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46
Special installations
5.2 Installation in intrinsically safe areas
Prior to installation the system must be planned and calculated in
detail. The planning and calculations must be documented and must
be approved by the EX protection representative. The results of the
calculations are used to document compliance with the interface pa-
rameters. All devices, cables, wires, connecting elements, sensors, etc.
used in the respective potentially explosive areas must be taken into
account and listed.
Data sheets, appendixes to prototype test certificates, etc. can be used
for the calculations.
Data sheets for coaxial cables often specify the characteristic imped-
ance and the capacity per unit of length, but not the inductance. In
this case, the cable inductance can be calculated using the following
formula:
Example for a system component with 4 MUXes (C
with a 100 meter coaxial cable RG 58 for connecting the MUXes.
Technical data for the coaxial cable:
Z = 50±2 Ohm;
C' = 101 pF/m;
Calculation of the linear line inductance L':
L' = Z² * C' = (52 Ohm)² * 101 pF/m = 273 nH/m
4 multiplexers à 18 nF = 72 nF;
inductance is negligible
4 coaxial cables à 100 m:
Capacitance: 101 pF/m * 100 m * 4 = 40.4 nF
Inductance: 273 nH/m * 100 m * 4 = 109.2 µH
Result:
Effective capacitance in the cable section: 40.4 nF + 72 nF = 112.4 nF
Effective inductance in the cable section: 109.2 µH
The values are within the permissible range for the limiting device VIB
3.550: 300 nF / 1000 µH.
The sensors, sensor cables, etc. connected to the MUXes must be cal-
culated and considered separately!
L = Z² * C
i
= 18 nF), each
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