CONTENTS
8.2.2 Polyethylene (Insulating)
8.2.5 Chlorinated Polyethylene
8.2.8 Low Smoke Zero Halogen (LSZH) (also Low Smoke Halogen Free, LSHF) Jackets
8.3 Thermosetting Jacket Materials
8.3.1 Cross-linked Polyethylene
8.3.3 Chlorosulfonated Polyethylene
8.3.5 Nitrile–Butadiene/Polyvinyl Chloride
8.3.6 Ethylene–Propylene Rubber
The terms “sheaths” and “jackets” are frequently used as though they mean the same portion of a cable. In this chapter, sheath is the term that applies to a metallic component over the insulation of a cable. An example is the lead sheath of a paper-insulated, lead-covered cable.
Various metals may be used as the sheath of a cable such as lead, copper, aluminum, bronze, steel, etc. A sheath provides a barrier to moisture vapor or water ingress into the cable insulation. It is necessary to use such a sheath over paper insulation, but it also has a value over extruded materials because of water ingress.
The thickness of the metal sheath is covered by ICEA and AEIC standards and specifications, but there are some constructions that are not covered. The thickness is dependent on the forces that can be anticipated during the installation and operation of the cable. Designs range from a standard tube to the ones that are longitudinally corrugated. The bending radius of the finished cable is dependent on such configurations.
To fully utilize the metal chosen, one should consider first cost, ampacity requirements—especially during fault conditions, and corrosion [1, 2].
The term jacket should be used for nonmetallic coverings on the outer portions of a cable. They serve as electrical and mechanical protection for the underlying cable materials.
There are many materials that may be used for cable jackets. The two broad categories are thermoplastic and thermosetting. For each application, the operating temperature and environment are important factors that must be considered.
Polyvinyl chloride (PVC) is the most widely used nonmetallic jacketing material in the wire and cable industry. Starting in 1935, when it first became available, the use of PVC grew rapidly because of its low cost, its easy processing, and its excellent combination of overall properties including fire and chemical resistance.
PVC belongs to a group of polymers referred to as vinyls. The unmodified polymer contains approximately 55% chlorine. It is fairly linear in structure (few side chains) with approximately 5%–10% crystallinity. The material must be compounded with additives such as fillers, plasticizers, and stabilizers to attain flexibility, heat resistance, and low temperature properties. General purpose jacketing materials normally possess good physical strength, moisture resistance (but not as compared to polyethylene), adequate oil resistance, good flame resistance, and excellent resistance to weathering and to soil environments. Both flame resistance and low temperature flexibility can be improved (within limits) by the use of additives.
General purpose PVC compounds are recommended for installation at temperatures above −10°C, but specially formulated compounds may be used as low as −40°C.
One of the limitations of PVC jacketed cable is its tendency to creep under continuous pressure. For this reason, cables that are to be supported vertically with grips should not have PVC jackets. Hypalon or Neoprene is recommended for such use.
In the low voltage field, PVC is widely used as a single layer of material where it functions both as insulation and as a jacket. Since PVC is usually a thermoplastic material, it cannot withstand high temperatures. Under high fault current conditions, the insulation can be permanently damaged by melting or can emit plasticizers and become stiff and brittle over a period of time. For this reason, it is not used for utility secondary network cable. Similarly, in industries that handle large amounts of heated material, or where there is the possibility of excessive heat, the use of PVC is avoided because of its tendency to melt or deform when heated to a high temperature. Under continuous direct current (DC) voltage in wet locations, as in battery operated control circuits, single-conductor PVC-insulated cables have frequently failed due to electroendosmosis (water vapor ingress created by voltage stress).
A large percentage of chlorine can be released during a fire. When combined with moisture, hydrochloric acid may be produced. It may leach into concrete structural members and attack and weaken the steel. This situation highlights one of the major problems that can result from the use of PVC. PVC has fallen out of favor for electric utility underground medium voltage power cables because the moisture resistance is not as good as polyethylene in the common case.
8.2.2 POLYETHYLENE (INSULATING)
Polyethylene has been widely used as a jacket for underground cables since it became commercially available in large quantities in about 1950. For use as a jacket, polyethylene may be compounded with carbon black or coloring material, and with stabilizers. Carbon black gives the material the necessary sunlight protection for outdoor use.
Polyethylene for jacketing is categorized under three different densities:
• Low density: 0.910–0.925 grams per cm3
• Medium density: 0.926–0.940 grams per cm3
• High density: 0.941–0.965 grams per cm3
Density generally affects the crystallinity, hardness, melting point, and general physical strength of the jacketing material. In addition to density, molecular weight distribution is important since it influences the processing and properties of the polymer.
Polyethylene jackets are an excellent choice where moisture resistance is a prime design criterion since it has the best moisture resistance of any nonmetallic jacket material. When polyethylene is used as a jacket material, it should be compounded with enough carbon black to prevent ultraviolet degradation. Linear, low density, high molecular weight (LLDPE) is the most popular jacket material since it has better stress-crack resistance than the high-density materials. High density provides the best mechanical properties, but may be very difficult to remove from the cable.
In evaluating fillers, both black and nonblack, it has been found that although many of these materials improve the aging characteristics, carbon black is by far the best. It has also been found that the aging resistance increases with carbon black loading from 2% to 5%. Normally, a 2.5%–3.0% loading is used.
Although polyethylene has good moisture resistance and good aging properties in its temperature limits, it has poor flame resistance. This discourages using it as a jacket in many circumstances. Polyethylene jackets have good cold bend properties since they will pass a cold bend test at about −55°C. They are extremely difficult to bend at low temperature because of their stiffness. Like PVC, polyethylene generally is a thermoplastic material and melts at elevated temperatures. This temperature will vary slightly with molecular weight and density, but melt occurs at about 105°C–125°C largely depending on density.
High-density polyethylene (HDPE) has been used extensively as the second (outer) layer for “ruggedized” thermoplastic in secondary and low voltage street light cables because of its toughness.
While black polyethylene for jacketing is most often an insulating material, with higher loadings of carbon black it can be a semiconducting material.
This material has been used for over 40 years in direct-buried applications to improve the grounding of the concentric neutral.
The semiconducting material used as a jacket for medium voltage underground residential distribution (URD) cable is a resistive thermoplastic composition. This requirement is met by a polymer containing a suitable conductive material as a filler (commonly carbon black). When semiconducting jackets were first employed in the 1970s, the only available thermoplastic semiconducting materials were the compositions being used for conductor and insulation shields. Several materials were available that were based on thermoplastic ethylene/ethyl acrylate (EEA) or ethylene/vinyl acetate (EVA) copolymers and carbon black. These polymers are relatively amorphous and can tolerate the high filler loading required for conductivity while providing reasonably good physical properties. Other polymers are used in special circumstances. ANSI/ICEA S-94-649 recognizes Type I and Type II semiconducting jackets. The basic difference being that the Type II jacket has a higher use temperature. This is the well-known deformation resistant thermoplastic (DRTP) shield that was used as a strippable insulation shield on URD cable prior to the introduction of the triple extruded cross-linked strippable insulation shield in the 1980s.
It is important that the neutral wires be encapsulated by the jacket. When encapsulation is complete, there is good surface contact and conductivity to ground is ensured. Without this contact, the grounding of the cable through the jacket is compromised. Also if a pin hole in the jacket should develop with well-encapsulated neutrals, there is no opportunity for ground moisture to travel longitudinally along the cable and pool at low spots. There is additional discussion of semiconducting compounds in Chapter 5.
The use of the available thermoplastic semiconducting materials for URD cable jackets in the 1970s and 1980s has been reasonably successful and many miles of cable with these semiconducting jackets are in service. There were several shortcomings that became apparent over time: some of the early semiconducting materials would become less conductive at elevated temperatures. ICEA Publication T-25-425 was developed to address this issue and improved, more conductive compositions were developed. When AEIC CS5 introduced the “Thermo-mechanical bending test,” these jackets often cracked and they were occasionally found cracked in the field as well. Finally, the moisture transmission through these jackets was in the order of 10 times greater than the LLDPE insulating jackets that reduce the progress of moisture-induced treeing in the insulation. To address this issue, a new class of thermoplastic semiconductive material was developed in the early 1990s. This material performs very nearly up to the values attained by LLDPE insulating jackets. Moisture transmission is nearly as low as with the insulating jacket and the new material consistently passes the thermomechanical test in all cable sizes and constructions. The new jacket material also does not adhere to the cross-linked insulation shield and no separator is required (Table 8.1).
TABLE 8.1
Properties of Jacketing Materials
Semiconducting |
Insulating |
|
Radial res., ohm-m |
40 |
na |
Tensile, psi |
1,700 |
2,200 |
Elongation, % |
400 |
600 |
Brittleness, °C |
−50 |
−60 |
Moisture vapor transmission |
1.5 |
0.8 |
Over the years, the availability of high quality thermoplastic semiconducting materials for jackets on URD cables has evolved. The current materials perform well in all respects.
Cables with semiconducting jackets have been successfully used with no reports of cable failures due to corrosion or corrosion failures of any adjacent equipment due to proximity with the semiconducting jacket.
Semiconducting jackets are being used over URD type cables as a deterrent to corrosion and to provide effective grounding to the cable during lightning strikes and faults. Semiconductive jackets reduce the neutral-to-ground impulse voltage by improving the grounding efficiencies.
Another value of a semiconducting jacket is that the jacket does not have to be removed for the installation of more frequent ground connections—only four per mile as required by the National Electrical Safety Code rather than eight. Such grounding points increase the chance of water penetration as well as other construction errors.
Polypropylene is a very rigid thermoplastic, typically harder than polyethylene and similar in electrical properties to polyethylene. It is commonly thought of as an alternative to HDPE. Polypropylene has a lower specific gravity than HDPE.
8.2.5 CHLORINATED POLYETHYLENE
Chlorinated polyethylene (CPE) can be made either as a thermoplastic or as a thermosetting jacket material. As a thermoplastic material, it has properties very similar to PVC, but with better higher temperature properties and better deformation resistance at high temperatures than PVC. CPE jackets also have better low temperature properties than PVC unless the PVC is specifically compounded for this property.
Thermoplastic elastomer (TPE) is a thermoplastic material with a rubber-like appearance. It is a form of crystalline polyethylene and it comes in various types. It can be compounded for use as an insulation or a jacketing material. By use of compounding techniques, a good electrical insulation can be developed with good moisture resistance properties.
Also, a jacketing material can be compounded to provide flame resistance, low temperature performance, good abrasion resistance, and good physical properties. This material is relatively new as compared with the thermoplastics previously mentioned, but appears to be a very versatile material.
Nylon is a thermoplastic with many properties that make it desirable for jacketing of wire and cable. Nylon has relatively high strength, and is tough but rather stiff especially in cold weather. Nylon also has good impact fatigue and, within limitations, good abrasion resistance. A very important feature is the low coefficient of friction in contact with conduit materials. This is an aid in pulling the cables into conduits. Nylon has excellent resistance to hydrocarbon fuels and lubricants as well as organic solvents. However, strong acids and oxidizing agents will attack nylon. The most common use of nylon in cable jacketing is the jacket on THHN and THWN building wire.
8.2.8 LOW SMOKE Zero Halogen (LSZH) (ALSO LOW SMOKE HALOGEN FREE, LSHF) JACKETS
LSZH jackets involve a number of polymers formulated to release limited smoke and no halogens when burned. LSZH jackets may be thermoplastic or thermosetting. This increases the escape potential from buildings or other spaces during a fire and reduced soot damage to facilities. The absence of halogens increases human safety (inhalation damage) and reduced corrosion of facilities associated with a fire. At this time of writing, pulling compounds must be selected with care as LSZH is more susceptible to jacket cracking from exposure to such. LSZH jackets commonly have large filler contents and may therefore have poorer chemical, mechanical, moisture, and electrical properties than non-LSZH compounds. LSZH compounds are widely used in Europe and North America and continue to grow in popularity.
8.3 THERMOSETTING JACKET MATERIALS
Thermosetting jackets are not widely used for underground distribution cables except for the special case of medium-or high-density cross-linked polyethylene that is used as the outer layer on two layer, ruggedized secondary cables. Thermosetting jackets are more commonly utilized in industrial and power plant applications.
8.3.1 CROSS-LINKED POLYETHYLENE
Cross-linked polyethylene, with the addition of carbon black to provide sunlight resistance, provides a tough, moisture-, chemical-, and weather-resistant jacket material. The medium-and high-density materials are especially tough and are widely used as the outer layer on two layer ruggedized secondary cables. Only limited use is found for other purposes.
Neoprene has been used as a jacketing material since 1950 for large power cables such as paper-insulated, lead-covered cables and portable cables. Compounds of Neoprene usually contain from 40% to 60% by weight of Neoprene that is compounded with other ingredients to provide the desired properties such as good heat resistance, good flame resistance, resistance to oil and grease, and resistance to sunlight and weathering. Moisture resistance can be compounded into the material when required.
Properties that can be varied by compounding techniques are: improved low temperature characteristics, improved physical strength, and better moisture resistance. Most Neoprene compounds have good low temperature characteristics at −30°C to −40°C. Special compounding can lower this to −60°C, but other properties, such as physical strength, have to be sacrificed.
Because of its ruggedness, tear resistance, abrasion resistance, flame resistance, and heat resistance, Neoprene is a widely used jacketing material in the mining industry. This is probably the most severe application for cables from a physical standpoint. The thermosetting characteristics of Neoprene are desirable in this application since these cables must withstand high temperature while installed on cable reels. Thermoplastic jacketing materials would soften and deform under such environments.
8.3.3 CHLOROSULFONATED POLYETHYLENE
Chlorosulfonated polyethylene (CSPE) is a thermosetting jacket compound with properties very similar to Neoprene. CSPE is unique in that colored compounds of this material, protected by sunlight stable pigments, have weather-resistant properties similar to black CSPE compounds. Hypalon is the trade name of the most commonly used material.
CSPE compounds are superior to Neoprene compounds in the areas of resistance to heat, oxidizing chemicals, ozone, and moisture. They also have better dielectric properties than Neoprene. The flame resistance of both materials is excellent. The superior heat resistance of CSPE as compared with Neoprene makes it the better choice for cables rated at conductor temperatures of 90°C.
Nitrile rubber compounds are copolymers of butadiene and acrylonitrile. They provide outstanding resistance to oil at higher temperatures. Since this is their only outstanding feature, they are generally limited to oil well applications where temperatures up to 250°C can be encountered. Their poor oxidation resistance in air limits their use for other applications.
8.3.5 NITRILE–BUTADIENE/POLYVINYL CHLORIDE
These jacket compounds are blends of nitrile rubber mixed with PVC to provide a thermosetting jacket similar to Neoprene. The advantage of this material over Neoprene is that colored jackets of nitrile–butadiene rubber (NBR)/PVC have properties comparable with that black jackets and can be compounded for physical properties and tear resistance similar to that of Neoprene.
8.3.6 ETHYLENE–PROPYLENE RUBBER
Ethylene–propylene rubber (EPR) is frequently used as an insulating material because of its balance of outstanding electrical properties. It can also be used for jackets, especially in low temperature applications where flexibility is required. These materials can be compounded for −60°C applications with reasonably good physical properties and tear resistance.
EPR is not generally used for a jacketing material in other applications. It is used as jackets in low voltage applications when flame resistance has been compounded into the material.
Armor is placed (or incorporated) into cables to protect them from physical abuse. Armor design and materials must be carefully selected.
• Armor must be considered when determining ampacity, cable loss, and impedance. High permeability materials such as steel may significantly impact these considerations.
• Armor can greatly add to cable weight.
• The corrosion resistance of the armor is a serious consideration.
• Armor will impact installation requirements such as cable bending radius.
This armor consists of a single metal tape whose turns are shaped to interlock during the manufacturing process. Mechanical protection is therefore provided along the entire cable length.
Galvanized steel is the most common metal provided. Aluminum and bronze are used where magnetic effects or weight must be considered. Other metals, such as stainless steel or copper, are used for special applications.
Interlocked-armor cables are frequently specified for use in cable trays and for aerial applications so that conduit and duct systems can be eliminated. The rounded surface of the armor withstands impact somewhat better than flat steel tapes. The interlocked construction produces a relatively flexible cable that can be moved and repositioned to avoid obstacles during and after installation.
An overall jacket is often specified in industrial and power plants for corrosion protection and circuit identification. Neither flat-taped armor nor interlocked armor is designed to withstand longitudinal stress, so long vertical runs should be avoided.
This construction consists of one or two layers of round wires applied over a cable core. For submarine cable applications, the wires are usually applied over a bedding of impregnated polypropylene or jute.
Round-wire armor is used where high tensile strength and resistance to abrasion and mechanical damage are desired. Vertical riser cables and borehole cables are made with round-wire armor when end-suspension from the wires is necessary for support for the longitudinal stresses. Round wires have less resistance to piercing than flat-tape armor or interlocked armor, but have superior tensile strength and abrasion resistance.
To calculate the strength of armor wires, the following calculation is suggested:
(8.1) |
where
W = total weight of cable in pounds
w = weight of cable to be suspended in pounds per 1,000 ft
ln = length of cable in thousands of feet.
Strength of steel armor wires:
(8.2) |
where
S = strength in pounds
N = number of armor wires
A = area of one armor wire in square inches
F = factor of safety (usually 5).
For copper or bronze, use tensile strength in ASTM Specifications instead of 50,000. If the strength is found to be less than the total weight W, the next step would be to select the next larger size of armor wires and recalculate the values.
For single-conductor cables, copper or aluminum wires have been used to minimize losses due to circulating currents. Such constructions sacrifice mechanical strength in order to achieve lower losses.
Armor wires can be made with the individual wires coated with polyethylene or other corrosion-resistant coverings. Since there is a portion of the circumference without metal protection, cables with such covered wires are usually made with two layers of armor wires with the second layer in the opposite lay to the first.
For installations in severe rock environments, two layers of steel wires, with no individual coverings, are applied in reverse lay. The outer layer frequently is applied with a very short lay to achieve optimum mechanical protection.
The number of armor wires for a wire-armored cable may be calculated from the following equation:
(8.3) |
where
N = number or armor wires, nearest whole number
D = core diameter of cable under armor in inches
d = diameter of armor wire in inches
F = lay factor (see Table 8.2)
D + d = pitch diameter or armor wire in inches.
Armor resistance may be calculated from the following equation:
(8.4) |
where
ra = DC resistance of one armor wire or tape per 1,000 feet at temperature t in ohms
F = lay factor (see Table 8.3)
N = number or armor wires
Note: For steel wire armor, increase Ra by 50% to obtain approximate alternating current (AC) resistance.
TABLE 8.2
Approximate DC Resistance of Armor Wire
Wire Size BWG |
Diameter inches |
Galvanized Steel, ohms/1,000 ft |
Hard Drawn Copper, ohms/1,000 ft |
Commercial Bronze, ohms/1,000 ft |
12 |
0.109 |
7.33 |
0.895 |
2.49 |
10 |
0.134 |
4.92 |
0.592 |
1.65 |
8 |
0.165 |
3.16 |
0.391 |
1.09 |
6 |
0.203 |
2.12 |
0.258 |
0.72 |
4 |
0.238 |
1.53 |
0.188 |
0.52 |
Basis: |
||||
Conductivity, temperature |
% IACS |
12.0 |
97.5 |
40.0 |
Coefficient of resistivity |
A |
0.0035 |
0.00383 |
0.00190 |
TABLE 8.3
Lay Factor for Round Wire Armor
Ratio of Length of Lay to pitch Diameter of Armor wire |
Lay Factor |
7 |
1.095 |
8 |
1.072 |
9 |
1.057 |
10 |
1.048 |
11 |
1.040 |
12 |
1.034 |
Note: Use 7 as a typical value if the ratio is unknown.
Other armor types include flat metal tapes, smooth or corrugated continuous metal sheath or twisted braid (see also Chapter 9).
1. Landinger, C. C., 2001, adapted from class notes for “Understanding Power Cable Characteristics and Applications,” University of Wisconsin–Madison.
2. ICEA S-93-639/NEMA WC 74-2006, “5-46 kV Shielded Power Cable for Use in the Transmission and Distribution of Electric Energy.”