—
KA!21CNALAlY31SCRYCOMHTTEEFORAERONAUTICS.
No.32
QAUSRSOF CRACKINGOF ZGlHTIf3NCABLE.
. . “ hJ. B. Dempsey,+F. B. Silsbee,
AeronauticPowerPlantsSection,
HXZEAUOF.STANDAM3S.
.—- .
.-
Febrmry,-2921.
.
4
. .
,
,
—
NATIONALADVISORYCONMITTEE.l!’O?lM3RCEJALTT1CS.
TECHNICALN~E NO.32,
CAUSESOFCRACKINGOF IGNITIONCABLE.
BY~. B. D&~pseyF; B. Silsbee,
AeronauticPowerPlantsSection,
HXREAUOF STANDARDS.
Resume.
NcYTE:-Theobjectof thisresumeisto presentinbriefandconvenientformthe sal iantfactsofthisreportforthebenefitM thosewhomay .izishtoutilizethe=sultsofthisinvestigation-tithoutstudyingthemoredetaileddescriptionofapparatusandmethods.
Theexperimentsdescribedinthisreportshonthatthe
crackingat sharpbends,observed-in theinsulationofhigh
tensionignitionitiresafterservice, isdueto a cheuical
attackupontherubberby theozonepro~cedby theelectric
dischargei/hichtakesplaceat thesurfaceof thecable.
Thiscrackingdoesnotoccurifthe insulatingmaterialis
not undertension,or ifthecableis surroundedby some
mediumotherthanair;butdoesoccurevenif theinsulation
isnotsubjeotedto electricstress~providsdtheatmosphere
nearthecablecontaintimone. Theextentof thiscracking
variesgreatlymiththeinsulatingmaterialused;andcan-
be materiallyreduoedby usingbraidedoableandby avoiding
sharpbendo.
-2-
CAUSESOFCRACKINGOF IGNI’TIONCABLE.
Hightensionignitioncables,usedto
sparkplugswiththedistributorblockson
bustionengines,occasicmallygivetrouble
connectthe
internalccm-
by crackingof
the insulation; whichisparticularlylikely
sharpbendsin thecable. Ithasbeenfound
turersof ignitionsystemsthata convenient
to occur at
by manufac-
testofthe
abilityof a cableto givesatisfactoryserviceis obtain-
edby wrappingthe”cablefirmlyapounda metalmandreland -
applyingvcltagefroma magnetoor sparkcoilbetweenthe
coreof thecableandtheman&e3..Inferiorqualitycable
willinthecourseof a fewhouxsshowcrackswhichbegin
at theoutersurfaceandrapidlybecomedeeperuntila
punctureof the insulationfinallyoccursthroughoneof
thesecracks.
TheBureauof StandardsW= requesteal‘~Ythe~;~au .
of AircraftProductionto locatemoredefinitelythecause
of thistrouble,andforthispurposean extensiveseries
of testswasmadeonthreesamplesof ignitioncable. Sam-
plesI andII wereverykindlysuppliedforthetest,by
theKeriteInsulatedWire-andCableCompaxqandmereinsu-
latedwiththeirKeritecompound.Sample111~asmibber
insulatedwitha mineralfillerandhadbeenfurnishedun-
derSignalCorpsSpecification28 003-AforHighTension
cable.Thepropertiesof thematerialsaregiveninthe
followingtable:
— — --
-3-
TM3LEI.
—
SampleNumber I 11 111—InsulationBraidsConductorsDiam.overDiem.overDielectricSet
Kerite Kerite RubberNone TWO TV7019# 26 12 # 26 19# 29
rub”oer 7.35mm. 6.29mm. 5.70mm,all 7.35mm. 7.64mm. 7.0?mm*constant. 3.7 3.6
::;4 4.6‘% 16 ‘%Ultimateelongation 320$ 310$ 360’$Eff.DielectricStrength 56 500v. 43 800v. 40 600v.
Themajozityof thetests=ze
whichthespecimenwassubjectedto
andelectricalstressfora certain
theendof eachofthesetests,the
enduranceruns,during
a ccmbinedmechanical
periodof time. At
samplesw-hichhadno%
puncturedwereimmersedin i-aterforapproximately24 hours
andthensubjectedto a breakdomtestusing60
voltage.Theresultsof thistestaregivenin
attached.
Inthistable,column11or 111,givesthe
cycleA.C.
TableII
lengthof
tiznethatthe
fromeither,a
firtiga 6 mm.
samplewassubjectedto theelectricalstress
magneto,runr.ingat about2000r~p.m.and
3-pointsparkgap;or from
givingthevoltageindicatedincoMmn TV
ColumnV givesthemediumsurroundingthe
umnVI thediameterof thearboron which
a transformer,
at 60 cycles.
cable, aadCOl-
it waswrapped.
ColumnVII statesby ~’yes~or ~’noflwhetherornotfailure
occurredduringtheendurancetest,andalso
ofsamplesto whichthisstatemntapplies.
stateswhetherornotcrackswerevisiblein
— --—
thenumber
ColumnVIII
the insulation
-4-..
at theendof theendurancetest. ColumnIXgivesthe
t 0%al
water
value
lengthof timethatthematerialhadbeensoakedin
previousto thebreakdomtest. ColumnX givesthe
of theeffective60-cyclebreakdom~olta~e&d is
theaverageof resultsobtainedupon
downsamplesindicatedin cblumnXI.
onlydataon’failuresWhichoccurred
thenumberofbreak-.
Thistablecontains
inthecenterportion
of thesamples.Alloaseswheretherewasanypossibility
of failurebeingproducedby unduemechanicalor electric-
al stressat theendsof thesampleor at thesurfaceofJthewaterhavebeenomitted.
Itappearsfromthisworkthatcrackingisproduced
in substantiallythesamemanner,but some-whatmorerap-
idly,whenthesaplesaretestedon 10000voltsalternat-
ingcurrent,thantientestedon a
of thecrackingisverymaterially
enceof thebraidandismuchless
Keriteinsulatorthanin thatwith
crackingisentirelyabsentifthe
magneto. Theamount
reducedby thepres-
in thematerialhaving
therubber.The
sampleis leftstraight,
or ifno electricalstressisapplied,or if surrounded
by waterorparaffin.Thecrackingisevidentlya pro- ‘
gressivephenomenon,requiringa combinationof several
factors,i.e., mechanicalstress,electrification,andthe
presenceof air.
Thetheoryofthisaction,whichsuccessfullyexplains
nearlyallof thephenomena,wasoriginallysuggestedby
-5-
Mr.Harrisof theKeriteCmpany as follows:Theelectri-
ficationproducesa brushdischargeat thssurfaceofthe
cable,whichin turnpzoducesozoneandoxidesofnitrc-
gen. Thisis shornby the
antetesthasbeenrunfor
surroundingthe wireswhen
prominenton theA.C.test
noticeableodoraftertheendur-
sometimeandthevislblsglow
inthedark. Thisglowismere
andFigs.1 md 2 arephoto-
graphsof thistakenwithabout15 minutes’exposureat s
13 000volts.Theelectricalstressin theairat the
surfaoeof theca-lle,as ccmputedfromthedimensionsof
thewireandthedielectricconstant,comesoutmuchin
excessofthebreakdomstrengthof aixandindicatedthat
coronaisto be expected.
It is wellknuwnthatozoneaqtivelyattacksrubber;
and whetitherubberisundermechanicalstresstheprod-
uctsofthereactionarepulledapart,thusexposingat
eachincipientcracka freshsurfaceof rubber,tiichis
in turnattacked.Thiseffectdoubtlessisresponsible
forthefactthattheactionislocalizedin definite
crackswhenthematerialisundertenqionbutdoesnot
penetratethematerialwhentherubberisunstressed.
Thatthepresenceof airisrequiredis
factthatno crackingoccurredinanyof the
ed underwateror whencoatedwithparaffin.
testsweremadeto demonstratetheeffectof
shownby the
samplestest-
A seriesof
theozone
by insertingsamplesofthecableinthebottomof-aglass
-6-
jarwhichwasprovidedat thetopwith
‘-”trodesfromwhicha highvoitagebrus’h
a pairof elec-
dischargecouZdbe
produced.The jarwassealedtightlyanda disc-ha.rge
fromthepointsmaintaineda copious
resultsobtainedwiththisapparatus
17 HOURRUN—- —’
KeriteTape: No tension;no
supplyof ozone.The
areas foliows:
effectnoticeable.
KeriteTape: 100$elongation;deepcrackson ex-posedside,particularlynearedges.Samplebrokeat grips.
ParaTape: No tension;softened,becamesticky,andcurledslightly.
ParaTape: 5C)fielongation;mappedon cable;crackedbadlyandfellapart.
OkoniteTape: No tension;no effect.
OkoniteTape: 50~elongation;wrappedon sable;orackedbadlyandfella~rt.
Threetypesof ignitioncablelaidstraightinbottomof jar;no cracking.
Onesampleof eachcablewrappedon 3/8t’diameterarbor.
Keritebraided:No cracksexceptat ends.
Keritebare: Badlycrackedantirubberentirely. strippedoff.
Rubbex: Deepandregularcracks.
Onesampleof eachcablewrappedonlf’diameterarbor:
Keritebraided:No cracksexceptat end.
Keritebare: Smallcrackson surface.Onelargecrackwhichallowedhalfof insulal-ationto peelaway.
Rubberbraided:Manyrathershallowcrocks.
-7-
138HCURRUN.
Onesampleof eachcablewrappedon 3/16~’diameter-arborandsubjectedto ozone:
Keritebraided:Crackeddeeplyanduniformlyexceptwhereparaifincoveredbraid.
Keritebare: Crackedandbrokein smallpieces,strippingfromcable~
Rubberbraided:Crackeddeeplyanduniformly.
Physicaltestsof samplesof cable‘whichhadbeen
restinglooselyintheozonetankfor138hourssho~d no
appreciablechangeinpermanentsetor in ultimateelonga-
tionas a resultof thetreatment.Thebreakdownstrength
(attheend of thisozonetreatment)isgivenin thefol-
Iowizzgtable:
TABLE111
Sample : BeforeOzoneTest ~After13&hrs.in (kone:Voltage Mo.Samples: Voltage No.Samples—
Keritebraided 43,SW ? 4’720G 3Keritebare 56 500 4 50100 3Rubberbraided 40 600 6 39 200 2.
Itwillbe notedthatin specimensNos.i-4-9-18-21
(Table11)failureoccurredduringtheendurancerum @hen
therewereno visiblecracksandwhentheconditionswere
suchthatno cracksweretobe expected.Themaximumvolt-
ageappliedto thespecimeninthesetestsprobablydidnot
exceed18 000volts,.whichisa relativelysmallfraction
of thebreakdownstrengthof theunaffectedcable,and
theseresultsimplya progressivedeteriorationof thema.t-
-8-
erialotherthanthatdueto cracking.Thesefailuresoc-
curredonlywhenbaresampleswerewrappedon thearbor,
ar.ddidnotoccurwitht-nebraidedmaterial.Specimens
Nos.67 and6S as comparedwithNo.5C forthemineralrub-
ber cableindicatet-hatsucha deteriorationtakesplace
withthisinsulation.A comparisonofspecimensNos.26
and27 withNo.25 showlittle,if any,effectofthis
natureintheKerite.
A comparisonof specimens
andNo.13 andNo.14 withNo.,
Nos.65 and66 with ifo.5G,
25 seemto indicatea deter-
iorationinbothclassesofmaterialnhensubjectedto
prolongedmechanicalstress.Thetimerequiredto produce
theseeffectsislongcomparedwiththatin whichcracking
developsinair,andtheyarethereforeoflessimportance
It ms at firstthoughtthatthe simultaneousappli-
cationofmechanictiandelectricalstressmight‘R the
causeof thefailuzes;butthisisnotthecase,as shown
by TableIV,which‘wasobtainedon Keriteinsulationin ‘
theformof strips,andwhichshowsno changegreateno~mh
to accountforthefailureofthespeoimen
voltageappliedcluingtheendurancetest.
at thelow
TAEUJEIV.
WithoutTension WithTension+Elongation o 100$Thickness- .080 .G56volts 33 400 26 70’3Voltsperroil. 420 480No.of samples 12 9
A possibiezemedyforthistroublewcuidbe to cover
thecablewitha closelyadheringcoatirgof conducting‘
materialsuchas conductingpaintor watersoakedbraid.
Thiswillp~eventthetendencyto fon coron,a,as i.sshown
in thephotographs.It is,however,o-ojectiona31ecnac-
countofthe increaseintheelectrostaticcapacity‘ofthe
leadswhichresuitsina reductionof themaximmz
producedby theignitionsystem,andalsobecause
formsofconductingpaintareliableto crackand
at thecracksan excessiveamountof corona.The
voltage
most
produce
useof ‘
an insulatorhavingas
dielectricconstantis
in theairoutsidethe
small& valueaspossibleforthe
advisakleas thevoltagegrqdient
cableistherebyreduced.Theze
seemsto W. considerabledifferenceintherapiditywith .
which’differentinsulationsareattackedchemicallyby
the@one- Theordinaryinpregaatedbraids helpverymat-
erially’inpreventingcracking,gmobablybecausetheyare
nearlyairtightandpreventtheozonefromreachimgthe
rubber.titermuchbending,however,theymillprobably
lose theirefficacy.Supportingthecableson insulating
materialsat a distancefromgro’mdedconductorsisbene–
ficialinreducingthecoronaandalsoreducingtheelec-
trostaticcapacity.It is,howsver,liableto causein–
duciiiveeffectsbetweenthedifferentcables,whichmay
leadto theproductionof sparksin thewrongenginecyl-
inder,unlessthecablesareplacedapartfromoneanother,
-10- -
as wellas fromtheground.Theiupoztanceof carefully
avoidingsharpbendsin thewiringisveryobvious.
It is to be concludedfromthisinvestigation,that
thefailureis causedby a chemicalattackon theriabber
by theproductsof the coronadischarge;butis locaiizea
in cracksonlywherstheinsulationis subjecttomechan-
icaltension.Failurea“stheresultof othercauses,suchas thecombinedapplicationofmechanicalandelectrical
stress,or theprolongedapplication05 electricalstress
alone,are showntobe absentor of minorimportance.
.
J_
Spec-imenNo.
1234
:
:9
101112131’4151617181920212223~~25262?28
-11”
TABLE11.
SUMMARYOFTESTOFHIGHTENSIONCABL~.
KE RI TE BARE
~ 111 ~r
ENDURAIWXZTESTOnWgnetoTimeRun
Hours ~.
22310.07.2.
None
None
1;0.16.None
0.030.0.025.0.0”0.0None
None
30:45.30.None
OnTransformerTimeRun S~Trounding
Hours ~. Voltage Material
—
None
0%o.1.5.None
o“o01.033.,33.25.
None156.326.326.
Watarn
— StraighttubeWaterAir ‘n
None u13,000 11!1 11n n
11,090 Fatern nNone Air158hrs.II n n n
13,000 Airn nN nn IIn
●n
n nn “n
Helix,parafiinHelix,air!1 n
None -11,000 “ W&tern nn Air
-la-
TABLE11(Ccmt‘d)
I
Spec-imenNo.
123~5G?89
1011121314151617181920212223ah25262’72&
_jXJMM.ARYOFT3ST0??HIGH‘TENSIONCABLES
yJ
Diameterof
ArbozOneinchn !1InfiniteOneinchR n!1 m!! nn tl“...n- n>~n IIn n“n n113132;3/8 n5/8 D13/16II@e “
n
1 7/8;7/8;n n
Infinite
Infinitenn
KERITEBA
v~FailurewithNo.ofSamDles
Yes,1Ko, 3NC, 3Yes,1Yes,1No, 3
No, 2Yes,1No, 1No, 1No, 3
Yes,2Ye~,2Yes,2Yes,1No. 1No, 1Y9S,1No, 1Yes,1No. 2
No, 3Ho, 3No, 3
VIII’
Cracks
No:tnnYesNo
Non
YesNonnnYesnnHonnRn’YesNo
Honn
13E
T& _x_ ~Hours Break-sctak- down N3. ofingin . volt- Samples\?atex
2.09240
24Nonen
18181’711718Ie
● -
&
4;24156 “326 ,3C
.
42,4004-5,10042,900
42,30050,300
43,;0049,0g(35El,80049,0-J()~~>Too37,CK)o
47;403
“45,50056,50048,We52,00052,400
J_
Speci-nen~To.
30313233
:;3637383940414243
505152535455565?5859606163636465
%6869
-L3-
TABLE11(Cent’d)
SWM.@YOFTEST03’~IGHTENSIONCABLES
KE RITE COVERED
11 111—ENJILR4NCETEST.—
on Magneto OnTransformerTimeRun T&e R~
Hours Min. Hours llin.
None None lime Noneo
3: 931 019 0 ._7 0
i- 5Q-. a 30
16 00
3:: 0156 0326 0326 G
MINERAL RUBBER
None None Hone None2 03031 :19 0
(~~=36(---55
[
4530
(== =1:16 c90 0
5
156 -=326 -326
gT_
VoltWe
None-
lzj,90c)nn ,000n r
n
n
n
n
None-
-.
15,Ocon11nnH
11,003n
v
SurroundingUaterial._
--
Tatern
StraighttubeRaterAirnn
WaternAirT%ter11Air
Yatern
StraighttubeWaterAirnn11n11n
Rater!1
n Air,no braid ._
21,000 Watern 11n Air
-14-
Speci-menNo.
3031323334353637383940414243
5051525354555657
.%6061626364
TABIJI.11(concluded)
SUMMARYOFTE~ OFHIGHTENSIONCABLES.
&E RITE COVERED. ..
Diameterof
Arbor
&e -%e<11 nInfiniteOneinchn n?l nn Itn n11 nnInfin;te
nn
Failure Hours Ereak-with soak- down No.ofNo.of ingin Volt- SamplesSamplesCracks Water age shot
xc)>3No, 3No, 1No, 3No, 3No, 3No, 3No, 3IJO,3No, 9No, 3No, 3No, 3
Non1?%!1nN
nnR
nnn
~INERAL RUBBER
Oneimhn n
InfiniteOneinch11 11n nn Ifn nn n1! nn nn w11 nn n
No, 3NC),3No, 1No, 3No, 3Yes.2Yes;~Yes,2No, 1Yes,1Yes,1No, 3No> 3Yes,1 -
NonItn
YesnnnnIIn
Non
Yes65 3/32’’,158hrs. ~66 ~ ‘,158hrs. -67 Infinite No, 3 No68 n No, 3 n69 U No, 2 n
24292401924
43,80039,60040,80042,90(344,30036,80043,10040,80047,8~o45,50046,00041,10043,00045,100
40,60C35,60032,90037,90036,20023,600
29,;00)
34,500)1
32,500-37,000
28,00037,20033-,80025,50043,700
Fm. s.d-orom onigdionmbl,.son I-inch arhor. Ex@welo corolmolily -wminn&sal13 .wovdUSA. C.UDpcr mandrel 1-14 to rlgbt, Kmb.e b% mtmial rabbtr, km-he cuvercd, AULbrte SPmimms
low= cndmWbcrc Calma npDcal’t.cwcmd with mr.dw+lns mhn, exe@ ●t
Iawer nundml ham left 10 rii’hh H@ wm, kerhe cvvwed, mineral rnbbrf. Spcctmem tmtremted.
FIG. 2 .—Corona on ignilion mblcs on I-iwh arbor. Evpo$uw to corona only IS minuk al 13 am wok A. C. and 10arhjicial light 5 samn.is 10$hom kmalwn ~ dirchqw,
u- mdml km leftWrighe Ko’ltc bare, mlnermlmblx.r, kcrie nmcmmd.I.nwrcrmandrel[mmM torichti Kai@ bmc, kcrltc mend, mlmral mljlxr.