Section 6: Insulators & Insulating · PDF fileInsulators & Insulating Covers. Insulators: ... • MVLC: Medium Voltage Line Cover ... Tyco Electronics Raychem GmbH

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Insulators & Insulating Covers.

Insulators:• EPBI:

Light Weight Polymeric Post Insulator

• EPBI:Light Weight Polymeric Stand Off Insulator

• EPCI:Light Weight Polymeric Tension Insulator

An Overview of Porcelain versus Polymeric Insulators

Introduction to Silicon Elastomer Insulators

Selection of Polymeric Insulators - Material Considerations

Insulating Covers: • BCAC:

Bushing Connection Animal Cover - up to 35kV

• BCIC:Raysulate Bird Protection Cover - up to 24kV

• BISG:Bus Insulator Squirrel ( Possum ) Guard

• MVLC:Medium Voltage Line Cover - up to 24kV

• OLIC:Overhead Line Insulating Cover - up to 24kV

• OLIT:Overhead Line Insulating Tape - up to 24kV

Creepage Extenders: • HVCE:

High Voltage Creepage Extenders - up to 66kV

Section 6:

Energy Divisionhttp://energy.tycoelectronics.com

EPBI R

Insulator for outdoor equipment

These lightweight polymeric insulators areideal for outdoor equipment applications suchas overhead line switches and fuse cut outs.

They provide equipment manufacturers andutilities with the benefit of a very lightweightand shatter proof construction without theneed to compromise on reliability or longtermperformance.

The insulators combine the advantages of amechanically strong and lightweight polymericcore with the Raychem HV material used in theouter insulator profile.

The core contains no glass fibres and is there-fore proof against wicking problems. The outerhigh voltage material has proven non tracking,UV stable properties backed up by 20 yearsfield experience of HV polymers in widely varying climatic conditions.

Features• Engineering polymer core with no

glass fibres• Proven HV materials, backed up by

20 years successful field performancethroughout the world

• Proven interfacial seal system• Stainless steel end caps

Benefits• Lightweight – easy installation, easy erection

and reduced transport costs for equipment• Shatterproof – breakages eliminated during

delivery and erection• Vandal resistant• Long term reliability.

High resistance to water ingress• High corrosion resistance

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All the above information, including drawings, illustrations and graphic displays, reflects our present understanding and is to the best of our knowledge and belief correct and reliable. It does, however, under no circumstance constitute anassurance of any particular qualities. Such an assurance is only provided in the context of our product specifications. Our liability for this product is set forth in our standard terms and conditions of sale.AMP, ELCON, ELO TOUCHSYSTEMS, HTS, MACOM, MADISON, NETCONNECT, RAYCHEM, SIMEL are trademarks of Tyco International Ltd.

Tyco Electronics Raychem GmbHEnergy DivisionHaidgraben 685521 Ottobrunn/Munich GermanyTel. (089) 6089-0 Fax (089) 609 63 45

Tyco Electronics CorporationEnergy Divisionc/o AMP Singapore Pte LtdNo. 26 Ang Mo Kio Industrial Park 2Singapore 569507Tel. 65-4836012Fax 65-4836031

Tyco Electronics CorporationEnergy Division8000 Purfoy RoadFuquay-Varina, NC 27526-9349, USATel. (800) 327-6996Fax (800) 527-8350

Members of the Tyco Electronics Corporation:

• Wedge technology products • Electrical connectors • Cable accessories • Asset protection • Surge arresters • Insulators • Fittings • Associated toolings

Technical SpecificationElectricalImpulse +ve (kV) 155 200

Impulse -ve (kV) 200 255

Creepage (mm) 740 1050

Dry A.C 50Hz (kV) 85 145

Wet A.C 50Hz (kv) 60 100

MechanicalMax loadF (N)+ 4500 4000

Max cantilever (Nm)+ 945 1380

Tensile (M16 pull out) (KN)+ 40 40

Torque withstand (Nm)* 200 200

Max M16 bolt torque (Nm) 50 50

DimensionsLength L (mm) 210 345

Diameter D1 (mm) 57 64



M16 bolt depth S (mm) 55 55

ø 6 mm pin spacing C (mm) 36 36

Ordering InformationDescription U.O.M. Weight (g)EPBI-0210/07-048/01 1 Insulator 1400

EPBI-0345/11-056/01** 1 Insulator 2400**The EPBI-0345/11-056/01 will be delivered with M16 bolts which have to be removed beforeinstallation

Vandal Resistance12 bore shot gun; full choke; 10 yards range:- no immediate electrical or mechanical failure. Droptest: 5 m on to concrete: – no mechanical failure.Further details including interfacial seal tests: Ref UVR 8150

ApplicationsEPBI-0210/07-048/01 is recommended for applications up to 24kV (line voltage).EPBI-0345/11-056/01 is recommended for applications up to 36kV (line voltage).Both insulators have sufficient creepage to operate in heavily polluted environments up to andincluding Class 3 IEC 815. EPBI insulators are designed for applications with high intermittentloadings such as overhead line switches and fuse cut outs.

InstallationThe end fittings are designed to be compatible with IEC 273 dimensions and require high tensileM16 bolts and 6 mm diameter anti-rotation pins. Bolts should be tightened to a maximum torqueof 50Nm.

EPBI Insulator for outdoor equipment

L

S

C

D1

D2

D3

* limited by end fittings + Intermittent loading using H.T boltsFurther details: Ref UVR 5166

EPCI 24 kV Tension Insulator

E L E C T R I C A L . P R O D U C T S . D I V I S I O N

The high tensile strength of glass fibrehas been combined with a Raychem HVshedded profile, to produce this rugged,lightweight tension insulator for overheadline applications up to 24 kV.

The glass fibre core provides high mech-anical strength with tensile values ofgreater than 70 kN. The Raychem insulator profile utilises the same materialstechnology that has been employed forover 25 years in Raychem’s high voltageterminations. Its proven track and erosionresistance and UV stability have givenoutstanding performance in the widestpossible range of climatic and pollutionconditions.

The construction consists of Raychem’scompact creepage design insulator pro-file which has alternating large and smalldiameter sheds to optimise the pollutionflashover performance. It is sealed to aglass fibre rod with track resistant visco-elastic mastic. The mastic remains mo-bile at service temperatures and ensuresthat an effective moisture barrier is cons-tantly maintained. The hot dip galvanisedsteel end fittings are crimped onto theglassfibre core providing high strengthcorrosion resistant fixing points.

Features• High strength glass fibre core• Raychem’s proven HV polymer tech-

nology with 25 years of successfulfield performance

• Proven interfacial seal system• Hot dip galvanised steel end fittings• Tested to IEC 1109 polymeric insulator

specification

Benefits• Lightweight – easy installation, easy

erection and reduced transport costs• Shatterproof – breakages eliminated

during delivery and erection• Vandal resistant• Long term reliability. High resistance to

moisture ingress• High corrosion resistance mastic

EPP 0538 22.08.2000 16:41 Uhr Seite 1

Technical Specification

Dimensions in millimetres

Electrical (tested according to IEC 383)Impulse withstand (positive) 158 kVImpulse withstand (negative) 190 kVCreepage (nominal) 600 mmDry withstand A.C 50Hz 91 kVWet withstand A.C 50Hz (horizontal) 75 kV(vertical) 60 kV

MechanicalSpecified Mechanical Load (S.M.L.) 70 kNRoutine Test Load (R.T.L.) 50 kNWeight 1300 g

Applications

The EPCI-0380/06-016/EE insulator is recommended for vertical or horizontal appli-cations at voltages up to 24 kV (system voltage). The insulator has sufficient creep-age to operate in heavily polluted environments up to and including Class 3 IEC 815.

Ordering InformationKit No. U.O.M. Weight (g)EPCI-0380/06-016/EE 1 PC Supplied as 3 pcs. 3900PCN: 851825-000 per box

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At Raychem we are committed tocontinuous quality improvement in every aspect of our business.

All of the above information, including illustrations is believed to be reliable. Users, however, should indepen-dently evaluate the suitability of each product for their application. Raychem makes no warranties as to theaccuracy or completeness of the information and disclaims any liability regarding its use. Raychem’s only obli-gations are those in the Standard Terms and Conditions of Sale for this product and in no case will Raychembe liable for any incidental, indirect, or consequential damages arising from the sale, resale, use or misuse ofthe product. Raychem Specifications are subject to change without notice. In addition, Raychem reserves theright to make changes in materials or processing, without notification to the Buyer, which do not affect compli-ance with any applicable specification.Raychem and RayBowl are trademarks of Raychem Corporation.

Raychem CorporationElectrical Products DivisionInsulator Group300 Constitution DriveMenlo Park, CA 94025-1164, U.S.A.Tel. (415) 361-5094Fax (415) 361-3195

Raychem CorporationElectrical Products Division8000 Purfoy Rd.Fuquay-Varina, NC 27526-9349, U.S.A.Tel. (800) 327-6996Fax (800) 527-8350

Raychem Ltd.Electrical Products DivisionFaraday RoadDorcan, SwindonWiltshire SN3 5HH, U.K. Tel. (07193) 528171Fax (07193) 572403

Raychem GmbHElectrical Products DivisionHaidgraben 685521 OttobrunnMunich, GermanyTel. (089) 6089-0Fax (089) 6096345

EPP 0538 22.08.2000 16:41 Uhr Seite 2

An Overview of Porcelain and PolymerElectrical Insulation


Overview 14.12.1999 21:14 Uhr Seite 1

IntroductionPorcelain as an insulating material hasover one century of service history, whilepolymer materials have three decades.Early generation polymer products didnot provide the expected service life,and users still have concerns aboutpolymer insulating material performance.

The chemical stability of porcelain resistsaging, however it also allows the surfaceto easily wet, which can lead to flashoverin contaminated locations. Polymeroutdoor insulation is organic in natureand it can age with exposure. In a worstcase condition, it can result in possiblesurface damage leading to surfacecracking and loss of mechanicalintegrity. It does, however, initially resistwetting. Adequate compoundformulation, combined with appropriateinsulator housing design, resistweathering and aging with the benefit ofenhanced flashover performance.

In this paper, consideration is given tothe relative advantages of each materialtype. Flashover mechanisms andmitigation techniques are alsodiscussed.

Porcelain Insulation

ExperienceThe vast majority of the installedelectrical power system insulationdirectly exposed to the environment isporcelain, with over 100 years of history.This material has proven itself to resistenvironmental aging, be self-supporting,and is used in a wide variety ofapplications. It is also inherently large,bulky and heavy, broken in handling,transit and by vandalism, and subject to flashover incontaminated environments. Despite themany advantages of porcelain, systemreliability can still suffer.

AdvantagesStabilityThe strong ionic bonding and closepacking of the atoms which constituteceramics, such as between silicon andoxygen in silica and silicates, yieldstructures which tend to be very stableand are not generally degraded byenvironmental stresses. This means thatthe ceramic housing should not bedamaged by UV, surface electricalactivity, humidity, etc. [1]

Mechanical strengthThe rigid nature of the ceramic materialimparts significant mechanical strength.

Insulators can be fabricated for bothtension and cantilever loads. Theporcelain housings employed for cableterminations, bushings and surgearresters are self-supporting and do notrequire other materials or componentsfor strength.

Low Raw Material CostsThe principal raw materials of porcelain,such as clays, feldspar and quartz, arerelatively inexpensive and readilyavailable.

Processing and Lead-timeThe manufacturing process for porcelaininvolves many steps. For largerhousings, long periods of waiting arerequired to reduce the extruded corewater content, prior to shaping andfiring. Lead-times tend to be long as aresult.

LimitationsBreakageCeramics are very brittle. This meansthat they are easily broken in handling,transit or installation. Vandalism is aprimary contributor to in-servicemechanical damage. It is commonpractice to include a breakage or lossfactor when purchasing porcelaininsulators for line construction, which isan added unit cost factor.

WeightThe very dense nature of the ceramicsmeans that porcelain bodies are veryheavy. As the voltage rating increases,there is a compounding effect. This notonly makes for difficult handling whichcan require cranes, but it also meansthat expensive and large structuralsupports are necessary. The large sizeand weight of porcelain products usuallydictates the least expensive and mosttime consuming means of transport, inorder to minimize cost. Thus, shipmentby sea or other less costly methods alsoextend delivery time.

Hollow Core Housing Failure ModeHollow core housings for insulators,bushings, instrument transformers, cableterminations and surge arresters canexperience a violent failure mode as aresult of an internal dielectric breakdown.When an internal power arc occurs,there is a rapid increase in the pressure.If the pressure cannot be relieved beforethe bursting strength of the housing isexceeded, the housing can shatter.When this occurs, pieces of porcelainare expelled with considerable force.This type of failure mode is well known


within the industry, especially fordistribution class surge arresters.Complex GeometryPorcelain housings tend to haverelatively low creepage distance per unitlength, mm/kV, because of the costassociated with producing weathershedswith significantly larger diameters thanthe main core section. The shapingprocess to create the more complexweathershed shapes, to increasecreepage distance and/or improve thecontamination performance, also addscost. For medium-voltage use, porcelainhousings tend to have a simpleweathershed design, and extra creepageis often obtained by using a highervoltage class rated housing. An exampleis the use of a 35 kV class insulator on15 or 25 kV class systems. Pole-typetransformers may also be specified withhigher voltage class bushings, whilesurge arresters may be assembled withhigher voltage class housings withinternal spacers to make up for theincreased longitudinal length. All of theseexamples seek to improve the pollutionperformance, often defined in terms ofthe mm/kV creepage distance.

Pollution PerformanceThe stable chemical bonds of theceramic material also mean that it hashigh surface free energy, a propertywhich describes the strength of thesurface adhesion of contaminants. [1]With a high surface free energy,porcelain is easily wetted. Water on thesurface tends to form filaments whichsheet or coat sections of the surface.Materials with such characteristics areknown as “hydrophilic”. Hydrophilicsurfaces tend not to perform well underpolluted conditions as the water filamentdissolves the conductive pollution,lowering the overall surface resistance ofthe insulation with a conductiveelectrolyte along a continuous path,which can initiate the flashover process.

Polymer Insulation

ExperienceInitial ProductsEarly polymer products did not providethe expected service life, primarilybecause of inadequate UV and trackingresistance. As a consequence of initialproduct failures, many users continue tobe concerned about the long termperformance of polymer materials.

More Recent HistoryHigh performance polymer materialshave been in use for about 30 years. [2]

During that time, the use of polymerinsulation has grown steadily, andpolymerics are now becoming theoutdoor insulation material of choice.Common applications include cableterminations, surge arresters, insulators,bus bar insulation, and bushings. Figure1 shows a highly protected creepageinsulator installed in a severelycontaminated site. This device is ahybrid design utilizing a porcelain corefor strength and environmental agingresistance about the terminals. Theelastomer housing provides thecreepage in a highly protected geometryas well as the weathering resistance.

Insulation enhancement products, suchas creepage extenders, are installed overthe weathersheds of porcelain insulatorsto improve the performance of porcelaininsulators in contaminated applications. [3] Figure 2 shows how polymer materialcan enhance porcelain insulatorperformance. Other retrofit techniquesare in use which apply polymer materialto enhance the surface properties ofexisting porcelain insulators to reduceflashover in contaminated locations.

Not only are polymeric productsdemonstrating their capabilities indiverse environments, but polymericdevices and materials are routinely usedtoday for contamination flashover of alarge installed base of porcelaininsulation.

AdvantagesPolymeric insulating materials offernumerous advantages over porcelain. A partial listing includes:

Light WeightThe density of polymer materials is muchlower than ceramic, which results insignificant product weight reduction. Theweight differences increase with voltageclass rating. Polymer devices tend not torequire cranes or other lifting devices forhandling or installation. The reducedweight also permits the use of lighterand less costly structures and mountingarrangements. The smaller size andweight result in lower shipping coststhan equivalent porcelain devices.Polymer insulators are advantageous touse in dense urban areas and offeradvantages in narrow rights of way.Non-ceramic insulators offer highstrength to weight ratio which permitless expensive structures and improvedvisual aesthetics. Polymer insulators alsofacilitate new compact transmission linedesign with reduced electromagnetic

Figure 1 15 kV class highly protected creepageinsulators with open shed polymer surgearresters and porcelain fuse cutoutssupplying a pole transformer in Brazil in aseverely contaminated coastalapplication. The highly protectedcreepage design of the insulatorenhances pollution withstand. Theintrinsic stress grading of the arresterprovides some assistance againstflashover, however, the fuse cutouthaving relatively little strike and creepagedistance is highly susceptible toflashover from the fuse contacts to thegrounded mounting bracket duringwetting conditions.

Figure 2 66 kV insulators energized at 33 kV in aWest Australia harbor were frequentlywashed because of routine flashoverfrom salt deposition from the sea, ironore dust from the handling equipment forore loading from rail cars to ships andcontamination from an adjacent coalfired generating station. Peak leakagecurrent pulse monitoring of insulatorsfitted with single creepage extendersshowed a significant reduction in leakagecurrent, with additional savings realizedfrom reduced washing expense.


field effects. Polymer distribution surgearresters may be hung directly off fusecutouts, reducing installation costs andimproving aesthetics. Porcelain andpolymer 66 kV terminations, installed inparallel, are shown in Figure 3, asevidence of the concurrent use of thesetwo technologies. The polymertermination requires cable support sinceit is not self-supporting.

Complex GeometryAs polymer insulating housings aretypically molded, it is not difficult tofabricate parts on a cost effective basis,which have higher creepage distanceper unit length than porcelain.Weathershed profiles can be made morecomplex without production or yieldproblems. Alternating diameterweathersheds (big-small) are nowcommonly supplied, which improves theAC wet flashover by avoiding bridging ofall sheds simultaneously during heavywetting conditions.

Pollution PerformancePolymer materials typically used foroutdoor insulating applications have lowsurface free energy. [4] When new andwithout exposure to the environment,polymer materials resist wetting and areinherently hydrophobic. Retention ofhydrophobic properties with exposure isa desirable attribute. Water on thesurface of hydrophobic materials formwater beads, so the conductivecontamination dissolved within the waterbeads is discontinuous. This conditionresults in lower leakage current flow andprobability of dry band formation, whichin turn requires a higher impressedvoltage to cause flashover. Figure 4shows the hydrophobic nature ofpolymer materials, where water tends tobead rather than form filaments alongthe surface. In severe conditions, allmaterials lose their hydrophobicity.

However, the reduced diameters andsuperior shed geometry of polymerproducts can still give inherently betterpollution flashover performance thanporcelain.Hollow Core Housing Failure ModeHollow core polymer housings are likelyto have a very different failure mode fromporcelain. The physical properties of thepolymer material means that it will notshatter like porcelain. With the initiationof an internal fault, the expected failuremode is a rupturing or bursting of thehollow structure with venting of theinternal pressure, leading to an externalflashover and dissipation of the faultenergy outside of the housing.

The actual failure mechanisms betweenhollow core porcelain and polymerhousings may differ depending upon theproduct design and function, and usersare cautioned against assuming allpolymer products are inherently safe.Depending upon the specific design,function, failure initiation mechanism andavailable fault current, internalcomponents can be expelled. However,the volume of available dense material,compared to a porcelain housing, iscommonly less.

ProcessingThe manufacturing process for polymerproducts is inherently shorter than forporcelain. Molding times typically are ofthe order of minutes, so the lead-timecan be considerably shorter than forporcelain devices.

LimitationsWeathering DegradationPolymer materials have differentchemical bonds than porcelain (covalentversus ionic), and they can be aged andchanged by the multiple stressesencountered in service. [5] With propermaterial development and productdesign, polymer insulating products canand do provide high performance withdesired service life as reported in Ref. 2.A complex formulation and designoptimization process must beundertaken in order to achieve desiredperformance and service life over adiverse range of service conditions. Thisrequires that the materials scientistformulate the polymer material with theappropriate additives in the necessaryconcentrations using appropriate mixingmethods. The product designer mustconsider the specific properties of theformulation in product design andapplication. The manufacturing process

Table 1Subjective Comparison of Porcelain and Polymer Material Properties. Relative rankings assigned.

Property Porcelain Polymer

Strength +++ -

Size -- +++

Weight -- +++

Breakage - +++

Aging Resistance +++ +

Creepage/unit length - +

Pollution Flashover - +

Figure 366 kV polymer terminations installed inEngland in parallel with existing porcelainterminations. Polymer terminationsrequire less structural support becauseof their light weight. Polymer productsare becoming more common installedadjacent to existing porcelain devices onexisting systems.

Figure 4 24 kV distribution class polymer surgearrester showing polymer materialhydrophobicity with water beading onsurface. Discontinuous electrolyteincreases voltage required for flashoveron polymer material surface comparedto porcelain.


conditions must be evaluated as well toinsure that compound degradation doesnot take place during production.

Performance and service life remain userconcerns. Polymer material formulationis a complex optimization process, butone which is attainable by aknowledgeable materials scientist withextensive experimentation and testing.Formulations can vary widely with basepolymer comprising 20-80% by weightof the material. Additives are used toextend and reinforce mechanicalproperties. Typical extending andreinforcing fillers include antioxidants,plasticizers, pigments, cure agents,catalysts, flame retardants, UVstabilizers, tracking and erosionresistors, processing aids and othersdepending upon the specific formulation.In many situations, it is the additives andfillers, both collectively and individually,whose performance determines theoverall material performance.

The structure of polymer materials, suchas flexible bonds and long chain mobility,provide many of the inherentadvantages. However, as all polymermaterial formulations are organic,continuous service stress can lead to deteriorationof the surface properties and pollutionwithstand characteristics, unlessreinforced with a proper additivepackage and a housing design whichlimits leakage current. It is therecognition of this fact, combined withmaterials science and product expertisewhich results in material formulation andproduct designs with proven high levelsof performance for diverse applications.

Service stresses, such as coronadischarge, UV exposure or chemicalattack, cause chemical reactions on thepolymer material surface. One result isthe formation of hydrophilic groupswhich allow the surface to wet, whichpermits increased leakage current flow.A material, which may experiencereduction in hydrophobicity, does notnecessarily continue to change, such asby tracking or erosion, during wettingconditions. It is an important part of thematerial scientist’s task to ensure thatloss of hydrophobicity does not cause adisastrous increase in the degradationrate of the material.

High Raw Material CostsPolymer raw material costs are muchhigher than porcelain raw materials.Fillers and additives are blended with the

base polymer material not only to reducecost, but also to enhance performanceand facilitate processing. The basepolymer may constitute of the order of20%-80%, by weight, of the endmaterial. Thus, the actual formulation ofcompounds claiming the same basepolymer will be different, which will havea direct effect on performance. Intoday’s highly competitive market,suppliers try to gain advantage via costreduction. Service experience of an oldergeneration of material may not predictperformance of a different formulation.

Mechanical StrengthPolymer insulation is typically neitherrigid nor self-supporting. For applicationssuch as cable terminations, the cablemust be supported by some othermeans such as clamping of the jacket(oversheath) to a structure, and/or rigidconnection to the phase conductor.Where intrinsic mechanical strength isrequired, ceramic cores, fiber reinforcedtubings or layers may be utilized, whichare covered by the polymer material forweathering resistance as illustrated byboth the hybrid insulators and polymersurge arresters in Figure 1.

Material CompatibilityPolymer products can have more thanone axial interface, depending upon theproduct function and specific productdesign. Use of different strength membercomponents results in different interfacialproperties between the polymer housingand other internal materials. Wheremultiple interfaces exist, the primers andadhesives/sealants selected are of greatimportance. The material and processingproperties must be known and carefullyanalyzed in order to assure long-term,stable performance over the broad rangeof conditions encountered in service.

The polymer formulation can suffer fromstress corrosion or brittle fracture, fromtypical service stresses, if improperlyformulated. Material developers mustensure that this aging mechanism is minimized.


Contamination Flashover

Common Porcelain Insulator Housing Flashover ProcessContamination flashover is a multi-stepprocess, which may result during anumber of conditions. [6,] The basicsteps in the more common process forporcelain insulator housing flashoverinclude:1. Contamination build up – wind drives

dust and/or other conductivecontaminants onto the surface of theinsulation.

2. Surface Wetting – High humidity, dew,mist or light rain wets the surface anddissolves the contamination, creatinga conductive electrolyte, which iscontinuous or nearly continuous alongthe insulator length. When theelectrolyte forms, the surfaceresistance of the housing falls andappreciable leakage current flows.

3. Ohmic heating – the leakage currentflowing through the electrolyte causesa decrease in resistance andcorresponding increase in current,since the electrolyte has a negativetemperature coefficient of resistance.There is accumulated energydissipation (I2Rt) heating which forceswater evaporation, ultimately leadingto a runaway increase in drying rate.

4. Dry band formation – the powerdissipation per unit area is the productof the electrical stress and currentdensity. The areas of the surface withthe highest power dissipation dry first.Geometry plays a role, and the currentdensity tends to concentrate in theregions with the smallest cross-sectional area, which is where dryingis accelerated. Drying increasespower dissipation because ofincreasing resistivity, leading to anunstable condition where dry bandsform. As dry bands are insulating,surface activity continues within theband region until the band grows tosufficient length to withstand theapplied voltage with only intermittentactivity.

5. Partial arcing and flashover – flashoveroccurs if one of the dry banddischarges extends across theremaining wetted surface of thehousing. Discharges usually extinguishjust before a voltage zero. If, however,the stress and leakage current arehigh enough, the discharges mayexpand along the entire housinglength and initiate flashover. Visiblesurface activity does not mean thatflashover will occur. Flashover canonly occur when the electrical stressin the discharge is less than the stressin the wet film. Note thatcharacteristics of a discharge arc are“inverse ohmic” – the higher thecurrent, the lower the stress.

Note that the above steps must all takeplace sequentially for flashover to occur.If the surface is altered by washing, suchas rain, then the electrolyte conductivityis decreased. If wetting is by dewformation, the rising sun will reduce thewetting conditions. In such cases, thechances of flashover will be reduced.

Additional Steps For PolymerFlashover and Effects of AgingThe analyses of laboratory data andliterature surveys suggest severaladditional steps that occur in theflashover of a hydrophobic polymerinsulator housing.1. Contamination deposition – same as

for porcelain.2. Wetting – high humidity, fog, dew

or light rain deposit moisture on thesurface which forms dropletsbecause of the hydrophobicproperties. Due to gravity,dropletsroll down sloped areas. Wheregravity does not encourage dropletmovement, discrete droplets remain.Salt and/or conductive pollutiondissolves in the water droplets,increasing the liquid conductivity.

3. The residual dry surface pollution isslowly wetted by the dropletmigration. This forms a highresistance conductive layer, andchanges the leakage current fromcapacitive to resistive.

4. Ohmic heating – same as forporcelain.

5. Electric field effect on hydrophobicsurface – the applied electric fieldcauses closely spaced droplets tojoin together into a larger singledrop, known as a filament. Flashovertends to take longer for ahydrophobic surface because of thetime to form a conductive path withfilaments. The local electric field hasto be sufficiently high to formfilaments as well.

6. Spot discharges on hydrophobicsurface – filaments reduce thedistance between housing terminals,increasing the electrical fieldbetween adjacent filaments. Whenthe stress is sufficient, surfacedischarge activity can occur.

7. Reduction in hydrophobicity –Discharge consumes the thinpolymer layer around the dropletsand reduces the hydrophobicity byrotation or breaking of the polymerchains. Loss or reduction of surfacehydrophobicity results in dropletdispersion and the formation of acontinuous conductive layer in a highstress area, allowing elevatedleakage current flow.

8. Dry bands form under the sameprocess as porcelain. The resultantactivity causes surface erosionwhose rate depends upon the


specific material formulationdischarge-free and promotes aging.

9. Full or partial recovery ofhydrophobicity may be possible ifthe material is discharge free for asufficient period of time. Recoveryability will depend upon the specificmaterial, formulation, housing designand service environment.

10. Repetition of the aging cycle causesfurther erosion of the surface, whichis enhanced by chemical reactionand local temperature rise. Local hotspots can be of the order of 400°Cduring heavy discharge activity.Other aging, such as UV damage,can cause surface crazing whichtraps and holds contaminants whichcan promote leakage current flowduring wetting.

11. Flashover ultimately occurs along thesame process as porcelain. Thesurface becomes hydrophilic, wetsout, dry bands form and thedischarge propagates to bridge thehousing terminals.

Hydrophobic surfaces (see Figure 4)present a higher resistance to leakagecurrent flow than hydrophilic surfacesand require higher leakage current andcommensurate energy dissipation toinitiate flashover. This is why polymerinsulators have higher flashover voltagesthan conventional porcelain insulators.As with porcelain, all of the above stepsmust take place sequentially for flash-over to occur. If the process isinterrupted, such as a change in wettingconditions or with surfacehydrophobicity recovery, flashover doesnot take place. Thus, visible activity doesnot always result in flashover.

Flashover Mitigation

PorcelainUsers can utilize severalcountermeasures to reduce flashoverwith porcelain insulating housings. Theyare:1. Creepage extenders – polymer sheds

(Figure 2) are installed directly overporcelain insulator weathersheds toincrease the creepage distance. [3]

2. Extra-creepage housings – housingswith extra-creepage, more creepagethan typically used for the specificsystem, will reduce flashover risk.

3. Washing – insulators may be washedlive or de-energized with highpressure water or with solid materialssuch as ground up walnut shells. Thisis a costly process which may need tobe carried out on a regular scheduleto be effective. In addition, the use ofsolid cleaning materials may abradeaway the protective glaze of theporcelain, exposing the underlying

substrate to the environment, whichcan then hold contamination.

4. Complex weathershed profiles –protected creepage and fog-typeweathershed profiles are available, ata premium cost, which have profileswhich resist contamination depositionin the protected areas. Such shapes,however, do not lend themselves tolive line washing.

5. Surface coatings - greases or polymercoatings may be applied to theporcelain housing to improve thepollution performance. Coatings havea finite life, are costly to install, andpotentially very costly to reapplybecause of the need to clean and/orremove prior applications.Performance improvement and timenecessary for recoating are highlydependent upon the quality of thecoating application.

PolymerSeparate or retrofit flashovercountermeasures for polymer are rarelyused. The development of standardproducts for a wide range of applicationsand conditions requires careful attentionto the material formulation and insulatorhousing design.

One important contamination applicationconsideration is if a polymer product canbe washed, either intentionally orunintentionally, along with porcelaininsulators. Polymer products which arenot firmly secured to the underlyinglayer, such as insulators or arresterswhich rely on grease as the housinginterfacial sealing system, may not besuitable for high pressure washing. Suchproducts may fail prematurely fromwashing, and users need to consider theuse of such products in their specificservice environment, if the supplier doesnot recommend washing. [7]

Polymer insulating housings need leakage current control. While somedegradation over service is likely tooccur, one defense mechanism againstflashover is retention of leakage currentcontrol. If the leakage current isinsufficient then the flashovermechanism cannot progress. This is aresult of the interaction of the polymermaterial and product design.


Conclusions1. Polymer insulating materials offer

significant advantages over porcelain. 2. Polymer materials have been proven

with over 30 years service history.3. Polymer material formulation must

resist service stress degradation by a complex optimization process ofmaterial formulation and housingdesign, which includes considerationof the manufacturing conditions.

4. All insulators exposed tocontamination can experience surfaceactivity which does not necessarilylead to contamination-inducedflashover.

5. Products need to be evaluated on anindividual basis. Because ofdifferences in formulation, productdesign and manufacturing conditions,polymer materials are not identical norare they generic. Users cannot predictthe performance of one polymerproduct based upon experience withanother, which will have a differentformulation and/or design.

6. The competitive nature of themarketplace is driving many suppliersto cost reduction and materialreformulation. Newer generationmaterials may be untested and pastservice experience may not indicatefuture performance of newergeneration materials.

7. Porcelain insulator flashoverexperience can be improved with theuse of polymeric materials. Coatingshave a limited service life, whereasother solutions, such as creepageextenders, offer long-termperformance.

References[1] “Insulators for High Voltage,” Looms,J.S.T., (Peter Peregrinus Ltd., London,UK, 1990), p. 17.

[2] Thornley, D. and Shockett, A., “25Years Experience of Outdoor PolymericInsulation”, 1994 IEEE Transmission andDistribution Conference, Chicago, IL,USA, April 10-15, 1994.

[3] Pack, G., “Creepage ExtenderImproves Insulator Performance”,Transmission & Distribution International,Vol. 3, No. 3, September 1992, pp. 22-25.

[4] Looms, p. 19.

[5] Ibid., p. 17.

[6] Karady, G., Shah, M. and Brown, R.,“Flashover Mechanism Of SiliconeRubber Insulators Used For OutdoorInsulation - I’” IEEE Trans. on PowerDelivery, Vol. 10, No. 1, October 1995,pp. 1965-1971.

[7] IEEE Std. 957 “Guide for CleaningInsulators”, 1995.

At Raychem we are committed tocontinuous quality improvement inevery aspect of our business.

All above information, including drawings, illustrations and graphic displays, reflects our present understandingand is to the best of our knowledge and belief correct and reliable. It does, however, under no circumstanceconstitute an assurance of any particular qualities. Such an assurance is only provided in the context of ourproduct specifications. Our liability for this product is set forth in our standard terms and condition of sale. Raychem is a trademark of Raychem Corporation.

Raychem GmbHElectrical ProductsHaidgraben 685521 OttobrunnMunich, GermanyTel. (089) 6089-0 Fax (089) 6096345

Raychem Ltd.Electrical Products438 Alexandra Road # 05-01 Alexandra Point Singapore 119958 Tel. 65-2774138Fax 65-2743611

Raychem CorporationElectrical Products8000 Purfoy Rd.Fuquay-Varina, NC 27526-9349, U.S.A. Tel. (800) 327-6996Fax (800) 527-8350

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An Introduction to Raychem’s Silicone Elastomer Outdoor Insulation Material


Introduction 14.12.1999 21:24 Uhr Seite 1

IntroductionElectrical insulating polymericformulations first introduced by Raychem30 years ago have been proven to haveexcellent long-term physical andelectrical properties. [1] These materialswere originally designed for fieldapplication by heat-shrinking and arebased on radiation-crosslinked, semi-crystalline polyolefin co-polymers. Whentheir excellent weatherability andmoisture-sealing properties weredemonstrated in cable accessories, theuse of these materials was expanded tosuch applications as surge arresters,insulators, insulation enhancement andbushings, supplied by Raychem andothers. Whereas polyolefin co-polymerbased formulations have definiteadvantages, other developmentalmaterials research has taken placewithin Raychem over many years inorder to fully exploit the propertiesachievable with elastomeric-based insulation materials.

At Raychem, materials development isrecognized as a complex formulationprocess where the entire package isoptimized considering each of the stepsinvolved in producing the final product.The base polymer grade and variousadditives which are used, combined withcompounding procedures, materialprocessing, product design andassembly all contribute to the overallproduct performance. The industryperception that polymer materials aregeneric or somehow similar inperformance only on the basis of theclaimed based polymer is incorrect.Performance is based on the specificcharacteristics of a unique formulation,utilizing a specific design producedunder a defined set of processconditions. With the extreme variationsand differences that exist in each keyperformance area, it is difficult tounderstand how materials could becommonly grouped and viewed asequivalent. Products need to beindividually evaluated by users.Consideration needs to be given to thereputation and experience of thesupplier, the material qualification andproduct performance testing conducted,who controls and is responsible for thepolymer material supply chain, and thelevel of detail and data that is suppliedabout the material and its fieldperformance.

The silicone-based formulations now inuse by Raychem have been speciallydeveloped for electrical insulatingapplications. They have undergone manyyears of development and optimizationto yield exceptional electrical andweathering performance properties,comparable to the Raychem polyolefinco-polymer materials. The purpose of

this paper is to highlight the advantagesof properly formulated silicone materialsin outdoor insulating applications and toanswer the question, “Why is Raychemalso using silicone-based elastomericinsulation?”

BackgroundRaychem’s development of polymer outdoor insulating materials was initiallybased on supplying product for use asan outdoor cable termination. As wasreported in early Raychem publications,[2,3] semi-crystalline materials withexcellent properties were developedalong with pioneering work on testing,evaluation and lifetime predictionmodels. Heat-shrink was selected in the1960’s as the field delivery system toprovide the ability to install the producton all cable types in different coreconstructions and conductor cross-sections. This was to simplify installationand improve reliability versus othertechnologies in use at the time.

The initial material was an alloy of a silicone elastomer with a polyolefin, thelatter required to provide the crystallinityneeded for supply in a heat-shrinkableform. Early terminations containing thismaterial are still performing well inservice today. The compounding andprocessing of this polymer-blendmaterial was a formidable task sincecontrols must remain extraordinarily tightto make consistent product. In the1970’s, [4] development work resulted ina polyolefin co-polymer formulationwithout the silicone elastomer, whichwas introduced in wide use to themarket in the 1980’s after an extensiveand rigorous test program. The revisedmaterial had enhanced mechanicalproperties, enabling it to be deliveredwith larger application ranges, furtheringthe inherent benefits of heat-shrinkmaterials with improved erosionresistance. The revised material providedimproved durability in highlycontaminated applications.

To complement our products containingthe polyolefin co-polymer formulation,Raychem has developed silicone-elastomer outdoor insulating materialswhich maximize the inherent materialcharacteristics through formulationexpertise to deliver good erosionresistance, weatherability, and ultimatelyexcellent product performance. Thesematerials can be moulded using aunique flashless process in openweathershed geometry or in a patentedhighly protected creepage weathershedfor severe contamination applications.For applications where the polymermaterial can be factory installed or forfield installation where all of the benefitsof heat-shrink may not be required,Raychem silicone elastomer products


can also be used on a cost-effectivebasis.

No one polymer material is universallysuperior than any other. The mechanicalproperties of the polyolefin co-polymermaterial are inherently better and offeradvantages where such characteristicsare needed, whereas the polyolefin co-polymer material cannot be as readilyproduced in more complex shapes asthe silicone-elastomer material, norutilized as a cold applied cabletermination.

As a materials science company,Raychem continues to develop high-performance materials and productswhich best serve the ever-changingneeds of our customers, on a cost-effective basis. Not every complexproblem is solved with the samesolution. Raychem is uniquely positionedto utilize its considerable skill and talentto prove the best solution for everyproblem every time.

What is “ Silicone” ?In the literature, many generic commentshave been made about silicone without atrue understanding of its chemistry ormicrostructure. The term “ silicone”refers to a polymer composed of aninorganic siloxane backbone (Si-O,silicon-oxygen link). The most commonsilicone is Polydimethylsiloxane (PDMS),which has a backbone of silicon andoxygen, but also contains two methylgroups (CH3) for every one silicone:

CH3|

-( - Si -O -)n-|

CH3

The above unit is linked with similar unitsto form a chain which is the polymer (ncan be in the thousands). Hydrocarbonside-groups other than methyl that arecommonly seen along the polysiloxanechain are ethyl, phenyl, and vinyl groups.There may be other types of side-groups(such as fluorine to form fluorosilicone orhydrogen such as in mono-substitutedsilicones), but the Si-O backbone is keyfor it to be called a “silicone”. Thus, withthe significant presence of carbon-containing groups, the concept thatsilicone is completely inorganic is nottrue. These chemical constituents andtheir placement along the siloxane chainwill determine many of the materialproperties of a particular grade of “silicone”. The length and distribution oflengths of the siloxane polymers canalso vary and additionally contribute tothe overall properties of the choice of“silicone” used.To form a “silicone elastomer” one mustbind the chains together in some way to

create a network that has rubberyproperties (i.e. it will snap back from astretched position). Vinyl and otherreactive groups are usually also presentas side-groups and end-groups; thesegroups allow crosslinks to form duringchemical reaction (many of these groupscontain multiple carbon atoms).Polymers can be joined at theirendpoints or along various sections of achain during a reaction, forming a“crosslink”. The nature and type ofcrosslink depends on the reactivegroups, crosslinking agents andinhibitors, and catalysts; one kind ofcrosslink is shown in Figure 1. [5] Beforecrosslinking, the material is termed a“silicone gum”. After crosslinking, it iscalled a “silicone elastomer” or “siliconerubber”.

“Silicone” elastomers usually alsocontain reinforcing silica (glass or quartz)to strengthen the polymer since itsmechanical properties are inherentlyweak when compared with otherpolymers. This silica has reactive groupswhich bind to the silicone polymer andinfluence the strength, hardness,toughness, and other physical propertiesof different silicone grades. Other fillerssuch as coarser grades of silica areadded to lower costs; these types offillers are termed “non-reinforcing” fillers.Still other fillers are included to influence other properties such as processingcharacteristics, mold shrinkage, thermalexpansion, tracking and erosionresistance, weathering performance, and color. These types of fillers chosen mustmeet two main criteria: 1) long termstability under expected serviceconditions and 2) chemical inertnesstowards the other components in theformulation such that the silicone retainsits functionality over time. [6]

“Silicone” must be thought of then as abroad-term covering materials with awide variety of achievable properties;some silicones are better for outdoorelectrical insulating applications,whereas others are not. However, oncethe base silicone polymer is chosen, itmust be properly formulated with theinclusion of additives to optimize

Figure 1. Schematic of a typical vinyltype crosslink in silicones. Note the number of carbon atoms present.

Figure 2. Protected creepage, double-bell, hybrid insulator design, 15 kV class.

Uncrosslinked Gum Cross-linked Example

CH3 CH3

-( - Si -- O - )n - -( - Si -- O - )n -

CH = CH2 CH2

vulcanization CH2

CH3 CH2

-( - Si -- O - )m - -( - Si -- O - )m -

CH3 CH3

Siliconeelastomerhousing withno longitudinalflash

High strengthceramic core

Cementedmetal base fitting


performance properties. At Raychem,candidate formulations are continuallydeveloped and evaluated prior toproduct introduction in order to assurethat the user will have a reliable andeconomic product in the long term.Raychem has found that silicone-basedformulations have both advantages anddisadvantages. Among the advantagesthat are exploited in outdoor insulationapplications are hydrophobicity andhydrophobic recovery, weatheringresistance (when adequately formulated),processability, and elastomericmechanical properties. Limitationsinclude high raw materials costs,softness, and relatively low mechanicalstrength. The following discussion willexplore a variety of these properties with respect to thechoice of silicone elastomers, using aRaychem hybrid insulator design as aproxy.

Hybrid Insulator DesignThe hybrid insulator consists of asmooth-bore ceramic core and suitableend fittings, depending upon the designand application, with a moldedweathershed housing. The housing mayhave conventional open weathershedsor a patented highly protected doublebell creepage design. Figure 2 showsthe schematic diagram of a protectedcreepage hybrid post insulator.

The synergistic performance realizedcomes about from maximum exploitationof the advantages of each material. Theceramic core provides mechanicalstrength for cantilever or tensionapplications. Because of its inherentchemical stability, [7] ceramic canwithstand weathering, chemical attackand surface activity without damage.However, the chemical nature alsoresults in high surface energy. Thispermits the surface to wet out easily,which can lead to flashover if thecreepage distance is not sufficient. Toovercome this intrinsic limitation, thesilicone-elastomer housing is installedover the ceramic core with a suitable,stable interfacial sealant to maintaindielectric strength. The very high leakagedistance and high surface resistance ofthe elastomer limit leakage current andprevent the onset of the flashovermechanism. In the event of elastomersurface damage and loss ofhydrophobicity, ceramic is exposed ateach terminal where there is the highestelectrical field and likely location forelectrical activity. With this design, theelastomer will not experience additionaldamage for electrical activity about theterminals. The reduced volume of theceramic core contributes to reducedweight and easier handling.

HydrophobicitySilicone’s property of hydrophobicrecovery has been much publicized inthe literature as a key feature for its usein outdoor insulation applications.Silicone’s low surface energy impartsvery good hydrophobicity which resultsin low leakage current during wettingconditions where contamination ispresent. [8] On a hydrophobic surface,water drops bead up and do not wet thesurface completely. This reduces theleakage current, which leads to higherflashover voltages. [9] The organicgroups which pack around the silicone-oxygen backbone are actuallyresponsible for silicone’s hydrophobicitywhich is similar to many organicpolyolefin co-polymers. In comparing thetwo Raychem materials, the silicone-elastomer has some advantage in powerloss, due to its faster hydrophobicrecovery.

The hydrophobic recovery property ofsilicone is attributed to the flexibility ofthe Si-O linkage and the presence ofvery mobile free silicone chains.Experimental studies indicated thatheavy pollution and simultaneouswetting produced surface arcing, whichdestroys surface hydrophobicity andincreases leakage current. However, thesurface recovers hydrophobicity after10-12 hours of a dry and arc free period.The recovery of surface hydrophobicity isdue to the diffusion of mobile lowmolecular polymer chains (LMW) fromthe bulk to the surface [9] and rotation ofsurface hydrophilic groups away fromthe surface. [10]

Surface hydrophobicity can be evaluatedin the laboratory by determining thecontact angle of a droplet of deionizedwater on a material’s surface.Hydrophobic recovery is monitored bytreating the surface with corona andmeasuring the resulting contact anglesas a function of time after exposure.Figure 3 shows the result of a studywhere 1cm x 3 cm of various siliconeslabs were exposed to corona for 10seconds (using a Model ED-20 coronatreater, Electro-Technic Products, Inc.).Average contact angles along theexposed regions were determined fromadvancing contact angle measurementsmade with a Rame Hart model 100Goniometer. In this figure, fourexperimental formulations (A,B,C and D),which contain the same siliconeelastomer but different filler types andlevels, are compared with a fifthcompound, a commercially availablesilicone (E). All of these siliconecompounds were found to haveexcellent hydrophobicity and allrecovered their hydrophobicity within 8hours after arc treatment. Theexperimental formulations (A,B,C, and D)


were comparable or better than thecommercially available peroxide curedsilicone compound which is highly filledwith alumina trihydrate (E). For surfacestreated with longer corona exposuretimes (e.g. 150 sec.), recovery stilloccurs and is complete after a 24 hourperiod.Beyond laboratory data which do notallow one to predict product performanceunder different and varyingenvironmental conditions, Raychem hasmonitored the leakage currents ofvarious hybrid insulator designs andceramic insulators, pairing them inservice under different types ofenvironments. We have been conductingtests at different locations around theworld. Figure 4 shows the typicalleakage current results from the PenghuDistrict in Taiwan. The pattern of leakagecurrent shows clearly that leakagecurrent increases during early morningtime and decreases during the middle ofthe day (The rising sun reduces dewformation). It can be inferred from theseresults and those taken at other sitesthat leakage current is higher on ceramicinsulators than the RayBowl silicone rubber insulator. The resultsdemonstrated the superior performanceof Raychem RayBowl hybrid insulator inthe severe winter season. These resultsalso confirm the results we haveobserved in other parts of the world.Average leakage currents on ceramicinsulators with complex designs aretypically six to eight times higher than onthe Raychem hybrid insulator. This ratioincreases with higher pollution levels.

Tracking and Erosion ResistanceWhile in a hydrophobic state, siliconeswill limit leakage current and surfaceactivity during wetting conditions withcontamination present. As hydrophobicproperties change, surface activity canoccur and become concentrated,rooting at a specific area. As locally, veryhigh temperatures are generated(>1000°C), the silicone polymer will startto degrade. Under these conditions,most neat silicones and certain siliconeformulations will generate a resistivetrack when exposed to rooted arcingactivity on their surfaces. The trackforms from the carbon that is present(for PDMS, there are at least 2 carbonatoms for every silicon atom), so itfollows that some silicones are moreresistant than others based on theconstituents which make up the side-groups and crosslink sites.Track resistant additives, such asalumina trihydrate (ATH), can be blendedinto the formulation. However, as part ofthe optimization process, the additiveloading level must be carefully evaluated.The addition of ATH enhances the trackresistance, but adversely affectsprocessing and dielectric strength.

Additional filler may also lessenhydrophobicity. The literature indicatesthat work by others with siliconeelastomer products found the need toreformulate and to increase the filler levelto improve weathering characteristics,demonstrating that the originaloptimization to facilitate manufacturingprocessing was inadequate. [8,11] A saltfog chamber study of different cabletermination technologies indicated thatof the samples tested, the siliconerubber product with the intentionally lowfiller loading performed poorly, [12]which may help explain the need for thereformulation discussed in Ref. 11.

While the literature devotes a great dealof discussion to hydrophobicity and itsrecovery, little attention is paid to erosion

resistance. It has been suggested thataccelerated tests need to include “rest”periods which permit hydrophobicityrecovery and various unsupportedclaims are made to justify the position.However, contamination does not occuron a regular schedule, nor has any effortbeen made to provide an engineeringbasis for the introduction of real-time“rest” periods into an accelerated test.“Rest” periods clearly seek to exploithydrophobicity recovery properties ofsilicone, yet such test data may misleadusers whose service environment mayconsists of little or no contamination

Figure 3. Advancing contact angle of thesurfaces of various silicone formulationsas a function of time after coronatreatment. Contact angles of untreatedsurfaces ranged from 105 to 112degrees. Samples A, B, C, and D areexperimental formulations which employthe same silicone grade but differentfillers. Sample E is a commerciallyavailable HTV silicone formulation.

Figure 4. Leakage current results fromTaiwan Power Company (PenghuDistrict)-Comparison of ceramic andpolymeric insulators. Average Current (inmA) is monitored with time for theRayBowl Protected Creepage, HybridInsulator (triangle) versus a ceramic fogbowl type insulator (square). Variations inleakage current correspond to dailyenvironmental cycles (i.e. highest peakoccurs during early morning fogperiods). The leakage current isapproximately 6 to 8 times lower usingthe silicone shed compared with theceramic insulator control.

Contact Angle(Degrees)

120

100

80

60

40

20

00 0,5 1 2 3 4 6 8Recovery Time (Hours)

ABCDE

AverageCurrent(mA)

0,7

0,6

0,5

0,4

0,3

0,2

0,1

0242 258 9PM 290 306 9PM 338 354 9PM 386 402 9PM 434 450 9PM 482

Time (Hours)

Hybrid Fogbowl


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