Electrical impedance is a measure of opposition to alternating current in a circuit. It is a complex ratio of voltage to current and has both magnitude and phase, unlike resistance which only has magnitude. The phase impedance of a single core cable is calculated using the formula that considers the AC resistance, reactance, frequency, and inductance. The characteristic impedance of twisted pair cables can also be calculated using formulas that include the mutual capacitance, conductor diameter, insulation diameter, insulation dielectric constant, stranding factor, and velocity of propagation. Tables are provided with typical values for dielectric constants, capacitance, velocity of propagation, and stranding factors.

1. = 1
0
IMPEDANCE
Siechem
Wires & Cables
Electrical impeda nce is the measu re of the opposition of an electric current to the
energy flow when voltage is applied. In quantitative terms, it is a complex ratio of the
voltage to the current in an alterna ting current (AC) circuit. Impedance extends the concept
of resista nce to AC circuits, and possesses both magnitude and phase, unlike resista nce
which has only magnitude
The phase impedance Z of a single core cables is given by
Z=.JR.2
+x2
(Q/km)
Where,
R= A.C. resistance at operating temperature (Q/km)
X= reacta nce (Q/km) (X = 2nfL x lQ-3)
f = frequency (Hz)
L= inductance (mH/km)
Balanced pair
For unshielded twisted pair
= 1016.ft
20
c
For shielded twisted pair
z 276 [l.2D]
0 .J& g fd
For overall shielded &cabled
z =347lo<>(I.SD]
0 .J& b fd
Where,
C = Mutual capacitance (pF/ft.)
d = Diameter of the conductor (in.)
D =Diameter over the insulation (in.)
E: =Insulation dielectric constant (seeTable 2)
f =Stranding factor (see Table 1)
Zo=Characteristic impedance

2. Table 1 Table 2
Siechem
Wires & Cables
Material Dielectric
Constant
t: fnF/ft)
Velocity of
Propagation
Yr (%)
ECTFE (Halar) 2.60 63
PFA (Teflon) 2.15 68
PVC 5.00 45
PVC (Semi-rie:id) 3.60 53
PVDF (Kynar,
SOLEF)
7.70 36
Polvethvlene 2.29 66
Polvoroovlene 2.25 67
Polvurethane 6.50 39
Rubber - butvl 4.0 50
Rubber - natural 5.0 45
Rubber - SBR 4.0 50
Rubber -silicone 3.1 57
TFE Teflon 2.1 69
TPE 5.0 45
Teflon 2.1
0 69
Tefzel 2.6 62
No.of
Strands
Stranding Factor
f
1 1
.000
7 0.939
1
9 0.970
37 0.980
61 0.985
91 0.988