Network Junctions; Safety Barriers - Siemens 7MF4333 User Manual

UMSITRPDS3-1
Example Calculation:
Assume a network consists of two field instruments (both C
Let R = 250Ω, C = 40 pF/ft, C
Then L = (65,000,000 / (250 x 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.
November 2010
= (1 + 1) x 5000 = 10,000
f
= 1).
n
Installation
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