Siemens SITRANS P Bedienungsanleitung PDF herunterladen (Seite 71)

UMSITRPDS3-1 Installation

November 2010

4-19

Example Calculation:

Assume a network consists of two field instruments (both C

= 1).

Let R = 250Ω, C = 40 pF/ft, C

= (1 + 1) x 5000 = 10,000

Then L = (65,000,000 / (250

40)) – ((10,000 + 10,000) / 40) = 6000 feet (1800 meters)

4.3.7 Network Junctions

A network junction is shown in Figure 4-6. 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 a HART

Communicator. Note the following:

• Network with barriers – Locate a junction anywhere along the network in the non-hazardous area.

• Network without barriers – A junction may be located anywhere along the network between the

power supply and transmitter.

• A junction should be a simple electrical series connection containing NO repeaters or other devices

(active or passive) that can degrade HART communications.

4.3.8 Safety Barriers

Installed safety barriers must comply with the following:

• Locate intrinsic safety barriers between the system power supply (e.g., Primary Master, if used)

residing in the non-hazardous area and the transmitter(s) in the hazardous area.

• Combined or separate supply and return barriers may be used.

• For an intrinsically safe application, the DC voltage applied to the safe side of the barrier must be 0.6

Vdc less than the rated barrier working voltage.

• An active supply barrier must be operated within its specified input working voltage.

• Barrier shunt impedance to ground to the HART range of frequencies (500 Hz to 2500 Hz) shall not

be less than 5000Ω.

• Barrier end-to-end resistance, stated by the manufacturer, is used in calculating the maximum

Network cable length and minimum and maximum network voltages.

• The barrier shall be installed and wired in accordance with the manufacturers instructions.

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