Siemens UM344-2 user manual Field Instrument Capacitance, Network Junctions

UM344-2 INSTALLATION

C:Cable capacitance per unit length between one conductor and the other conductor connected to the shield. C may be in pF/ft or pF/meter.

Cf: Total input terminal capacitance of Field Instruments; the Primary Master is excluded. Cf is given by the following formula:

Cf = (sum of all Cn values) x (5000)

Where Cn is an integer (e.g., 1, 2, 3) corresponding to the input terminal capacitance of a Field Instrument. Cn values are read from the following table. For Field Instruments without Cn values, use Cn = 1

FIELD INSTRUMENT CAPACITANCE	Cn VALUE
Less than 5000 pF	1
5000 pF to less than 10000 pF	2
10000 pF to less than 15000 pF	3
15000 pF to less than 20000 pF	4
20000 pF to less than 25000 pF	5
25000 pF to less than 30000 pF	6 (Model 344)

Example Calculation:

Assume a Network consists of a 344 and a Field Instrument (Cn = 1 and Cn = 6).

Let R = 250Ω, C = 40 pF/ft., Cf = (1 + 6) x 5000 = 35,000

Then L =	65,000,000	-	35,000 + 10,000	= 5375 feet (1612.5 meters)
	(250)(40)		40

4.3.6 Network Junctions

A Network Junction is shown in Figure 4-1. It is a wiring junction installed at a convenient point in the loop to facilitate wiring, testing, and troubleshooting. Typically the Junction is a conventional terminal block mounted on a panel with a protective cover, cabinet, or junction box to enclose and protect wiring terminals.

Multiple Junctions can be installed to provide field access terminals for the connection of an MXC.

Note the following:

ŸNetwork with Barriers - Locate a Junction anywhere along the Network in the non-hazardous area between a barrier and the Current Sense Resistor.

ŸNetwork without Barriers - A Junction may be located anywhere along the Network between the Current Sense Resistor and Transmitter.

September 1995 4-11