senso s
Re iew
A c T acking Con ol in Insula ion Sys ems o
Ae onau ic Applica ions: Challenges, Oppo uni ies,
and Resea ch Needs
Jo di-Roge Riba 1,* , Ál a o Gómez-Pau 2, Manuel Mo eno-Eguilaz 2and San iago Boga a 1
1Elec ical Enginee ing Depa men , Uni e si a Poli ècnica de Ca alunya, 08222 Te assa, Spain;
[email p o ec ed]
2Elec onics Enginee ing Depa men , Uni e si a Poli ècnica de Ca alunya, 08222 Te assa, Spain;
[email p o ec ed] (Á.G.-P.); [email p o ec ed] (M.M.-E.)
*Co espondence: [email p o ec ed]; Tel.: +34-937-398-365
Recei ed: 14 Feb ua y 2020; Accep ed: 13 Ma ch 2020; Published: 16 Ma ch 2020


Abs ac :
Nex gene a ion ai c a s will use mo e elec ical powe o educe weigh , uel consump ion,
sys em complexi y and g eenhouse gas emissions. Howe e , new ailu e modes and challenges a ise
ela ed o he equi ed ol age inc ease and consequen ise o elec ical s ess on wi ing insula ion
ma e ials, hus inc easing he isk o elec ical a c appea ance. This wo k pe o ms a c i ical and
comp ehensi e e iew conce ning a c acking e ec s in wi ing insula ion sys ems, unde lying
mechanisms, ole o ma e ials and possible mi iga ion s a egies, wi h a special ocus on ai c a
applica ions. To his end an e alua ion o he scien i ic and echnological s a e o he a is ca ied
ou om he analysis o heses, esea ch a icles, echnical epo s, in e na ional s anda ds and whi e
pape s. This e iew pape also epo s he limi a ions o exis ing insula ion ma e ials, s anda d es
me hods and mi iga ion app oaches, while iden i ying he esea ch needs o comply wi h he u u e
demands o he ai c a indus y.
Keywo ds:
A c acking; pa ial discha ges; co ona e ec ; insula o ma e ials; polyme s; powe sys em
p o ec ions; ae onau ic wi es
1. In oduc ion
The nex gene a ion o mo e elec ic ai c a s (MEAs) is being de eloped wi h he inal goal o
e ol ing owa ds all-elec ic ai c a s (AEAs), since hey a e en i onmen ally iendlie [
1
]. To his end,
i is equi ed o op imize he pe o mance, eliabili y and powe densi y, by lowe ing he weigh [
2
],
educing Ope a ions and Main enance (O&M) cos s, educing sys em complexi y, uel consump ion and
g eenhouse gas emissions o he new ai c a s. MEAs and AEAs will use mo e elec ical powe han hei
p edecesso s, since hey eplace pneuma ic and hyd aulic sys ems by elec ical ones [
3
] o educe he
weigh . The e o e, hey a e equi ed o ope a e a highe ol age le els [
4
], abo e 1 kV [
5
], highe dV/d ,
highe powe densi ies and dec eased wi e dis ances. These modi ica ions lead o new ypes o
ailu e modes, including a highe isk o elec ic a c appea ance, which can be po en ially ha m ul [
6
].
A o emos conce n ela ed o elec ic a c o ma ion in ae onau ic en i onmen s is he p esence o
o eign objec deb is, since a conduc o objec can induce an elec ic a c when sho ing ou wo wi es [
6
].
Elec ical sys ems well-designed o g ound ope a ions can gene a e pa ial discha ges (PDs) when
ope a ing a lowe p essu es; his e ec is equen ly in ensi ied due o mois u e condensa ion caused
by p essu e changes du ing apid ascen s and descen s [
1
]. The e o e, ae onau ic and ae ospace
insula ion sys ems a e mo e suscep ible o PDs because o he low p essu e ope a ing en i onmen [
7
],
wi h incep ion ol ages lowe han hose ound a sea le el [
2
], especially when ope a ing a highe
ol age le els [8,9].
Senso s 2020,20, 1654; doi:10.3390/s20061654 www.mdpi.com/jou nal/senso s
Senso s 2020,20, 1654 2 o 16
Since elec ic and elec onic de ices a e being inc easingly applied in ae onau ics and ae ospace
sec o s, polyme insula ion ma e ials a e inexo ably exposed o a wide ange o en i onmen s. Howe e ,
such demanding applica ions equi e ma e ials and de ices wi h e y eliable ope a ion unde a wide
ange o en i onmen al and ope a ing condi ions [10].
PDs a e elec ical discha ges ha do no comple ely b idge he insula ion be ween wo
elec odes [
11
]. They a e igni ed by h ee aul ypes, i.e., in e nal, ex e nal and co ona. Pe sis en PDs
can p oduce se e e insula ion damage due o he chemical ans o ma ions p oduced by he low o
elec ons in he de ec . Such ans o ma ions can p oduce a conduc i e pa h, which allows he low
o an elec ic cu en and a ise o he empe a u e, hus damaging he insula ion and p omo ing he
o ma ion o an a c o comple e b eakdown due o he weakening o he insula ion [
12
]. A cing can be
gene a ed be ween elec ical wi es sepa a ed by a solid insula ing medium when he la e one becomes
ca bonized. When he ca boniza ion is gene a ed because o an elec ic cu en , he a cing p ocess is
known as a c acking.
Elec ical acking is o igina ed by su ace PD ac i i y on an o ganic ma e ial, which deg ades he
polyme , hus mo ing om he insula o o conduc o s a es due o powe ul he mal shocks p oduced
by an in ense bomba dmen o elec ons [
13
], which con ibu e o b eak he polyme ic chains and,
ul ima ely, o gene a e ca bonized conduc ing pa hs. New discha ges ex end exis ing acking pa hs,
hus educing he egion o heal hy insula ion. The applied ol age will gene a e o he ca bonized
pa hs on he heal hy insula ion su ace be ween he high ol age elec ode and g ound, hus leading o
a lasho e o a b eakdown [
14
], wi h he consequen i e haza d [
15
]. The e o e, a c acking can be
unde s ood by sel -sus ained a cing be ween wo o mo e wi es ac oss a ca bonized pa h p oduced due
o deg ada ion o he insula ion, which can induce igni ion o wi ing [
16
], e en in 12 ol au omo i e
wi ing sys ems [
17
] o explosions [
3
]. Figu e 1shows an ini ial co ona discha ge and he e ec o a c
acking in a conduc o .
Senso s 2020, 20, x FOR PEER REVIEW 2 o 16
Since elec ic and elec onic de ices a e being inc easingly applied in ae onau ics and ae ospace
sec o s, polyme insula ion ma e ials a e inexo ably exposed o a wide ange o en i onmen s.
Howe e , such demanding applica ions equi e ma e ials and de ices wi h e y eliable ope a ion
unde a wide ange o en i onmen al and ope a ing condi ions [10].
PDs a e elec ical discha ges ha do no comple ely b idge he insula ion be ween wo
elec odes [11]. They a e igni ed by h ee aul ypes, i.e., in e nal, ex e nal and co ona. Pe sis en PDs
can p oduce se e e insula ion damage due o he chemical ans o ma ions p oduced by he low o
elec ons in he de ec . Such ans o ma ions can p oduce a conduc i e pa h, which allows he low
o an elec ic cu en and a ise o he empe a u e, hus damaging he insula ion and p omo ing he
o ma ion o an a c o comple e b eakdown due o he weakening o he insula ion [12]. A cing can
be gene a ed be ween elec ical wi es sepa a ed by a solid insula ing medium when he la e one
becomes ca bonized. When he ca boniza ion is gene a ed because o an elec ic cu en , he a cing
p ocess is known as a c acking.
Elec ical acking is o igina ed by su ace PD ac i i y on an o ganic ma e ial, which deg ades
he polyme , hus mo ing om he insula o o conduc o s a es due o powe ul he mal shocks
p oduced by an in ense bomba dmen o elec ons [13], which con ibu e o b eak he polyme ic
chains and, ul ima ely, o gene a e ca bonized conduc ing pa hs. New discha ges ex end exis ing
acking pa hs, hus educing he egion o heal hy insula ion. The applied ol age will gene a e o he
ca bonized pa hs on he heal hy insula ion su ace be ween he high ol age elec ode and g ound,
hus leading o a lasho e o a b eakdown [14], wi h he consequen i e haza d [15]. The e o e, a c
acking can be unde s ood by sel -sus ained a cing be ween wo o mo e wi es ac oss a ca bonized
pa h p oduced due o deg ada ion o he insula ion, which can induce igni ion o wi ing [16], e en
in 12 ol au omo i e wi ing sys ems [17] o explosions [3]. Figu e 1 shows an ini ial co ona discha ge
and he e ec o a c acking in a conduc o .
(a)
(b)
Figu e 1. (a) Ini ial co ona discha ge a 10 kPa unde 600 V–50 Hz. (b) E ec o a c acking in a
conduc o .
The e is a lack o echnical li e a u e ocused on he analysis o a c acking, and mo e speci ically
cen e ed on ae onau ic applica ions. This sho age o da a is due o he di icul y o a c
cha ac e iza ion because o i s e y unp edic able beha io [18].
This pape e iews he s a e o he a ela ed o a c acking, wi h a special emphasis on
insula ion wi ing sys ems o ai c a elec ical and elec onic sys ems, ocusing on he cu en ends
in MEAs and AEAs. Due o he inc eased p opo ion o elec ical powe in nex gene a ion ai c a s,
insula ion ma e ials ha e o wi hs and inc easing le els o elec ical s ess, hus being mo e p one o
elec ical discha ge ac i i y. The pape e iews he cu en unde s anding ela ed o he mechanisms
leading o a c acking, as well as he e ec s and possible emedial ac ions o neu alize o minimize
a c acking occu ence. This e iew pape aims a pe o ming a comp ehensi e and c i ical analysis
o such e ec s unde ae onau ic en i onmen s, while de ec ing he esea ch needs and challenges o
be aced. This wo k is based on in o ma ion and da a ound in in e na ional s anda ds, ecen
scien i ic and echnical publica ions, including jou nals, con e ence pape s, heses, whi e pape s and
echnical epo s.
The es o he a icle is o ganized as ollows. Sec ion 2 is de o ed o p esen ing he ac o s
a ec ing a c acking wi h special emphasis in ae onau ic wi e insula ion ma e ials. A p o use e iew
Figu e 1.
(
a
) Ini ial co ona discha ge a 10 kPa unde 600 V–50 Hz. (
b
) E ec o a c acking in
a conduc o .
The e is a lack o echnical li e a u e ocused on he analysis o a c acking, and mo e speci ically
cen e ed on ae onau ic applica ions. This sho age o da a is due o he di icul y o a c cha ac e iza ion
because o i s e y unp edic able beha io [18].
This pape e iews he s a e o he a ela ed o a c acking, wi h a special emphasis on
insula ion wi ing sys ems o ai c a elec ical and elec onic sys ems, ocusing on he cu en ends
in MEAs and AEAs. Due o he inc eased p opo ion o elec ical powe in nex gene a ion ai c a s,
insula ion ma e ials ha e o wi hs and inc easing le els o elec ical s ess, hus being mo e p one o
elec ical discha ge ac i i y. The pape e iews he cu en unde s anding ela ed o he mechanisms
leading o a c acking, as well as he e ec s and possible emedial ac ions o neu alize o minimize
a c acking occu ence. This e iew pape aims a pe o ming a comp ehensi e and c i ical analysis
o such e ec s unde ae onau ic en i onmen s, while de ec ing he esea ch needs and challenges o be
aced. This wo k is based on in o ma ion and da a ound in in e na ional s anda ds, ecen scien i ic and
echnical publica ions, including jou nals, con e ence pape s, heses, whi e pape s and echnical epo s.
Senso s 2020,20, 1654 3 o 16
The es o he a icle is o ganized as ollows. Sec ion 2is de o ed o p esen ing he ac o s
a ec ing a c acking wi h special emphasis in ae onau ic wi e insula ion ma e ials. A p o use e iew
o he s a e o he a is p o ided, disclosing he condi ions and ci cums ances o a c acking in
ae onau ic wi es. Sec ion 3 ocuses on he wo majo si ua ions o ae onau ic wi e a c acking, namely,
we -wi e and d y-wi e a c acking. Sec ion 4exposes he main ypes o wi es used in he ai c a
indus y, disclosing hei p os and cons as well as highligh ing he ups and downs su e ed o e
he yea s in he sea ch o new and be e ma e ials. Sec ion 5is de o ed o he discussion o he
exis ing s anda ds o assessing he a c acking p e en ion capabili ies o comme cial ae onau ic
cables. Sec ion 6explains he main a c acking sensing, de ec ion and p ocessing mechanisms ound
in he ela ed li e a u e. Thei p inciples o ope a ion a e unco e ed, as well as he adop ed ae onau ic
indus y implemen a ions a e discussed. Sec ion 7lis s he iden i ied esea ch needs in ega d o he
s udied ield, aiming o se e as u u e esea ch guidelines ha will mee he challenges disclosed
h oughou his e iew a icle. Finally, Sec ion 8summa izes he main ideas and concludes he pape .
2. Fac o s A ec ing A c T acking
Polyme ic ma e ials wi h low weigh and s anding elec ical and mechanical p ope ies a e widely
u ilized in elec ic insula ion applica ions [
15
,
19
,
20
]. Howe e , mos o such o ganic ma e ials can
be damaged by acking due o he change in he ca bon bonds in hei inne s uc u e, and hus,
he polyme esis ance owa ds elec ical discha ges is much lowe compa ed o ino ganic insula o
ma e ials such as glass o po celain [
21
]. Resea ch on acking esis ance o insula ion ma e ials is
a key poin o ensu e he long- e m eliabili y [
15
] and sa e y o elec ical insula ion sys ems [
22
].
Recen s udies show ha polyme ic nanocomposi es end o o e conside ably enhanced p ope ies
compa ed o con en ional and mic on-sized pa icle- illed polyme s [
23
,
24
]. I is p o ed ha wi h
he addi ion o nano- and mic o- ille s hyd ophobici y inc eases, whe eas we abili y, su ace ene gy
and in e ac ion pa ame e s end o dec ease [
23
]. In [
24
] i is shown ha he esis ance o acking
and e osion, he esis ance o high- ol age a cing and he esis ance o he mal deg ada ion can be
imp o ed subs an ially wi h low nano- ille con en s. Such imp o emen s, which appea o ille
con en s usually below 10 w .% [
24
], a ise om he s ong in e acial in e ac ions and ul a- ine phase
dimensions, hus imp o ing hyd ophobici y, e osion and acking esis ance [
23
], as well as esis ance
o PDs and eeing pe o mance [
24
]. The in e phase plays a key ole, since i can p esen di e en
p ope ies om hose o he unin luenced polyme [24].
Insula ion ma e ials a e subjec ed o mechanical, elec ical and en i onmen al s esses [
25
],
as well as elec os a ic discha ges, which may gene a e c acks in wi ing insula ion ma e ials [
26
].
In pa icula , en i onmen al s esses such as ul a iole adia ion om sunligh , mois u e, con aminan s
and pollu ion such as ozone o acid ain [
27
] and aged polyme ic insula ion ma e ials [
28
] could
deg ade he ma e ial due o su ace acking [
29
] and e osion [
30
]. Wi e insula ion ma e ials p esen
di e en unan icipa ed ailu e modes, such as a c acking, a cing and inally insula ion lasho e in
o de o inc easing se e i y [31].
An elec ic a c is de ined as a luminous high empe a u e elec ic discha ge h ough a medium
such as cha ed insula ion o ac oss a gap [
32
] unde AC o DC supply [
33
]. Inne a c empe a u es
a e in he ange o se e al housands o Kel in deg ees, al hough hey depend on se e al ac o s,
including ol age d op, cu en , geome y o ma e ials in ol ed [
34
]. Usually, adia i e losses lessen as
p essu e dec eases, and hus a c empe a u e inc eases [
4
]. A c beha io , igni ion and ex inc ion depend
on he condi ions o he elec ical ne wo k, ol age le el and ype (AC, DC, shape o he wa e o m,
pola i y and di e en equencies), as well as on o he ac o s such as en i onmen al condi ions,
speci ically p essu e, wa e condensa ion, gases, conduc o ma e ial, hickness and composi ion o he
insula ion. In [35] i is s a ed ha DC a c cha ac e is ics a e also in luenced by he chemical na u e o
he we ed laye a he su ace o he solid insula o . O he impo an ac o s a e wi e design and aging
pa ame e s. To sus ain he a c, minimum cu en and ol age condi ions a e equi ed. I hose h eshold
alues a e no eached, he a c will no be igni ed [
16
]. In [
36
] an a emp o model he beha io
Senso s 2020,20, 1654 4 o 16
o DC a cs is made unde ae onau ic p essu e condi ions, al hough di e en assump ions mus be
ca ied ou o simpli y he inhe en complexi y o he a c na u e, including ha he a c is supposed a
one-dimensional adial p oblem, he a c has cylind ical shape, he a c column is a he modynamic
equilib ium and he elec ic ield wi hin he a c column is uni o m, con ec ion is omi ed so only powe
losses due o adial he mal conduc ion a e aken in o accoun , and he a c powe in he elec ode
su ace can be exp essed as he p oduc o he elec ical cu en by he equi alen ol age due o he
a c. Acco ding o his simpli ied one-dimensional model, he a c ol age V
a c
e sus he leng h o he
a c column leng h can be exp essed as,
Va c =VA+VC+Zd
0
E(x)dx,
whe e V
A
is he ol age d op in he anode, V
C
is he ol age d op in he ca hode, Eis he mean elec ic
ield s eng h in he a c column and dis he leng h o he a c.
In [
37
] a non-adiaba ic Fini e Elemen Me hod (FEM) model o he a c is p oposed, which can be
applied o model d y and we a c acking e ec s.
In AC ci cui s, he a c ex inguishes a each cu en ze o. Unde acuum condi ions, he a c las s
less, s opping as a e he i s cu en ze o, al hough he e ec s o he a c a e mo e se e e compa ed
o ambien p essu e [
38
]. Unde DC condi ions, a minimum cu en is equi ed o s abilize he a c,
o he wise i ends o sel -ex inguish. When he minimum h eshold cu en is applied, he a c sus ains
p o iding ha he cu en is no swi ched o by he p o ec ions. Since he a ed cu en o ac i a e he
p o ec ions usually depends, p ima ily, on he diame e o he wi e, g ea e damage is expec ed o
la ge diame e s [16].
Expe imen al esul s o DC acking conduc ed unde di e en p essu e le els conclude ha
he ela ionship be ween acking ailu e and p essu e and he ype o damage depends on he ype
o insula ion ma e ial. Some ma e ials unde go acking- ype damage, whe eas o he s unde go
e osion- ype damage. E osion occu s in polyme ma e ials ha do no gene a e ca bonized acks,
he su ace being e oded p og essi ely due o he gaseous discha ge, inally leading o dielec ic
b eakdown [
22
]. I was epo ed ha he AC acking esis ance is highe han he DC one, and ha
he DC acking esis ance dec eases when dec easing he p essu e [22].
T acking ypically a ises due o su ace con amina ion [
39
]. A c acking usually equi es a
polyme ic insula ing su ace con amina ed wi h humidi y, liquids, conduc i e dus s o sal s [
19
,
32
],
placed in a ci cui wi h a su icien ol age applied, con amina ion gene a ing a closed elec ical ci cui
ac oss he polyme su ace, and ime. A c acking co esponds o he p opaga ion o he a c along he
wi es due o abla ion o he me allic co e and deg ada ion o he insula ion [
4
]. The discha ge pa h can
be p oduced due o he exis ence o deb is o conduc i e ma e ials on he su ace o e en due o he
conduc i i y o he insula ion i sel . When he discha ge de elops wi h a su icien leng h, i can e ol e
o lasho e . These sho du a ion sho -ci cui a cs be ween a aul y polyme ic insula ed wi e and
ano he wi e [
40
] can pa ially py olize ( he mal ca boniza ion) he insula ion [
41
], hus gene a ing an
elec ical conduc ance along he su ace o he insula ion. Small leakage cu en s lowing ac oss he
con amina ed a ea deg ade he base ma e ial, p oducing a c discha ge, cha ing o igni ing combus ible
subs ances nea by he a c. This p oblem, ypical o high- ol age sys ems, has also been desc ibed
in 120/240 V AC sys ems [
32
]. The cha ed polyme becomes conduc i e, being able o sus ain a
sho -ci cui a c, which may sp ead along he wi e due o a con inuous py olyza ion o he insula ion
ma e ial, leading o he a c acking e ec [
41
] due o he o ma ion o a conduc i e ca bon pa h along
c acks wi hin he insula ion. In he case o mul iple wi e bundles in a ha ness, he insula ion laye s
o o he wi es wi hin he bundle may also become he mally ca bonized o cha ed, hus s a ing o
a c ack, and ul ima ely leading o a comple e aul o he en i e bundle o e en he ha ness [
42
],
p oducing a massi e cu en leakage and leading o he lash-o e [
43
] and ul ima ely esul ing in
se e e unc ionali y loss [
44
]. A c damage in a wi e bundle de elops in a hyb id a angemen including
insula ion ma e ials, me allic conduc o s, and a c plasma. Depending on elec ical ne wo k pa ame e s
Senso s 2020,20, 1654 5 o 16
including he powe sou ce and he equi alen impedance, such aul s may lead o s abilized a cs as
long as he p o ec ions a e no ac i a ed and a sel -ex inguishing p ocess is ini ia ed. Once he a c has
been ex inguished and he sys em eene gized, he phenomenon can eeme ge, depending on applied
ol age, load cu en , chemical composi ion and hickness o he pa ially damaged insula ion [45].
Because o he elec ic esis ance o he ca bon pa h, he empe a u e along he conduc i e
pa h inc eases due o he localized hea gene a ed by he low o a leakage cu en , hus py olyzing
he insula ion. Despi e he small size o a cing, i can igni e combus ible gases p oduced on he
su ace [
41
]. A c acking occu s because he polyme su ace becomes conduc i e, hea ing up he
polyme ic ma e ial and inally causing igni ion [
46
]. The e o e, a c acking ans o ms an insula ing
ma e ial om non-conduc i e o conduc i e h ough a su ace deg ada ion p ocess [
46
]. In [
47
],
he mal spo s wi h empe a u es abo e 400
◦
C a e epo ed. This hea ul ima ely leads o a chemical
b eakdown o he insula ion a ea su ounding he ca bon pa h, gene a ing mo e ca bon along he pa h,
hus sel -p opaga ing he phenomenon and ending o g ow wi h ime. A c acking du a ion may
luc ua e be ween some en hs o second and ew seconds, gene a ing cu en and ol age pulses in a
wide spec al ange [
48
]. In [
49
] i is p o ed ha du ing he inclined plane es , d y band discha ges
(DBDs) ha e di e en s ages. Fi s , DBDs in he o m o s eame discha ges and pa ial a cs end o
s ay a he same posi ion un il he con aminan is comple ely e apo a ed. Nex , DBDs occu as e
and change hei loca ion due o he educ ion o he sample esis ance because o he gene a ed
ca boniza ion. Finally, he discha ges a e longe , emi mo e ligh and he leakage cu en inc eases
compa ed o he p e ious s ages.
P oneness o acking is hea ily in luenced by he chemical composi ion o he insula ion [
16
]. I has
been ound ha some insula ion ma e ials can be inadequa e o use in high al i ude applica ions [
22
].
Apa om he e ec s o he con aminan s, a c acking can also be igge ed by di e en ac o s,
including adial c acking, imp in labeling, ab asion by ubbing be ween wi es o be ween a wi e and
he ai ame [
32
] o by sho ci cui s. A c acking can also be accele a ed due o unde ec ed damage
in inaccessible a eas o also by using unsui able insula ion ypes in a eas p one o se e e wea he
and mois u e condi ions [
43
]. The minimum cu en equi ed o ini ia e a c acking depends on he
speci ic insula ion, since insula ion ma e ials ha e di e en a c acking p opensi y, ypes and es
condi ions. Ma e ials p one o ca boniza ion, i.e., a oma ic polyme s, he mose plas ics, o ma e ials
including al e na ing double bonds, a e mo e suscep ible o a c acking, al hough he e a e no plas ics
o polyme s comple ely esis an o a c acking [17].
I is known ha su ace ea men s may inc ease he a c acking esis ance o polyme ic ma e ials.
In [
25
] i is p o ed ha by luo ina ing silicon ubbe (SiR) samples by means o an F
2
/N
2
mix u e,
lasho e es s show an imp o emen o he acking esis ance. SiR insula ion sys ems a e addi i a ed
wi h di e en ille ma e ials ha a e used o inc ease he hyd ophobici y. Resis ance o UV adia ion
can also in luence acking and e osion beha io [
50
]. The adi ional solu ion o p e en a c acking
consis s in he use o coppe wi es wi h an insula ion composed o poly e a luo oe hylene (PTFE)
luo oca bon polyme [18].
S udies pe o med on insula ions combining c oss-linked polye hylene (XLPE) and SiR indica e
ha inc eased alues o he in e acial p essu e can delay in e acial discha ges and acking ailu e,
since i con ines he ini ial in e ace discha ges and es ic s he expansion o ca boniza ion pa hs [
51
].
Howe e , when inc easing he in e ace p essu e, he size o he discha ge channel educes, and he
damage inside he samples inc eases [
52
]. Such s udies also indica e ha he minimum ol age o
ini ia e su ace discha ges inc eases wi h he ambien p essu e, which also impac s he dis ibu ion
cha ac e is ics o he ligh emi ed by he discha ges and he acking pa e n a he in e ace [53].
3. We - and D y-Wi e A c T acking
Polyme s equi e a con inuous and la ge he mal ene gy supply o decompose, i.e., o e ol e
ola ile molecules [
54
]. High empe a u e discha ges in polyimide (PI) ma e ials end o p oduce
he mal deg ada ion due o he ca boniza ion o he a oma ic polyimide. Ini ially, ca boniza ion akes

Senso s 2020,20, 1654 6 o 16
place a localized spo s bu as he eac ion con inues, a ca bon pa h can be o med be ween conduc o s,
so he hea p oduced by he high elec ic cu en lowing h ough he ca bon pa h will cause se e e
a cing, e en ually leading o lasho e [
55
]. I was shown ha by means o hin luo opolyme
coa ings, ce ain polyimide insula ion (Kap on
®
) can be imp o ed o endu e we a c acking [
55
].
When ageing, Kap on
®
insula ion becomes mechanically mo e ulne able and b i le, hus gene a ing
c acks in he insula ion and a o ing a c acking appea ance. The e o e, he endency is o discon inue
Kap on
®
wi es in new ai c a s [
56
]. Once he Kap on
®
laye becomes c acked, he wi ing is suscep ible
o h ee po en ial issues, i.e., hyd olysis (insula ion c acking and b eakdown due o exposu e o
mois u e), we a c acking, and d y a c acking [
44
]. Mos alipha ic luo opolyme s used in wi e
insula ion, excep c oss-link-E hylene e a luo oe hylene (XL-ETFE), do no gene a e ca bon deposi s
due o he mal deg ada ion, and some polyimide- luo opolyme insula ion ma e ials can esis we
a c acking [
55
]. Bo h we and d y a c acking, when occu ing in wi es wi h a oma ic polyimide
insula ion (Kap on
®
), can lead o ca bon a c acking. We and d y a c acking can p oduce e y
ha m ul e ec s in insula ion sys ems and lead ul ima ely o a eal haza d o he whole ai c a [
44
].
I is known ha PVC-insula ed wi es a e p one o ail due o sel -induced we - acking, especially
when placed in e y low humidi y a mosphe es [57].
We -wi e a c acking (WWAT) is a speci ic ype o a c acking ha occu s when leakage cu en s
o igina ed on a we insula ing su ace a e able o apo ize i s mois u e, o ming d y spo s, which o e
a high esis ance pa h o he low o he elec ic cu en . The leakage cu en can gene a e su ace
discha ges, esul ing in he deg ada ion o he ma e ial [
58
]. As a esul o he leakage cu en , a ol age
will be induced ac oss he spo s due o he leakage cu en , hus de eloping iny su ace discha ges,
i s ly appea ing as su ace scin illa ions. Scin illa ions a e a cing su icial elec ical discha ges,
being low-ene gy and low- empe a u e e en s. Bu i he p ocess con inues un es ic edly, i may lead
o ull-de eloped a cing in he ai gap be ween wo wi es, ul ima ely gene a ing high empe a u es and
being qui e des uc i e [
17
]. Howe e , scin illa ions can gene a e spo empe a u es o abou 1300 K,
hus he mally deg ading he insula ion o a ce ain ex en , depending on he na u e o he insula ion
ma e ial [
55
]. The e o e, we a c acking can occu when mois u e o ai c a luids induce a sho
ci cui be ween wo adjacen exposed wi es o be ween an exposed wi e and he ai c a s uc u e,
which a e a a di e en elec ic po en ial. Con e sely, d y a c acking occu s mainly due o insula ion
ab asion o damage o bad ins alla ion p ac ices unde d y condi ions, he e o e gene a ing a sho
ci cui be ween wo adjacen wi es [
44
]. D y a cing gene a ed by nea by ca bonized a eas p omo es
he mal deg ada ion, hus p oducing acking and e osion [47,59].
Hyd ophobici y ends o delay leakage cu en de elopmen [
59
]. Howe e , due o he p esence
o mois u e and con amina ion, he ou e laye o he insula ion can change om hyd ophobic o
hyd ophilic [
25
]. Since his we laye is conduc i e, a leakage cu en is gene a ed in his new
conduc i e laye . When he wi e d ies, his conduc i e laye ends o b eak, and d y bands a e o med
wi h a consequen ol age d op ac oss each d y band. I he ol age applied is high enough, he ai
su ounding he su ace will b eak down, hus o ming d y band a cs [60].
In [
18
] i is p o ed ha 50%–65% o he powe associa ed wi h a c acking be ween adjacen
wi es is ans e ed o he wi es, hus mel ing and pa ially apo izing he me allic co e and insula ion,
being adia ed o conduc ed. The emaining ene gy is ans e ed o he a c column, being conduc ed,
con ec ed o adia ed.
I was shown ha low-ene gy co ona discha ges and UV adia ion can b eak C–H and Si–CH3
bonds o SiR insula ion, hus con e ing o hyd ophilic he ini ially hyd ophobic polyme su ace [
25
].
Once con amina ed o oxidized, SiR canno easily eco e hyd ophobici y. This e ec inc eases he
leakage cu en on he con amina ed su ace unde we condi ions, acili a ing d y band a cing,
which is a highly ene ge ic phenomenon and can se iously damage he su ace by acking and e osion,
hus educing he mechanical s eng h and insula ing p ope ies signi ican ly [47].
Bo h we and d y a c acking can lead o e y ha m ul e ec s in insula ion sys ems and ul ima ely
o a eal haza d o he whole ai c a .
Senso s 2020,20, 1654 7 o 16
4. Wi e Types o Ai c a Applica ions
Ai c a wi ing is a key elemen in ansmi ing elec ical powe and signals along he ai c a .
Wi h he de elopmen o new MEA sys ems, ai c a wi es a e becoming inc easingly demanding
in e ms o ligh -weigh , long-li e equi emen s and elec ical and physical cha ac e is ics [
61
].
Ai c a wi ing is o en subjec ed o di e se ac o s such as empe a u e and p essu e changes,
ib a ion, mois u e, o cha ing. Whe eas many ai c a s pa s a e pe iodically inspec ed o enewed,
due o i s complexi y, he wi ing sys em emains usually un ouched, hus a o ing he condi ions o
he occu ence o elec ical a c aul s [62].
Insula ion wi e damage can induce a high-cu en a c be ween wo me allic conduc o s, which can
cause an elec ical b eakdown o he whole sys em [
38
]. The e o e, aul a cs gene a ed in ae onau ic
ci cui s mus be e adica ed o con olled o limi hei e ec s [
4
]. Today, wi ing sys ems a e a ocus o
conside a ion due o di e se ai c a acciden s. Acco ding o he FAA [
31
], wi e insula ion ma e ials
a e selec ed by balancing mechanical, elec ical, he mal and chemical p ope ies, whe eas low
smoke and non- lammabili y a e a plus. Wi e insula ion ma e ials a e chosen based on he ype o
ins alla ion, so hei ea u es a e selec ed depending on he ope a ing en i onmen , including a c
esis ance, dielec ic s eng h, co osion and ab asion esis ance, luid esis ance, mechanical s eng h,
impac s eng h, cu - h ough s eng h smoke emission, hea dis o ion empe a u e, o lame esis ance,
among o he s [
63
–
66
]. Hea -aging beha io mus be conside ed when selec ing elec ic wi es, since hey
a e o en placed in a eas unde ex eme ope a ing condi ions (cu en loading, empe a u e and p essu e
anges) wi h in equen inspec ion o e long pe iods o ime. Thus, hea esis ance is a key ac o in
he selec ion and a ing o ai c a wi e [63].
A he end o he six ies, e a luo oe hylene (TFE) and PTFE (Te lon
®
) we e he mos used
ai c a wi e insula ion ma e ials because o he excellen physical, elec ical, mechanical and chemical
p ope ies, al hough limi ed o ope a ing a 200
◦
C. ETFE (Tez el
®
) appea ed du ing he se en ies
as wi e insula ion ma e ial since i allowed a g ea educ ion o he wi e weigh and diame e while
exhibi ing good physical, elec ical, and co osion p ope ies, al hough i is a so ma e ial, hus making
i s ins alla ion mo e di icul . Howe e , ETFE use was limi ed o ope a ing below 150
◦
C. Du ing he
la e se en ies, he adia ion c osslink me hod was success ully adap ed o ETFE insula ion, so XL-ETFE
became widely applied due o an ope a ing empe a u e o 200
◦
C, and be e mechanical p ope ies
han PTFE [
67
]. Today, XL-ETFE is s ill b oadly applied. PI wi e insula ion ape was de eloped du ing
he ea ly se en ies [
68
]. PI ilm insula ion exhibi s appealing ea u es such as low weigh , high dielec ic
s eng h, non- lammabili y, and high empe a u e capabili y [
69
]. PI is a sel -ex inguishing polyme ,
has a decomposi ion empe a u e beyond 500
◦
C, and can ope a e a 250
◦
C~280
◦
C, while o e ing
excellen physical, elec ical, and mechanical p ope ies and gene a ing low smoke densi y when
bu ning. Howe e , se e al acciden s due o i s endency o hyd olyze, esul ing in low mois u e
and a c esis ance, gene a ed awa eness o i s widesp ead use. Upon a c ini ia ion, deg aded PI
insula ion becomes conduc i e, and he a cs can p opaga e along he wi ing ha ness [
69
]. The e o e,
some ai c a manu ac u es banned i s use. Du ing he nine ies, ai c a wi e manu ac u e s combined
PTFE wi h PI o p oduce PTFE/PI ape co e ed wi h PTFE ape, allowing a e y hin insula ion laye ,
hus sa ing weigh compa ed o o he wi es. PTFE/PI ape w apped wi e has an ope a ing empe a u e
o 260
◦
C [
61
]. Insula ion ma e ials wi h a high PI a io exhibi a sho e a c du a ion wi h espec o
wi es wi h ETFE and PTFE insula ion, he las ones being cha ac e ized by a high a c esis ance due o
he sel -ex inguishing ea u es o he a c and he longe ime o b eakdown. The e o e, wi es wi h
ETFE and PTFE insula ion a e mo e s able agains a aul a c han hose wi h PI insula ion [38].
Cu en ly, he wo mos applied wi es in comme cial ai c a s a e insula ed wi h Kap on
®
(a oma ic
polyimide) o wi h a luo opolyme (TFE, FEP, ETFE, o o he luo opolyme esins) [
55
]. Al hough used
in he pas and s ill ins alled in old ai c a s, insula ion ma e ials such as PVC/nylon, Kap on
®
o
Te lon
®
a e almos no longe ins alled in new ai c a s. New ai c a designs mainly use insula ion
ma e ials made o Te zel
®
, TKT (Te lon
®
/Kap on
®
/Te lon
®
) and PTFE/Polyimide/PTFE [
63
]. Table 1
summa izes he ypical wi ing used in comme cial ai c a s, acco ding o he FAA [54].
Senso s 2020,20, 1654 8 o 16
Table 1. Common ai c a wi es.
Designa ion Insula ion Ma e ial
M5086/1,2 PVC/Nylon
M81381 A oma ic Polyimide (Kap on®)
M22759/34 C oss-Linked ETFE
M22759/80-92 PTFE/Polyimide/PTFE composi e (TKT)
M22759/11 Te lon®(PTFE)
M22759/18 Tez el®(ETFE)
The AS50881 ae ospace s anda d [
70
] guides how o size bo h he conduc o and insula ion in ended
o ae ospace wi ing. Conduc o size depends upon he cu en ca ying capabili y, aking in o accoun
he use o conduc o bundles and he ope a ing al i ude, whe eas insula ion hickness elies on he
a ed ol age, so ha i mus wi hs and PD occu ence [71].
5. Compa a i e T acking Index (CTI)
The CTI is applied o measu ing he a c acking p ope ies o an insula ion ma e ial. The es
me hod is de ailed in he IEC s anda d 60112 [
72
], which de ines he CTI as he maximum measu ed
ol age exp essed in ol s, a which a ma e ial wi hs ands 50 d ops o pollu ed wa e wi hou gene a ing
acking and a pe sis en lame. The ASTM in e na ional s anda ds o ganiza ion has an equi alen
s anda d, he ASTM D368 [73].
The in e na ional s anda ds ASTM D2303 [
74
] and IEC 60587 [
39
] a e widely applied o anking
he esis ance o e osion (loss o ma e ial due o elec ical discha ge o leakage cu en ) and acking
in insula ion ma e ials, by means o a liquid con aminan and an inclined plane [
75
]. The inclined
plane s anda d es me hod [
39
,
74
] is commonly used o measu e e osion and acking esis ance o
insula ion ma e ials du ing he design p ocess bu no o moni o ing pu poses, since i emula es
he he mal deg ada ion by elec ic a cing. Howe e , i is a complex and ime-consuming me hod,
hus e y di icul o pe o m in ield inspec ion [
76
] and i has an inhe en limi a ion since only a
4-mm gap can be applied, due o he pollu ed liquids d opping sys em de ined in he s anda d [71].
The inclined plane es p esen s o he issues. I was epo ed ha he e ec s o DC d y a cing on
polyme ic ma e ials a e se e e han hose due o AC d y a cing, al hough i was a ibu ed mos ly o
ac o s including elec ohyd odynamic e ec s, es elec odes co osion, o deposi s o med by liquid
con aminan s. The e o e, i is equi ed o alida e he sui abili y o he me hods and appa a us speci ied
in he s anda d AC inclined plane es ( he mos widely applied sc eening me hod) o conduc DC es s,
and o conclude whe he he g ea e ha shness o such DC d y a cing e ec s, ela i e o AC d y a cing,
is due o he appa a us and me hods applied o due o undamen al unde lying mechanisms [
75
].
The e o e, he e is a need o de elop an equi alen s anda d es me hod o DC.
6. A c T acking De ec ion Me hods
Today, mode n ai c a s use a c aul ci cui b eake s (AFCBs) as p o ec ion agains a cing e ec s.
AFCBs a e based on elec onic componen s. Due o elec ical a cing e en occu ence, by analyzing
he cu en wa e o m, he ci cui b eake ips wi hin 100 ms, acco ding o he AS6019 s anda d [
77
].
When he aul occu s, he wa e o m is al e ed, and he AFCB mus iden i y he a c aul and disconnec
he ci cui a ec ed by he aul . Howe e , AFCBs ac once he a cing occu s bu no be o e, so hey
can be imp o ed o educe he damage le el and sho en he eac ion ime. Wi h he nex gene a ion
o MEAs, which will depend mo e and mo e on elec ical and elec onic sys ems, he use o cu en
AFCBs is ques ionable [62], so i is equi ed o de elop new app oaches o an ea ly de ec ion.
Ea ly symp oms o a c acking include pa ial discha ges (PDs) and co ona. PDs a e
elec ic discha ges which only pa ially b idge he insula ion ma e ial be ween wo elec odes [
11
].
PDs de e io a e he insula ion due o he bomba dmen o discha ge ions and he ac ion o chemicals
o med du ing he discha ge. In mos MEA sys ems, he discha ges occu in ai [
71
], so hey occu in
Senso s 2020,20, 1654 9 o 16
a gaseous medium known as co ona discha ges. PDs and co ona can be ini ia ed when inadequa e
hickness o he wi e insula ion allows enough elec ic ield s eng h in he gap be ween he wi e and a
g ounded elec ode [
71
], and also due o he p esence o wa e d ople s, which agg a a e he si ua ion.
PDs and co ona usually do no gene a e an app eciable aul cu en , hus hinde ing powe sys em
p o ec ions ope a ion, al hough hey end o con inually damage insula ion sys ems un il p oducing
comple e ailu e [71].
PDs and co ona usually gene a e sound, ligh (mainly UV bu also isible), wide equency ange
adio in e e ence ol age, elec omagne ic wa e ene gy, hea and chemical eac ions. Unde low
p essu e en i onmen s sound p opaga es wi h di icul y, so elec omagne ic and UV- isible adia ion
can be used o de ec co ona [
1
,
5
]. A cing in sho gaps usually is p eceded by co ona discha ges
occu ing a he egion wi h highes elec ic ield s eng h [
11
,
78
,
79
]. Co ona discha ges a e key sou ces
o p ema u e ailu e o insula ion sys ems ope a ing a high al i ude [
5
], occu ing ea lie a low
p essu e han a a mosphe ic condi ions [
13
,
80
], so su ace discha ges a e among he main causes o
ea ly ailu e o elec onic equipmen ope a ing in low p essu e en i onmen s [
13
]. This is because ai
densi y is di ec ly ela ed o he de elopmen o elec ical discha ge de elopmen ; any educ ion o ai
densi y leads o a d op o he h eshold alue o he ai ioniza ion elec ic ield, by which he a c is
ini ia ed [
81
]. In [
5
] i is clea ly demons a ed by means o long-exposu e pho og aphs ha co ona
incep ion ol age (CIV) dec eases wi h dec easing alues o he a mosphe ic p essu e. Ea ly ailu e
could be ini ia ed because o he as oxida ion due o he ozone gene a ed by he co ona discha ge [
82
],
al hough ea ly co ona s ages may no show any isible e ec s. Co ona e ec s on insula ion ma e ials
ypically lea e a whi e dus due o he b eakdown o he ma e ial, bu as he condi ion agg a a es,
ca bon acks can de elop. O he e ec s include pi ing, discolo a ion as well as mic o-c ack s ains
on he insula ion. Unde he wo s case scena io, wi es could be se e ely damaged. I is known ha
co ona incep ion ol age has an impo an pola i y dependency, since he CIV is lowe o nega i e
compa ed o posi i e pola i y co ona, bu b eakdown o posi i e pola i y occu s a lowe ol age
han o nega i e pola i y [
71
,
78
]. CIV is also in luenced by he na u e o ol age applied, i.e., DC, AC,
swi ched DC o pulsed among o he s [5].
Aging e ec s due o PDs and de ec s in insula ion ma e ials ha e mo e impac in AC ci cui s
compa ed o DC ci cui s. The likelihood o a c igni ion/ e-igni ion inc eases wi h he cu en and
ol age le els, being usually highe in AC ci cui s. Due o he change o pola i y in AC ci cui s, he a c
ends o ex inguish wi h a subsequen e-igni ion. The du a ion o he a c ends o inc ease unde
educed p essu e condi ions.
PDs and co ona can be de ec ed o - and on-line. O -line de ec ion me hods equi e he ci cui o
be de-ene gized, being usually ca ied ou in special labo a o ies equipped wi h ol age sou ces ee o
discha ges and PD de ec o s [
83
]. O -line de ec ion me hods canno be applied o moni o ing PDs and
co ona ac i i y in eal ime o in he ield, so hey a e no a ac i e o an ea ly de ec ion o a c acking
ac i i y in eal ai c a en i onmen s. On-line de ec ion me hods a e based on senso s such as high
equency cu en ans o me s, i.e., Rogowski coils, ansien ea h ol age senso s, high equency
senso s including HF/VHF and UHF senso s [
84
], o acous ic [
85
] and ul asonic senso s, which can be
used in ene gized ci cui s, so some o hem can be applied in eal ime. Co ona can also be de ec ed by
means o op ical spec opho ome e s [
86
], UV senso s [
87
] o isible-UV came as [
1
,
78
,
88
–
90
]. Howe e ,
he disad an ages o some o hese me hods a e hei sensi i i y o in e e ence om hemsel es and
he su oundings, he di icul ies in de e mining he exac PD-co ona loca ion [
83
], hei cos and size.
In [
12
] a simple me hod is p oposed o de ec PDs ha is compa ible wi h ae onau ic en i onmen s
since i uses a small low-cos s ipped coaxial cable as an an enna senso , allowing i o each small
places whe e bigge senso s canno i , al hough i is also suscep ible o ex e nal in e e ences. In [
1
]
a low-cos and small-sized senso ha allows loca ion o he co ona spo s and is compa ible wi h
ae onau ical en i onmen s is applied o de ec and loca e he co ona poin s in he ea ly s age. In [
91
]
a echnique o disc imina e be ween no mal and a c aul ope a ion is p oposed, which is based on
analyzing he magne ic ield equency–domain ea u es using a cos -e ec i e magne o esis i e senso .
Senso s 2020,20, 1654 16 o 16
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