Siemens Pacemaker UM344-2 Uživatelský manuál stáhnout pdf (Strana 49)

UM344-2 INSTALLATION

September 1995

resistance change of RTD per 1°C

Total length (L) =

resistivity of #24 AWG per foot

0.39 ohms

L = = 14.8 feet or 7.4 feet for each lead.

0.0262 ohms per foot

A 0.3° offset error is caused by an extension lead wire (#24) length of 2.23 feet.

As the calculations indicate, extension lead wire added to a 2-wire RTD can cause serious offset error. A

2-wire RTD should not be used without determining that the results are acceptable.

The effects of lead wire resistance are compensated for by the Transmitter when 3 and 4-wire RTDs are

used.

4.3.11 Shielding and Grounding

The preferred method of grounding the loop cable shield is illustrated in Figures 4-1, 4-2, and 4-3.

The following guidelines represent proven grounding practices that will reduce magnetically coupled

interference:

Ÿ Ground the cable shield at ONE point, as shown.

Ÿ Preferably, ground the cable shield at the Network power supply.

Ÿ When the cable shield is grounded at the power supply:

a) The cable shield should remain open (not connected) at the Field Instrument (Transmitter).

b) The shields of both cables at a Network Junction should be spliced. Alternatively, connect both

to a terminal in the Box or Panel, provided that the terminal is isolated from ground.

Ÿ Point-To-Point Network

Other permissible single point grounding schemes are:

a) The cable shield may be grounded at the Ground Connection in the signal terminal compartment

of the transmitter's electronic housing. It is recommended that a separate ground wire be run

from this Ground Connection to building ground to ensure a dependable ground.

The power supply (+) and (-) connections must be floated and the shields at Network Junctions

must be connected as discussed above.

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Siemens Pacemaker UM344-2 Uživatelský manuál Strana 49