Solartron Mobrey 7828, Техническое руководство

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Electrical Installation 7828 Density Transmitter Technical Manual
3-2
78285000_AB
3.2 INSTALLATION CONSIDERATIONS
3.2.1 POWER SUPPLY
The power supply to 7828 must have the following specification:
Voltage: Nominally 24V dc, but in the range 20 to 28V dc
Current: >50mA.
If several 7828 transmitters are to be used within a local area, one power supply can be
used to power them all; where the transmitters are distributed over a wide area and
cabling costs are high, it may be more cost effective to use several smaller, local power
supplies.
The 4-20mA output is NOT self-powered, and requires a 15-28V dc power supply (see
section 3.4.6). The main transmitter power supply can be used, if necessary.
If an RS232 to RS485 converter is to be used (for example to connect to a serial port on a
PC), this may also require a power supply. The transducer power supply can be used to
power the converter, or it may be more convenient to use a separate power supply.
Care should be taken where there is the possibility of significant common-mode voltages
between different parts of the system. For example, if the 7828 is local powered from a
power supply which is at a different potential to the RS485 ground connection (if used).
3.2.2 EMC
To meet the EC Directive for EMC (Electromagnetic Compatibility) it is recommended that
the transmitter be connected using a suitable instrumentation cable containing an overall
screen. This should be earthed at both ends of the cable. At the transmitter, the screen
can be earthed to the transmitter body (and therefore to the pipework), using a conductive
cable gland.
3.2.3 GROUND CONNECTIONS
It is not necessary to earth the transmitter through a separate connection; this is usually
achieved directly through the metalwork of the installation.
The electronics and communications connections (RS485/Modbus and Analog 4-20mA)
of the 7828 are not connected to the body of the transmitter. This means that the negative
terminal of the power supply can be at a different potential to the earthed bodywork.
In the majority of applications, it is not necessary to utilise the RS485 ground connection.
In areas where there is a significant amount of electrical noise, higher communications
integrity may be obtained by connecting the negative power terminal (pin 2)of the 7828 to
the communications ground. If this is done, it is important to ensure that the possibility of
ground loops, caused by differences in earth potential, are eliminated.
3.2.4 CABLING
Although it is possible to connect separate cables to the 7828 for power, RS485 and the
Analog output, it is recommended that all connections are made through one
instrumentation-grade cable. Connections for the Analog and Modbus signals should be
individually screened twisted-pairs with an overall screen, foil or braid for the cable.