Siemens Sitrans IS applications maximum cable length, Interconnection, Example, 121 - 65= 56 nF

2.Calculate the length of cable allowed, by subtracting the capacitance value that the device presents on the loop from the total capacitance. SITRANS LC 500 has no measurable capacitance value, but assume 5 nF. Then use the maximum capacitance limit (1 m /10 nF) to determine the cable length.

121 – 5= 116 nF

116 ⋅ 10= 1160 m

IS applications: maximum cable length

In an IS application, the IS side of the barrier allows for only 70 nF.

Example:

Subtract the capacitance for the device:

70 – 5= 65 nF

65 ⋅ 10= 650 m

This allows for a maximum 650 meters on the IS side.

On the other side of the barrier:

121 – 65= 56 nF

which allows for 560 meters on that side.

Note: The resistance of this length, 650 + 560 meters, could reach 145 Ohm (at 120 Ohm / km), which is too much. In this case, use a thicker cable with lower resistance.

IIB type/class hazardous area applications: maximum cable length

In IIB type/class hazardous area applications the maximum allowed capacitance value is 330 nF, as long as you are not using HART. If you are using HART, the maximum cable length will be limited. Depending on cable specifications, the maximum length lies between 1 and 3 km.

Multi-drop applications: maximum cable length

In a multi-drop application, the total capacitance of all the devices must be calculated. With five devices, at 5 * 5 nF, the allowable cable length will be considerably limited.

Notes:

•If the device is part of a multi-drop setup, the SITRANS LC 500 sets the current to 4 mA, which inhibits analog signalling, including fault signalling.

•Multi-drop is a HART mode where devices are set to a fixed current, and the device is interrogated periodically. The maximum number of devices on one loop is 15, one of which can be an analog mode device.

Interconnection

7ML19985GE01 SITRANS LC 500 – INSTRUCTION MANUAL Page 25