5
There are two basic design requirements:
1. In normal operation the voltage and
current applied to terminals 1 and 3 of the
BA304ND must not exceed:
Ui
=
4V dc
Ii
=
30 mA dc
Providing the maximum input current in
normal operation is less than 30mA, the
maximum voltage between the indicator
terminals will automatically be limited by
the internal safety components.
2.
The 4/20mA loop must be able to tolerate
the additional 1.1V required to operate the
indicator.
In practice it is only necessary to ensure that in
normal operation the maximum current flowing in
the loop is less than 30mA. In the example shown
in Fig 2 this current is determined by the maximum
current from the transmitter.
The sum of the maximum voltage drops of all the
components in the loop must be less than the
minimum power supply voltage. Considering the
example shown in Fig 2:
Minimum operating voltage of 2-wire Tx
10.0
Maximum voltage drop caused by controller
5.0
Maximum voltage drop caused by BA304ND
1.1
Maximum voltage drop caused by cables
0.4
_____
16.5V
Therefore at 20mA the power supply in this
example must have an output greater than 16.5V
4.2 Remote indication
A BA304ND indicator may be driven from any
4/20mA signal to provide remote indication. Fig 3
shows a typical application in which the output
from a gas analyser drives a BA304ND. Again it is
necessary to ensure that the loop complies with
the two design requirements listed in section 4.1
Fig 3 Remote indication
5. INSTALLATION
5.1 Location
The BA304ND indicator is housed in a robust IP66
glass reinforced polyester (GRP) enclosure
incorporating an armoured glass window and
stainless steel fittings. This makes it suitable for
exterior mounting in most industrial applications,
including off-shore and waste water treatment.
Please contact BEKA associates if high vibration is
anticipated.
The BA304ND is surface mounting, but may be
pipe or stem mounted using the accessory kits
described in sections 8.5 and 8.6 of this manual.
The field terminals and the two enclosure mounting
holes are located in a separate compartment with
a sealed cover allowing the instrument to be
installed without exposing the display assembly or
controls. Terminals 2 and 4 are internally joined
and may be used for linking the return 4/20mA wire
- see Fig 2. Similarly, terminals 5 and 6 are
internally joined and may be used for linking the
cable screens. The BA304ND earth terminal is
connected to the internal EMC filters. For
maximum radio frequency interference rejection
this terminal should be connected to a local earth,
or to an earthed cable screen. To prevent
circulating currents, cable screens should only be
earthed at one point in the safe area.
The BA304ND enclosure is supplied with a
bonding plate to ensure electrical continuity
between the three conduit / cable entries.
