Siemens Sitrans Manual De Usuario

Page 24

SITRANS LC  500 – INSTRUCTION MANUAL

7ML19985GE01

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you need to know the cable length, the barrier type (if applicable), and the 
measurement resistance

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select a cable that will give you a capacitance time constant of less than 65 µSec

1.

Calculate the capacitance for a time constant of 65 µSec, using the following 
formula: 

 (time constant = Resistance * Capacitance) 

 is the sum of the load resistor and cable resistance.

 is the sum of the cable capacitance and the capacitances of the connected 

device/devices.

2.

Determine the cable length allowed, by subtracting the capacitance value of the 

device (or devices) on the loop from the total capacitance, and using the maximum 
allowable limit of 100 pF per meter (or 1 nF per 10 meters).

Example 
1.

Calculate the cable capacitance which will give a time constant of 65 µSec:
A twisted pair cable with a conductor cross-section of 1 mm

2

 (AWG 18 equivalent) has 

a copper resistance of 73.6 Ohm/km and a capacitance of 100 pF/m (or 1 nF/10m). 

For a standard 28 V 280 Ohm barrier and a 250 Ohm measuring resistance, with a 
100 meter cable:

Resistance = 280 (barrier) + 250 (sensing device) + 7.36 (cable)= 537.36

 s = 

 nF

nF

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To maintain reliable transfer of the HART modem signals, the RC

1

 time constant of 

the connections should be less than 65 µSec. 

•

Cable capacitance must also be considered when selecting cable for intrinsically 
safe installations.

•

For output signals (from the SITRANS LC 500), only the cable and barrier 
resistance are relevant. For input signals the measurement resistance is also 
relevant.

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Use twisted pair cable, screened as a pair.

2

1.

RC = Resistance * Capacitance

2.

Or, if you use a common screen over a cable containing multiple twisted pairs, 
do not use other pairs for signals that could interfere with HART signals.

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×

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65 10 6

–

×

537.36 C

×

65 10 6

–

×

537.36

⁄

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121

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