materials must remain functional for a use life of 50 or more years. Cable jacket blends are not basic commodities. Specific polymer structures and additive blends are combined to produce long-lasting performance. There are many formulations of cable jacket material depending on the intended use and unique performance characteristics.

The COF value is not a key design factor in cable jacket design. For some low or medium voltage cables, additives are either combined with the polymer or coated on the cable surface to reduce friction. In these cases, it is important that the cable does not slip on the reel and can be safely handled. Use of lubricating agents cannot interfere with the cable jacket strength. Even when cable jackets include slip additives, high-performance lubricants further lower COF.

Understanding Differences in COF Across Cable Jacket Materials

The materials used in the cable jacket and conduit determine the COF. Polywater testing shows the importance of high-performance cable lubricants. Compiled data from years of testing show the typical properties of different material and lubricant combinations. In the next two graphs, a portion of these data are shown.

Tested cable jacket materials include:
• CPE (chlorinated polyethylene)
• CSPE (chlorosulfonated polyethylene)
• HDPE (high-density polyethylene)
• LLDPE (linear low-density polyethylene)
• Polypropylene
• PVC (polyvinyl chloride)
• XLPE (cross-linked polyethylene)

Harder materials like HDPE and polypropylene have naturally lower COF values than other cable jacket materials. CPE and CSPE tend to be higher friction “softer” polymers. PVC is dependent on the additive package and has a bigger range of COF values. No two cable jackets will have the same performance. In all cases , lubricants dramatically reduce the COF and “level the playing field.”

Graph 1 shows the COF of various jacket materials on PVC conduit. The COF of unlubricated cables ranges from 0.35 to 0.73 with a difference of 0.38 from lowest to highest. Lubricated COF values range from 0.06 to 0.16 with a difference of 0.10. Lubrication lowers and minimizes COF variance.

Graph 1

Graph 2 shows the COF of various jacket materials on Rigid Steel conduit. The COF of unlubricated cables ranges from 0.42 to 0.73 with a difference of 0.31 from lowest to highest. Lubricated COF values range from 0.09 to 0.29 with a COF difference of 0.20 from lowest to highest. Rigid steel shows higher variability in COF performance. This graph shows average values. Laboratory testing has produced higher variance with results depending on the steel surface treatment and the smoothness of the cast. On average, high-performance lubricants reduce the COF by 70% on the different rigid steels tested.

Graph 2

Polywater has tested many more combinations of cable and duct than shown in Graphs 1 and 2. Besides PVC and Rigid Steel ducts, HDPE, Fiberglass, Electrical Metallic Tubing (EMT), and Aluminum materials have been tested. These include different manufacturers and, in some cases, specialty textures or treatments. Other cable jackets tested include Medium Density Polyethylene (MDPE), Nylon (THHN), Thermoplastic and Natural Rubber, Cross-linked Polyethylene (XLPE), and Lead .

Measuring the COF on different surfaces and understanding what kind of performance to expect is key to developing good cable lubricants. High performance lubricants do an excellent job of COF reduction and act as an equalizer. No matter what cable jacket is chosen, a high-performance lubricant lowers the COF effectively during installation.

COF Variations Within Material Types: The Case of PVC Cable Jackets

The preceding graphs show average COF values for each material type. Several polymeric blends show high variability within this material type. A good example of this variability is PVC. PVC can be compounded into both flexible and rigid forms, which results in a large variety of different formulations. Recently, Polywater received three 1/C 750 KCMIL power cables with PVC jacketing for testing on the Friction Table — each cable manufactured by a different cable company. The cable jackets blends are unique to each cable design. The cable samples themselves have a different appearance. Notice the differences in surface sheen and texture in Photo 1.

Photo 1

Cables were tested with two different high-performance cable lubricants. Results in Graph 3 show that the PVC jackets all have different unlubricated COF values. The effect of each lubricant varies as well. The PVC with the lowest unlubricated COF value shows the highest lubricated value.

Graph 3

COF Testing Enhances Installation Planning and Reduces Risk

COF is unique to the materials tested. Performance cable lubricants in combination with the cable jacket formulation and conduit type will produce a unique value. COF can vary within material type depending on the specific formulation. For important installations, it is worthwhile having the COF tested ahead of the pull. Understanding what kind of COF to expect can increase predictability. Better cable tension prediction lowers risk.

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