1077-2618/09/$26.00©2009 IEEE

An overviewof cold applied technology

for medium-voltage cable accessories

COLD-SHRINK AND HEAT-SHRINK

technologies have been around for many

years, but how they work is quite different

and not widely understood. This article

offers an overview of the shrink technologies and their

advantages and disadvantages. This article also discusses

the different methods of electrical stress control and how

they perform relative to terminations and splices. Specific

considerations for both terminations

and splices are also addressed. We dis-

cuss high-dielectric stress control and

geometric stress control and present the advantages and

disadvantages of each.

Shrink Technologies

Cold-shrink products are typically made from either eth-

ylene propylene diene monomer (EPDM) or silicone

rubber, while most heat-shrink products are made from

ethylene vinyl acetate (EVA). All three of these materials

are crosslinkable, which is what allows them to be

stretched and then shrunk onto the cable. To better under-

stand some of the differences between cold shrink and heat

shrink, let’s look at a brief description of how the different

materials and products work.

In both cold-shrink and heat-shrink products, cross-

linking forms bonds between the mol-

ecules of the material that act like

springs when the material is stretched.

These bonds try to return the material to its original

diameter. The more the material is crosslinked, the stron-

ger the bonds become and the closer the material will

return to its original diameter when it is shrunk. However,

it will never recover to its original diameter, and this dis-

tance between the original diameter and the diameter that

the material recovers to when shrunk is called permanent

set. All cold-shrink and heat-shrink products are sizedDigital Object Identifier 10.1109/MIAS.2009.933406

B Y W I L L I A M L . T A Y L O R& J O E N . W H I T E H O U S E

© DIGITAL VISION

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based on this permanent set of thematerial so that they will shrink ontothe cable on which they are installed.

Cold-shrink products that arestretched and then allowed to shrink onthe cable always exert an inward pres-sure on the cable as they try to shrinkback to their original diameter. Thisinward pressure provides an excellentenvironmental seal and also improveselectrical performance. Cold-shrinkproducts are typically expanded be-tween 30% and 150% when installedon a cable. When they are loaded on aremovable core, they are typicallyexpanded between 200% and 300%.Because of this living seal, cold shrink isan excellent choice in cases wherethermal cycling occurs due to changesin load and ambient conditions. As thecable expands and contracts with these

differences in temperature, the cold-shrink product will expand and con-tract with the cable to maintain anexcellent seal.

Heat-shrink products are also cross-linked, which allows them to beexpanded and then shrink back to asmaller diameter. The difference is inthe material used for heat-shrink prod-ucts. The material has crystallineregions that keep it expanded at nor-mal temperatures without a mechani-cal core. When the material is heatedabove the crystalline melt tempera-ture, the crystalline regions melt andallow the material to be expanded.After it is expanded, it is cooled belowthe crystalline melt temperature,which allows the crystalline regions tocrystallize and become rigid again.The product is shipped in this state.For installation, the product is heated

to a temperature above the crystalline melt temperature,typically more than 100 �C, which melts these crystallineregions and allows the product to shrink. However, whenthe source of heat is removed, after the product has shrunkonto the cable, the crystalline regions resolidify. Underoperating conditions of the cable, these regions remain incrystalline form and, therefore, exert no inward pressureon the cable. That is why most heat-shrink products usehot melt adhesives and mastics to environmentally sealthe cable, since the heat-shrink material does not expandand contract with the cable.

Advantages and DisadvantagesCold-shrink products do not require special tools to install,while heat-shrink products require a torch as well as moreskill. Cold-shrink products install consistently withrespect to length and insulation thickness. Heat-shrinkproducts will install with a fairly consistent length, butthe insulation thickness depends on the skill of the instal-ler. If the heat is applied mainly on one side of the tubeinstead of evenly all around the tube, then the thickness ofinsulation may be half as thick on one side compared to theother for a low-voltage insulating tube, which could causean electrical failure.

During installation, additional care must be taken toensure that the torch does not damage the cable, otherworkers, or anything else in the immediate area. In a man-hole, there is the possibility of combustible gases entering,which could cause a possible explosion. The gases from theflame must also be exhausted out of the manhole and theoxygen replenished for worker safety. Obviously, cold-shrink products are a safer option when working in man-holes and other areas where gases may concentrate. It willalso normally take a longer time to install a heat-shrinkproduct than a comparable cold-shrink product.

Heat-shrink products are much tougher and more rigidthan cold-shrink products and perform better in applicationswhere abrasion resistance is required, such as terminationson mining cables. Typically, cold-shrink products are muchsofter and more easily damaged than heat-shrink products.

50%

60%

80%

40%

20%Shield

Equipotential Lines

Electric Flux Lines

Insulation Conductor

1Electrical stress at the end of cable semicon.

Electric Flux Lines

Conductive Cone

Shield

20%40%

50%

60%80%

EquipotentialLines

Cone InsulationInsulationConductor

2Geometric stress control.

COLD SHRINK ISAN EXCELLENT

CHOICE WHERETHERMALCYCLING

OCCURS DUE TOCHANGES INLOAD ANDAMBIENT

CONDITIONS.

20%

Hi-K TubeShield

Equipotential Lines 40%

60%

80%Insulation

Conductor

3

ElectricFlux Lines

High-dielectric constant stress control.62

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Electrical Stress ControlThis basic understanding of how the materials work willhelp the end user choose the correct accessory for the appli-cation to provide long life. In addition to the materialselection, the type of electrical stress control must be con-sidered. Electrical stress is defined as the concentration ofelectrical potential over a defined distance and is measuredin kV/mm. The electrical stress in a medium-voltagepower cable is concentrated at the end of the cable insula-tion shield. For all cable accessories, this electrical stressmust be controlled, or it will cause an electrical failure.Figure 1 shows this electrical stress concentration with nostress control.

As can be seen in Figure 1, the equipotential lines arevery close together at the insulation shield discontinuity,which indicates that the electrical stress is very high. Thereare several methods of controlling this electrical stress. Onemethod is geometric stress control, a method used to reducethe stress at the shield discontinuity by extending theshield and gradually increasing the thickness of insulationunder the shield. This method overcomes the high electri-cal stress with additional insulation, as shown in Figure 2.

Another method of controlling the electrical stress atthe end of the insulation shield of the cable is high-dielec-tric constant stress control [1]. Adevice made with high-dielectric con-stant (K) material is used to reduce theelectrical stress at the cable insulationshield discontinuity by field refractionbecause of the difference in K values ofthe two adjoining dielectric layers. Asseen in Figure 3, this method ofelectrical stress control refracts theelectrical stress or equipotential lines,which spreads them out along thecable insulation interface. Electricalstress is much greater where the elec-trical stress lines are closer together.By spreading out these electrical stresslines, the surface stress of the termina-tion is greatly reduced, which improvesthe performance of the termination.

Figure 4 shows how high-dielectricelectrical stress control works for spli-ces, and Figure 5 shows the geometricelectrical stress control for splices.

These stress control plots are repre-sentative of the different types ofstress control for splices and are usedfor comparative purposes. Both high-dielectric and geometric stress controlwork well for medium-voltage cablesup through 35 kV. Above 35 kV, geo-metric stress control performs betterfor splices. With high-dielectric stresscontrol, you will get some dielectricheating in the stress control area,which adds additional heat to thesplice. Splices with high-dielectricstress control are not recommendedfor installations in systems that are tobe field tested with a tan delta test

system, according to the IEEE 400 standard. For theseapplications, geometric stress control is recommendedsince it does not influence the test results.

Termination Design ConsiderationsWhen considering the best termination to use for a specificapplication, there are many issues to consider [2]. Some ofthese include the environment in which the terminationwill be functioning, the altitude of the installation, and ofcourse the operating voltage of the system. Since termina-tions rely on air for part of their performance, when termi-nations are installed in higher elevations, air has a lowerdielectric strength and the termination must be derated.For heavily contaminated areas, it is recommended toinstall a higher voltage class of termination. This providesa longer creapage distance and better performance.

Some other important considerations for terminationsare as follows: 1) environmental sealing, 2) filling the semi-con step and electrical stress control, 3) track and environ-mental resistance, and 4) creapage distance and overalldesign. Each of these considerations will be briefly discussed.

Environmental sealing is critically important in keep-ing moisture out of the termination and cable. Mastic istypically used to seal around the ground coming out of the

100%

E1

E4

30%

50%

70%90%

E1 Maximum Stress Over the Connector Shield 46.7 V/milE2 Maximum Stress Along the Interface at the End ofConnector Shield 18.3 V/milE3 Maximum Stress Along the Interface at the End ofCable Insulation Shield 7.8 V/milE4 Maximum Stress in the Cable Insulation 72 V/mil

CableInsulation

Conductor

10%

E3E2

Hi-K

4High-dielectric stress control for a splice.

Insulation Shield

E1 E2

E4

E3

10%20%

30%40%

50%60%

70%80%

90%

E1 Maximum Stress Over the Connector Shield 31.1 V/milE2 Maximum Stress at the End of Connector Shield 36.3 V/milE3 Maximum Stress Along the Interface 12.7 V/milE4 Maximum Stress in the Cable Insulation 72 V/mil

CableInsulation

Conductor

0%

100%

5Geometric stress control for a splice.

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base of the termination under the bot-tom seal of the termination. At the lugend of the termination, mastic or tapeis typically used to provide an environ-mental seal. For both of these types ofseals, cold-shrink-type materials workextremely well because of the continu-ous inward pressure that they exert.This inward pressure and flexibility ofthe material allow it to move with cableand maintain the environmental seal.Cold shrink relies on this inward pres-sure for environmental sealing, whereasheat shrink, because it exerts no inwardpressure, relies on mastics and adhe-sives for an environmental seal.

Since the highest electrical stress ina termination is at the insulationshield step, the filling of this step toeliminate voids is critical to the termination performance.This step is typically filled with high-dielectric mastic orgrease. The high-dielectric mastic provides much betterelectrical stress control and provides much better partialdischarge levels. The mastic along with the high-dielectricstress control tube provides much better electrical stresscontrol and, therefore, lowers the electrical surface stresson the termination. The lower surface stress helps to re-duce the chance of surface tracking.

For surface tracking on a termination to occur, threethings must be present: electrical potential, moisture, andcontamination. Since electrical potential is always present,the termination material should be formulated to minimizemoisture and contamination. Silicone materials do thisextremely well, as they repel water and cause it to bead up onthe surface. This never allows a continuous wetted-out sur-face on the termination and thus minimizes tracking. Thematerial and physical design are important in minimizingsurface contamination. Physical skirt design assists in mini-mizing surface contamination and continuous moisture onthe surface. Ultraviolet resistance is also critical to the longlife of a termination. The sun’s ultraviolet rays do not affectmost silicone materials. Other materials typically have addi-tives to make them repel water and become more ultravioletresistant. It is also common for termination materials to haveadditives to make them more resistant to surface tracking.

Creapage distance is the total distance from the high-voltage lug to ground at the base of the termination. Thisincludes the distance around the skirts, if skirts are used.For highly contaminated areas, special skirt design or addi-tional skirts may be required to provide a longer creapage

path and long life. Extra insulation ontop of the electrical stress control willalso reduce the surface stress and im-prove performance. Improved electricalstress control will also increase theelectrical performance.

All of these things must be consid-ered to ensure long life and excellentperformance for terminations.

Splice Design ConsiderationsSplices, as seen in the stress plots,unlike terminations, contain all of theelectrical stress in the splice, becauseof the grounded shell of the splice.There are three main areas to considerwhen discussing splices. These areas,as shown in Figure 6, are the electrodearea, the insulation, and the insulation

shield, part of which is the end seal. Each of these areaswill be discussed in detail.

The most critical area of a splice is the area around theconnector. The gaps between the connector and the cableinsulation and the gaps created by crimping the connectoror from the set screws must be handled so that corona dis-charges do not occur. These gaps can be controlled by fill-ing the gaps entirely or by creating a Faraday cage aroundthe connector. It is difficult to consistently fill all of thevoids using mastic, but it can be done. The Faraday cageworks by transferring potential from the connector to thesemiconductive inner electrode. Since the conductor andthe inner electrode are now at the same potential, there isno potential difference across the gaps, which eliminatesthe chances for breaking down of the air.

The inner electrode has to be long enough to cover theconnector and the gap between the connector and the cableinsulation, so that the ends are on the cable insulation.The end geometry of the inner electrode should also bedesigned to minimize electrical stress.

The primary insulation has several important functionsand design considerations. It obviously must provideadequate electrical insulation, but it should also be capa-ble of dissipating the heat from the connector. The begin-ning of the ramp of the insulation geometric stress controlshould have a minimum step. A step of more than 1 mmcould cause a high electrical stress area.

The splice insulation to cable insulation is also thecritical interface area of the splice. The interface area pres-sure has also been shown to greatly enhance the perform-ance of the splice. The higher this interface pressure, thebetter the electrical performance of the splice. The researchfor this information was performed by the Institut deRecherche d’Hydro-Quebec (IREQ) test lab in Montreal,Canada, and is documented in an article titled ‘‘InterfacialDischarges in Underground Cable Accessories’’ [3]. Thetype of splice material, the length of interference betweenthe splice and the cable insulation, and the type of spliceaffect the interface pressure. As we know from our earlierdiscussion of heat-shrink materials, these splices providevery little interface pressure. Cold-shrink splices providethe highest interface pressure and do not lose any of thisinterface pressure as they go through load cycles.

ENVIRONMENTALSEALING ISCRITICALLY

IMPORTANT INKEEPING

MOISTURE OUT OFTHE TERMINATION

AND CABLE.

SplicePrimary Insulation

CableInsulation Shield

SemiconductiveInner Electrode

SemiconductiveEnd Seal

6Components of a splice.

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Push-on splices also provide much less initial interfacepressure than cold-shrink products and tend to lose much oftheir interface pressure as they go through load cycles. Sincethe initial interface pressure of push-on splices is lower, theymust have a much longer interface to provide the sameelectrical performance as a cold-shrink splice. Table 1 showsthe percent of interface pressure that is lost during loadcycling and compares push-on splices to cold-shrink splices.This comes from research by IREQ and is documented in anarticle titled ‘‘The Effect of Thermal Cycling on SpliceInterface Pressure’’ [4]. As seen from this table, push-onsplices tend to lose much of their initial interface pressureafter thermal cycling. This is due to the small amount ofinterference that they have and the material properties.

The insulation shield and end seal area is important aswell, since it keeps the electrical stress in the splice andkeeps the outside of the splice at ground potential. The endseal provides an additional environmental seal to make surethat no moisture gets into the interface area of the splice.

The second part of the insulation shield is the metalliccomponent, which is typically a tinned copper sockattached to the metallic cable shield with constant forcesprings. The metallic portion is required to carry the faultcurrent and leakage current of the system.

Over the metallic shield is a splice jacket that replacesthe cable jacket. The splice jacket is critical for environ-mental sealing and physical protection of the splice body.There are numerous options for the splice jacket, such asa tape system, cold-shrink tubes, or heat-shrink tubes. Atape jacket typically consists of multiple layers of rubbertape to provide an environmental seal, with at least twohalf-lapped layers of vinyl on the outside to provide abra-sion resistance. The combination of the two types of tapesalso provides physical protection. The cold-shrink jacketprovides an excellent environmental seal, in conjunctionwith mastic, and physical protection. If additional physi-cal protection is required, then a hard resin typecast mate-rial can be applied. Heat shrink provides good physicalprotection and, in conjunction with the mastics and adhe-sives, an environmental seal.

For aerial splices, additional considerations need to betaken into account. The splice will need to be more flexibleand maintain the interface pressure and an environmentalseal as the cable moves. Cold-shrink splices will be able tohandle this movement better than tape, heat shrink, orpush-on splices. The other major consideration for aerialsplices is that the jacket system must be resistant to thesun’s ultraviolet rays. Vinyl tape meets this requirement ina tape jacket system. For a cold-shrink jacket tube, it shouldalso be covered with two layers of vinyl tape to obtain theultraviolet resistance. A second option for jacketing a cold-shrink splice is to use a silicone cold-shrink tube. Mostheat-shrink tubes are formulated for ultraviolet resistanceand can be used as a jacket over the installed splice.

Performance of AccessoriesAll accessories manufactured and sold today meet thestringent requirements of the IEEE 48 standard for termi-nations, the IEEE 404 standard for splices, and the IEEE386 standard for modular accessories, such as tee bodies andelbows. These standards cover voltages beginning at 2.4kV and include transmission voltages. For the distribution

voltages that we typically deal with, the standard providestest levels to ensure that accessories will provide excellentfield service if they meet these standards.

The standards include ac test levels up to four and one-half times the operating voltage the accessory will see,impulse tests, dc tests higher than any field tests, and cur-rent cycle tests. In addition to these qualification tests, thesplice and modular standards require that all premoldedaccessories such as push on and cold shrink be 100% factorytested to provide additional assurance of performance.

Heat shrink and cold shrink have accessory products upthrough 69 kV, and push-on products are used for mediumand transmission voltages.

Because of the excellent material designs and rigidqualification tests, all accessories when installed correctlywill perform in the field. It is up to the end user to look athis requirements and choose the best fit for his application.

ConclusionsThere are many issues to consider when selecting termina-tions and splices. Just a few of these have been consideredin this article. The material properties and how they func-tion, as well as the actual application, must be consideredto make the proper selection. Because of cold-shrink mate-rials’ unique properties, cold-shrink accessories move withthe cable as it goes through its load cycles while continu-ing to provide excellent electrical performance and envi-ronmental sealing, thus providing long life and reliability.

References[1] P. N. Nelson and H. C. Hervig, ‘‘High dielectric constant materials

for primary voltage cable terminations,’’ in Proc. IEEE UndergroundTransmission and Distribution Conf., 1984, pp. 347–352.

[2] L. A. Johnson, ‘‘Polymeric terminations, present and future,’’ presented at

the IEEE Transmission and Distribution Conf., New Orleans, LA, 1989.

[3] C. Dang, ‘‘Interfacial discharges in underground cable accessories,’’

Canadian Electricity Association, Montreal, Quebec, CEA No. 3060932,

1997.

[4] N. Amyot, D. Fournier, and D. Lalancette, ‘‘The effect of thermal cycling

on splice interface pressure,’’ in Proc. Int. Conf. Advances on Processing, Test-ing and Application of Dielectric Materials, 2001, pp. 197–200.

William L. Taylor (wltaylor1@mmm.com) and Joe N.Whitehouse are with 3M Austin Center in Austin, Texas.Taylor is a Senior Member of the IEEE. This article firstappeared as ‘‘An Overview of Cold Applied Technology forMedium-Voltage Cable Accessories’’ at the 2007 Petroleumand Chemical Industry Conference.

TABLE 1. PERCENT OF LOST INTERFACE PRESSURE.

Splice Type Push-On Splice Cold-Shrink Splice

After first thermal cycle

At 75 �C 49 106

At 90 �C 38 94

At 130 �C 7 107

After stabilization of thermal cycles

At 75 �C 41 105

At 90 �C 33 99

At 130 �C 6 107

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