Endress+Hauser PROline promag 53 - Page 60

Proline Promag 53 FOUNDATION Fieldbus
24 Endress+Hauser
<
0
Bus cable type 3076 from Belden (shielded bus cable):
• Nominal values:
R’ = 24 Ω/km (loop resistance)
C’ = 82 nF/km (capacitance per unit length between the two wires)
L’ = 623 µH/km (inductance per unit length of the two wires)
C’
LS
= 147 nF/km (capacitance per unit length wire against shielding)
Bus termination:
• Nominal values:
U
i
=30 V
P
i
= 1.2 W
C
i
= negligible
The effective capacitance of the bus line is derived first from the capacitance per unit
length C’, which is effectively the capacitance between the two wires. In the case of the
shielded wires the order of capacitance “wire against shielding” and “shielding against
wire” must be observed. The total capacitance of the bus line is thus calculated as fol-
lows:
C’
eff/cable
= (C’ + 0.5 x C’
LS
) x L
SEG
The total length of the bus line L
SEG
is made up of the length of the main cable and the
length of all spurs. Using the system structure illustrated in Fig. 8 the following data rel-
evant to safety emerges for the bus line:
L
SEG
=L
main
+ Σ L
spurs
= 580 m + 35 m
=615 m
C’
eff./cable
= (C’ + 0.5 x C’
LS
) x L
SEG
= (82 nF/km + 0.5 x 147 nF/km) x 0.615 km
= 95.6 nF
L’
eff./cable
= L’ x L
SEG
=623 µH/km x 0.615 km
=383.1
µH