Cables for Installations and Industry - Draka

42
DATA TRANS MIS SION CA BLES
All auto ma tion ca bles, such as in stru men ta tion, sig nal and
data net work ca bles, are data trans mis sion ca bles.
They are cop per ca bles with a maxi mum op er at ing
volt age of 75 V or op ti cal fi bre ca bles.
They trans mit in for ma tion in an elec tric or op ti cal form.
The trans mit ted in for ma tion may con sist of speech, sound,
mov ing or still pic tures, text, data, meas ur ing or con trol
data or it may be the sig nal of a fire de tec tor or
a bur glar alarm.
The trans mit ted sig nal may be ana log or digi tal.
The ca pac ity of an ana log trans mis sion is meas ured by
a band width which is the fre quency range re quired to
trans mit a cer tain sig nal. In digi tal trans mis sion a cer tain
number of sig nals are trans mit ted in a time unit and this is
called trans mis sion speed. A cer tain trans mis sion speed
cor re sponds to a cer tain band width. That is why the term
band width can also be used in digi tal trans mis sion.
The band width re quired by a digi tal sig nal is al ways
wider than that of a cor re spond ing ana log sig nal.
Ca ble de sign
Data trans mis sion ca bles are di vided into three groups:
sym met ri cal ca bles, co ax ial ca bles and op ti cal fi bre
ca bles.
a)
b)
c)
conductor
insulation
inner conductor
insulation
outer conductor
core
shell
(a) The ba sic struc ture of a sym met ri cal pair, (b) a co ax ial
pair and (c) an op ti cal fi bre.
A sym met ri cal ca ble con sist of one or sev eral sym met ri cal
pairs. A pair con sists of two or four iden ti cal in su lated
con duc tors which are mutually twisted into a pair.
The con duc tors of a pair are alike in their elec tric
dis po si tion. Sym met ri cal ca bles are either screened or
un screened.
A co ax ial pair is the ba sic ele ment of the co ax ial ca ble.
It con sists of two con duc tors, an in ner con duc tor and
an outer con duc tor. These are dif fer ent in their struc ture
and elec tric dis po si tion. The cy lin dri cal outer con duc tor
is con cen tri cally around the in ner con duc tor.
In an op ti cal fi bre ca ble, the sig nal pro ceeds in the form of
light along the op ti cal fi bre. The fi bre, con sist ing of a core
and a shell, is made from quartz glass or plas tic.
The re frac tive in dex of the core is higher than that of
the shell, and this causes a re flec tion when the light hits
the shell.
GE NE RAL
De pend ing on the dif fer ent in stal la tion con di tions,
the ca bles are also di vided into in door ca bles and out door
ca bles. Sel fex tin guish ing and bend abil ity are typi cal
prop er ties of in door ca bles. They are also lighter in
con struc tion and smaller in size than the cor re sponding
out door ca bles. The serv ice tem pera ture range of out door
ca bles is wide. They are re sis tant to e.g. hu mid ity, sun light
and han dling in out door con di tions. De pend ing on
the in stal lat ion site, out door ca bles can also be di vided
into duct, bur ied, sub ma rine and aer ial ca bles.
ELEC TRI CAL PROP ER TIES
The trans mis sion ca pac ity of the ca ble is es sen tially
ef fected by re sis tance, ca paci tance, im ped ance,
at tenua tion and cros stalk.
Re sis tance
Re sis tance is caused by the con duc tor to
the elec tric cur rent and it is de pend ent on the cross
sec tion and the ma te rial of the con duc tor,
the tem pera ture, the sig nal fre quency and the metals
close to the con duc tor.
For twisted- pair ca bles it is given as loop re sis tance which
in cludes the ef fect of both the sup ply con duc tor and
the re turn con duc tor. The re sis tance is given per unit of
the length, i.e Ω /km.
Ca paci tance
Ca paci tance is in er tia to volt age changes be tween
the con duc tors of a pair. It is de pend ent on the di ame ter of
the con duc tors, the dis tance be tween them,
the in su la tion, the ma te rial around the in su la tion and
the elec tric po ten tial of the in su la tion.
The ca paci tance in creases if the dis tance be tween
the con duc tors is re duced or if the di ame ter of
the con duc tors in creases. It is di rectly pro por tional to
the di elec tric con stant which is spe cific to every in su la tion
ma te rial. The ca paci tance is given per unit of the length,
i.e. nF/km.
Im ped ance
Im ped ance gives the ra tio of the volt age be tween
the con duc tors to the cur rent car ried therein. It is also
known as surge im ped ance or char ac ter is tic im ped ance.
With trans mis sion ca bles all these terms have the same
mean ing. The unit of im ped ance is ohm (Ω).
The im ped ance of ho mo ge ne ous pair is con stant across
the whole length of the pair. The volt age and the cur rent
are at tenu ated but their ra tio is con stant, i.e. equal to
the im ped ance. The im ped ance is there fore one of
the char ac ter is tics of a pair. At low fre quen cies,
the im ped ance is sig nifi cant only with long ca bles.
For ex am ple at a fre quency be low 29 kHz it be gins to have
sig nifi cance if the ca ble is longer than 1,000 m.
At a fre quency of 1,000 kHz or higher, the im ped ance is
very im por tant for match ing. This means that
the im ped ance of the unit or com po nent con nected to
the pair must be equiva lent to that of the pair.