The experiments described here show that the cracking at sharp bends, observed in the insulation of internal combustion engine high tension ignition wires after service, is due to a chemical attack upon the rubber by the ozone produced by the electric discharge that takes place at the surface of the cable. This cracking does not occur if the insulating material is not under tension, or if the cable is surrounded by some medium other than air. But it does occur even if the insulation is not subjected to electric stress, provided that the atmosphere near the cable contains ozone. The extent of this cracking varies greatly with the insulating material used. The cracking can be materially reduced by using braided cable and by avoiding sharp bends

Silsbee, F B

English

NASA Technical Reports Server

• 
• 
TECHHICAL KOTES 
NA'::'IGNAL ADVISORY COMi~iITTEE FOR AERONAUTIOS . 
, 
• No . 32 
• 
CAusrs OF CRACI:ING OF IGlHTION CAB:UE . 
By 
F . B . Silsbee, 
Aerc:nau.t ic PO~';er Plant s Section , 
BUREAU OF STANDA.::-:D8. 
' . I 
' .. 
1 i 
https://ntrs.nasa.gov/search.jsp?R=19930080826 2020-03-17T05:02:03+00:00Z
• 
• 
• 
• 
• 
• 
• 
• 
NATIONAL ADVISOR! COHkITTEE F'OR .AER0iJAlif!CS. 
TEcmnCAL NOTE NO. 32. 
CAUSES OF OR/.OKING OF lGITITION __ QAJ3LE. 
By 
F-. B. Silsbee, 
Aeronautic .t'o·,ver Plants Section, 
BUREAU OF STANDARDS . 
Resume. 
NOTE:- 7he object of this resume is to present ifr brief 
and convenient forl:. the salient facts of this report for 
the benefit of those ',/ho may ',-;ish to utilize the results 
of this jLvestiga-i:iion without studying the more detailed 
description of apparatus and methods. 
The experiments described i~ this report sho~ that the 
cracking at sharp bends, observed in -Ghe insulat ion of high 
tension ignition .vires after service, is due to a cher::ical 
attack upon the rubber by the ozone produced by the electric 
discharge ":;hich takes place at the surface of the cable . 
This cracking does not occur if the insulating material is 
not under tension, or if the cable is surrounded by some 
medimn other than air; but does occur even if the insul~tion 
is not subjected to electric stress, provided the atmosphere 
near the cable containc ozone . The extent of this cracking 
varies greatly with the insulating material used; and can -
be materially reduced by using braided cable and by avoiding 
sharp benda . 
• 
• 
• 
- 2 -
CAUSES OF CRl\.CKING OF IGNITION CABLE. 
High tension ignition cables, used to connect the 
spark plugs with the distrioutor blocks on internal com-
bustion engines, occasionally give trouble by cracking of 
the insulation; which is particula~ly likely to occur at 
sharp bends in the cable. It has been found by manufac-
turers of ignition systems that a convenient test of the 
ability of a cable to give satisfactory service is obtain-
ed by '.VTapping the cable firmly around a metal mandrel and 
applying VOltage f_offi a magneto or spark coil between the 
core of the c~ Ie and the ~andrel . Inferior quality cable 
will in the course of a few hours show cracks which begin 
at the outer surface and ra?id~y become deeper ~~til a 
puncture of the insulation finally occurs through one of 
these cracks . 
The Bureau of Standards I~S request3d oy the Bureau 
of Aircraft Production to locate more defi~itely the cause 
of this trouble, and for this purpose an extensive series 
of tests was made on three samples of ignition cable. Sam-
ples I and II were very kindly supplied for the test, by 
the Kerite Insulated Wire and Caole Company and ITere insu-
lated with their Kerite compaQlld. Sample III \~s rJboer 
insulated with a mineral filler and had been furnished un-
der Signal Corps Specification 28 003-A for High Tension 
cable . The properties of tile materials are given in the 
following table: 
• 
• 
• 
- 3 -
TAB:E 1. 
Sample Numoe r 
Insulation 
Braids 
Cond-Jctors 
Diam. over rubber 
Diam. over all 
Dielectric constant. 
Set 
Ultimate elongation 
Eff . Dielectric Strength 
o 
.1. 
Kerite 
None 
19 #= 26 
7.35 mm. 
7.35 mm. 
3.4 
5.7 % 
3<20 % 
56 500 v. 
I: 
Kerite 
T:.vo 
12 # 26 
6.29 mm. 
7.64 mm. 
3 . 7 
4 . 6 % 
310 % 
43 800 v . 
III 
Rubber 
Two 
19 #= 29 
5.70 mm. 
7.07 mrn. 
3.6 
16 % 
360 7~ 
40 600 v. 
The maj ority of the test s i.-iere endurance !Uns, during 
whi ch the specimen was subjected to a combined mechanical 
and electrical stress for a certain period of time. At 
the end of each of these tests, the samples whic~ had not 
punctured were irr~ersed in vater for approximately 24 hours 
and then subjected to a breakdolID test using 60 cycle A.C. 
vOltage. The results of this test are given in Table II 
attached. 
In this table, column II or III, gives the length of 
time that the sample was subjected to the electrical stress 
from either a magneto, running at about 2000 r~p.m. and 
firing a 6 mm. 3-point spark gap; or from a transformer, 
giving the voltage indicated in column IV at 60 cycles. 
Column V gives the medium surrounding the cable, and col-
umn VI the diameter of the arbor on which it was wrapped. 
Column VII states by "yes ll or Hnoll "lVnether or not failure 
occurred during the endur~nce test, and also the number 
of samples to which this statement applies. Column VIII 
states vmether or not cracks were visible i~ the insulation 
• 
- 4 -
at the end of the endurance test. Column IX gives the 
total length of time that the material had been soaked in 
water previous to the breakdown test. Column X gives the 
value of the effective 60-cycle breakdo"n voltage and is 
the average of results obtained upon the number of break-
down samples indicated in column XI. This table contains 
only data on failures which occurred in the center portion 
of the samples. All cases where there was any possibility 
) 
of failure being produced by undue mechanical or electric-
al stress at the ends of the sample or at the surface of 
J 
the v~ter have been omitted. 
It appears from this work that cracking is produced 
in substantially the same manner, but so~e what more rap-
idly; when the samples are testeu on 10000 volts alternat-
ing current, than when tested on a magneto. The amount 
bf the cracking is very materially reduced by the pres-
ence of the braid and is much less in the material having 
Kerite insulator than in that with the rubber. The 
cracking is entirely absent if the sample is left straight, 
or if no electrical stress is applied, or if surrounded 
by water or paraffin. The cracking is evidently a pro-
gressive phenomenon, requiring a combination of several 
factors, i.e . , mechanical stress, eleotrification, and the 
presence of air. 
The the ory of this action, which successfully eXplains 
nearly all of the phenomena, ~as originally suggested by 
- 5 -
Mr. Harris of the Kerite Company as follows: The electri-
fication produces a brush discharge at the surface of the 
cable, whicn. in turn produces ozone and. oxid.es of nitrc-
gen . This is sho~n by the noticeable odor after the endur-
ance test has been run for some time and the vis~ble glow 
surrounding the vYires '[men in the dark. This glow is more 
prominent on the A. C. test and Figs . 1 and 2 are photo-
graphs of this taken with about 15 minutes' exposure at 
13 000 volts. The electrical stress in the air at the 
surface of the cable, as computed from the d.imensions of 
the wire and the dielectric constant, comes out much in 
excess of the breakdovm strength of air and indicated that 
corona is to be expected . 
It is well known that ozone actively attacks rubber; 
and when the rubber is under mechanical stress the prod-
ucts ' of the reaction are pulled apart , thus exposing at 
each incipient crack a fresh surface of rubber, Which is 
in turn attacked . This effect doubtless is responsible 
tor the fact that the action is localized in definite 
cracks when the material is under tension but does not 
penetrate the material when the rubber is unstressed. 
That the presence of air is required. is shown by the 
fact that no cracking occurred in any of the samples test-
ed under water or when coated with paraffin. A series of 
tests were made to demonstrate the effect of the ozone 
by inserting samples of the cable in the bottom of a glass 
jar which 'lIas provided at -Ghe top with a pair of elec-
-- --trodes from -,vhich a high vOltage brush discharge could be 
produced. The jar was sealed tightly and a discharge 
from the points maintained a copious supply of ozone. The 
results obtained with this apparatus are as follows: 
Kerite Tape: 
Kerite Tape: 
Para Tape: 
Para Tape: 
Okonite Tape: 
Okonite Tape: 
No tension; no effect notioeable. 
100% elongation; deep cracks on ex-
posed side, particUlarly near edges. 
Sample broke at grips . 
No tension; softened, became stickY3 
and curled slightly. 
50% elongation; ~ apped on cable; 
cracked badly and fell apart . 
No tension; no effect. 
50% elongation; wrapped on cable; 
cracked badly and fell apart. 
Three types of ignition cable laid straight in bottom of jar; 
no cracking. 
One sample of each cable wrapped on 3/8 11 diameter arbor. 
Kerite braided: No cracks except at ends. 
Kerite bare: Badly cracked and rubber entire l y 
stripped off. 
Rubber: Deep and regular cracks. 
One sample of each cable wrapped on 111 diame ter arbor: 
Kerite b r aided: No cracks except at end . 
Kerite bare: Small cracks on surface. One large 
crack which al1o~ed half of insulal-
ation to peel away. 
Rubber braided: Many rather shallow' cracks. 
- 7 -
138 HOUR RUN. 
One sample of each cable wrapped on 3/16" diameter 
arbor a.nd subjected to ozone: 
Kel'ite braided: Cracked deeply and uniformly except 
i1here paraffin covered braid. 
Kerite bare: CrackeQ and broke in small pieces, 
stripping from cable. 
Rubber braided: Cracked deeply anQ u~iformly . 
Physical tests of samples of cable which had been 
resting loosely in the ozone tank for 138 hours shov~d no 
appreciable change in permrulent set or in ultimate elonga-
t:on as a result of the treatment. The breakdo~m strength 
(at the end of this ozone treatment) is given in the fol-
lowing table: 
Sample 
Keri te braid.ed 
Kerite bare 
Rub"oer braided 
TABLE III 
Before Ozone Test 
: Vol t .s.ge J"o . Sampl e s 
43 800 
56 500 
40 600 
7 
4 
6 
After 13B hr s . in Ozone 
Vol tage 
47 200 
50 100 
39 300 
No . Samples 
3 
3 
3 
It will be noted that in specimens Nos. 1-4-9-18-21 
(Table II) failure occurred during the endurance runs when 
there were no visib~e cracks and when the conditions vre re 
such that no cracks i:~ere to be expected . The maximu.'1l vol t-
age applied to the specimen in these tests probably did not 
exceed 18 000 volts, which is a relatively small fraction 
of the breakdown strength of the unaffected cable, and 
these results imply a progressive deterioration of the mat-
r 
- 8 -
erial other than that due to cracking . These failures oc-
curred only when bare sacnples were wrapped on the arbor, 
and did not occur vnth tne craid.eci material. Specimens 
Nos. 67 and. 68 as compared VJ i t:1 l\~o . 50 for the mineral ru-;)-
ber cable ind.icate that such a deteTiorat~on takes place 
with this insulation. A comparison of s~:>e cimens Hos. 26 
and 27 with No. 25 show little) if any, effect of this 
nature in the Kerite. 
A comparison of specimens Nos. 65 and 66 with No. 50) 
and Ho . 13 and No. 14 wit~ No . 25 seem to indicate a deter-
ioration in both classes of material rmen subjected to 
prolonged mechanical stress. The time required to ?roduce 
these effects is long compared with that in wh ich cracking 
develops in air, and they are therefore of less importance. 
It was at first thought that the simultaneous appli-
cation of mechanical and electrical stress might be the 
cause of the failures; but this is not t~e case, as shown 
by Table IV, which was obtained on lerite insulation in 
the form of strips, and which shows no change great enough 
to account for the failure of the specimen at the lo w 
voltage applied during the endurance test. 
Elongation 
Thickness 
Volts 
Volts per mil. 
No. of samples 
TABLE IV. 
Without Tension 
o 
.080 
33 400 
420 
12 
With Tens ion 
100 % 
.056 
26 700 
480 
9 
- ~ - - -~ --'-- ~---...--.~--
- 9 ,... 
A po ss ible ::.~emedy for trlis t~o1..:ble \i,'()uld be to co'rer 
the cable with a close:y adherinb coatir.g of COllducting 
uaterial such as conducting pain~ or water soakeQ braid. 
This will prevent the tend$ncy to fo~m corona, as is shown 
in the photographs . It is , howeve~ , oo:e c: iona~le o~ a c-
.Jount of the increase in the electrostatic capac:.i.ty 'of the 
l eads wfiicD r esults i n a reductio~ cf the maximuu voltage 
produced by the ignition syst em , ar-d also because most 
forms of conducting paint ~re liable to crack a~d produce 
a Jc the cl'acks an exce ss i ve amount of corona . The use of 
an insulator having as small a value as poasible for the 
dielectr ic c onstant is advisable as the voltage gradient 
i n the air outs i de t he cable is t he reby reduced . There 
seems to be considerable differe~ce in the rapidity wit h 
·Nhich different ill sul at ions are at.tacked chen:ically b~T 
t he ozone. The ordinar y impregnated braids he l p very mat -
erially i!l preventing cracking, probably oecause they a re 
nearly ~ir tight and prevent the ozone f r om reaching the 
rubber . After muce bending , howeve r, they ;iill :;:>roba".Jly 
lose their efficacy . Support ing the cables on insulating 
materials at a distance from grmlnded conductors is bene -
ficial in reducing tie corona and also reducing the ele c-
trostatic capacity . It is , ho v~ver, liable to cause in-
ductive effects betv~en the Qifferent cable s, which may 
lead to the production of s~arks in the wrong engine cyl -
inder,- unless the cables are placeQ ~part f~om one another, 
- 10 -
as well as from the ground. The iffiportance of carefully 
avoiding sharp cends in the i>7iring is very obvious . 
It is to be concluded fro!"{1 this investigat~on, ~hat 
the failure is caused by a chemical attack on the rubber 
by the products of the corona discharge; 'but is localiz.ed 
in. c:::-acks only ; ,~ere the insulation is S'.l0ject to mechan-
ical tension. Failure a 's the result of other causes, such 
as the combined application of mechanical and electrical 
stress, or the prolonged application of electrical stress 
alone, are shOwn to be absent or of minor importance . 
- 11 -
TABLE II. 
SUMMARY OF TEST. OF HIGH TENSION CABLES. 
K E R I T E B A R E 
I II III IV V 
ENDURANCE TEST 
Spec - On _Magneto On TransfoTr.:l8 r 
imen T irr.e Run Time Run Surrounding 
No . Hours Min . Hours Min . Vol tage Material 
1 2 0 . 0 Wata.r 
,.., 2 30. II .:::; 
3 31 0 . 0 Straight tube 
4 0 . 0 25 . Water 
5 7. 0 . 0 Air 
6 2. 0 . 0 II 
7 None None J\one None None " 8 43 13 , CiOO " 9 0 . 0 50 " tI 10 O. 30 It 11 
11 l. 0 11,0')0 Water 
12 5. 0 II tI 
13 l~one None None ~Tone None Air 158 hrs . 
14 11 11 II tI 11 II 
1 5 0 8 13,000 Air 
16 0 9 11 11 
1 7 0 19 11 If 
18 1.0 0 II II 
19 33 . 0 
" 
11 
20 33 . G 11 II 
21 25 . 
° 
II II 
33 15 30. Helix ,l:araffin 
33 O. 45 . Helix, air 
24: 16 . 30. II II 
25 None None None None None 
35 156 . 11,OJO Water 
37 326 . 
" 
11 
28 326 . 11 Air 
I 
Spec-
imen 
No. 
1 
2 
3 
4 
5 
S 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
?oli '-J _ 
25 
26 
27 
28 
12 -
TABLE II (Cont1d) 
.§UMMARY OF T~ST OF HIGH TENSION CABLES 
K E R I T E BAR E 
VI VII VIII IX X 
Failure Hours Break-
Diamet er with Soak- down 
of NO.of ing ir.. Vc1t-
Arbor Samples Cracks Water age 
----
One inch Yes~ 1 No 2 . 0 42,400 
II 11 No, 3 !I 92 45~100 
Infi!lite No, 3 !I 40 42,900 
One inch Yes, 1 II 
II II Yes, 1 Yes 24 
!: II No, 3 No None 42,300 
II Il II 50,300 
II n No, 2 No 
' ,II n Yes, 1 II 18 43 ,100 
- - > '-
II II No , 1 Yes 18 49,000 
II II No , 1 No 17 58,800 
II II No, 3 II 117 49,OJO 
II II II 18 47,700 
3/32 II II 18 37,JOO 
3/8 Yes, 2 Yes 
5/8 II Ye 3, 2 !l 
13/16 II Yes: 2 II 
One II Yes, 1 No 
n II No . 1 fI 
1 7/8 II No, 1 II 
fI n Yes, 1 II 
7/8 n No, 1 II 40 47,400 
I! II Ye;:; , 1 Yes 
Infin:ite No . 2 Ii ° 40 45 , 500 
24 56 , 500 
Infinite No, 3 No 156 48,900 
II No, 3 II 326 52,000 
II No, 3 II 30 52,400 
XI 
No. of 
Samples 
Shot 
3 
3 
3 
3 
3 
2 
1 
1 
3 
3 
2 
1 
1 
4 
3 
3 
2 
I 
Speci-
Den 
rTo . 
30 
31 
32 
33 
34 
35 
36 
37 
38 
39 
40 
41 
42 
43 
50 
51 
52 
53 
54 
55 
56 
57 
58 
59 
60 
61 
62 
63 
64 
65 
66 
67 
68 
69 
( 
( 
I 
\ ( 
( 
- :L3 --
SUMMA~Y OF JEST _O? HIGH TENSION CABLES 
KERITE COVE::tED 
II III 
ENDURANCE TE ST 
----' 
On Magneto 
Time Run 
Hours Min. 
None 
.8 
38 
31 
19 
'7 
11 
}Jone 
.8 
30 
31 
19 
7 
-
.i 
None 
C) 
'J 
o 
o 
o 
N ERA 
None 
0 
0 
0 
J 
0 
On Transformer 
Ti::.:e Run 
Hours Min. 
L 
None 
0 
J 
16 
90 
366 
156 
336 
326 
R U 
... Jone 
16 
90 
156 
326 
386 
B 
None 
50 
30 
0 
0 
0 
0 
0 
0 
B E 
none 
30 
55 
45 
30 
4: 
18 
C 
o 
5 
R 
IV 
Voltage 
None 
l~.)OO 
II 
11,000 
n 
n 
II 
11 
11 
None 
13 , OCO 
II 
II 
fI 
II 
II 
11,000 
n 
II 
11,000 
rr 
n 
v 
Surrounding 
,1aterial 
Water 
II 
Straight tube 
Water 
Air 
II 
II 
Water 
II 
Air 
Water 
11 
Air 
"tJat e r 
" Straight tube 
Water 
Air, 
Air 
n 
11 
11 
II 
!1 
II 
Water 
II 
no braid 
Water 
" Air 
- 14 -
'!'ABLE . I I (Concluded) 
8mJmARY OF TEST OF HIGH TENSlm~ CABLES. 
KERITE C 0 V E. RED. 
Failt!.re Hours Break-
Speci- Diameter with Soak- down No. of 
men of No. of ing in Volt- Samples 
No. Arbor Samples Cracks Water age Shot 
.. 30 24 43,800 7 
31 One inch No, 3 No 2 39,600 3 
32 II 11 No , 3 Jl 92 40,800 3 
33 Infinite No, 1 n 40 427900 2 
34 One inch No, 3 f1 19 44,300 3 
35 " 11 No, 3 II 24 36,800 2 36 11 " No, 3 II 18 43,100 2 37 11 11 No, 3 n 18 40,800 3 
38 II II No, 3 II 17 47,800 3 
39 II II .No, 3 II 117 45,500 3 
40 11 II No, 9 II 30 46,000 9 
41 Infinite No, 3 11 156 41,100 3 
42 11 No, 3 " 326 43,000 3 43 
" 
No, 3 " 30 45,100 2 
M I NE R A L RU B B E R 
50 24 40,600 6 
51 One inch No, 3 No 2 35,600 3 
52 II 11 No, 3 II 92 32,900 3 
53 Infinite No, 1 II 40 37.1 900 3 
54 One inch No, 3 11 19 36,200 3 
55 
" " 
No, 3 Yes 24 23,600 3 
56 II II Yes, 2 11 
57 II II Yes, J. II 
58 II 1\ Yes, "- n 18 29,600 ;3 <:) 
59 II 
" 
No, 1 11 18 ) ) 
60 
" 
II Yes, 1 If 18 \ 34,500) 3 J 
61 11 II Yes, 1 II 18 ) ) 
62 II \I No, 3 No 17 32,500 3 
63 If If liTo, 3 II 117 37,000 3 
64 II 11 Yes, 1 Yes 
65 3/32 11 ,158 hrs. 18 28,000 2 
66 1 ",158 hrs. 18 37,200 3 
67 Infinite No" 3 No 156 3J. ,800 .... 
" 68 II No, 3 
" 
326 25,500 3 
69 II No, 2 
" 
30 43,700 3 
,,- I" 
FIG. 2.-Corona on ignition cables on I-inch arbor. Expos ure to corona only I5 minutes at 13200 va lls A. C. and to 
artificial light 5 seconds to sho w location of discharge. 
Upper nlandrel fro111 left to right : Kerite bare, mineral rubber, kerile uncovered. 
Lower n13ndrel from left to right : Kerite bare, kerite covered, mineral rubber . 
:. 
· c 
F IG. I. - Corona on ignition cables on I - in ch arbor. Expos1tre to corona only 20 minnles at 13 2 00 valls A. C. 
U p per mandrel from left to right: K erite bare, m in eral ru bber, kerite covered. All th ree specimens covered with cond uctin g pain t, except at 
lower ends where corona appears. 
Lower mandrel fr0111 left to righl : K eri lc ba re, kcri le covered, mi nera l Tu bber. Specinlc .ls un treated. 


Causes of Cracking of Ignition Cable

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