9.4 4/20mA output
The BA314NG Tachometer can be supplied with a
factory fitted galvanically isolated 4/20mA output
which may be configured to represent any part of the
Tachometer speed display.
9.4.1 Ex nA certification
The optional 4/20mA output is a separate galvanically
isolated Ex nA circuit with the following input
parameters:
Ui
=
30V dc
This allows the 4/20mA output to be connected to any
dc circuit providing that in normal operation the
maximum supply voltage is not greater than 30V dc.
Providing that the BA314NG Tachometer is correctly
installed in Zone 2 the 4/20mA current output may be
used to transmit to suitably protected equipment
located in Zone 1 or 2 of a hazardous area, or to
equipment located in a safe area.
Fig 17 Application of 4/20mA output
(Shown without recommended screened cables)
Fig 17 shows a typical application in which a Zone 2
mounted BA314NG Tachometer is transmitting a
4/20mA current to the safe area.
To comply with the requirements of EN 60079-14
Electrical installations design, selection and erection
,
each of the wires entering the hazardous area
should be individually fused and contain a means of
isolation. These two requirements may be satisfied
by installing DIN rail mounted terminals incorporating
a fuse which can easily be opened as shown in Fig 2
to provide isolation. Clear identification of, and easy
access to the means of isolation is essential for their
effective use. It is also necessary to ensure that the
maintenance procedure makes sure that
unauthorised re-closure of the switches does not
occur. It is not considered necessary to have a
means of isolation or electrical protection for cable
screens.
For some applications Ex nA instrumentation
energised by a current limited power supply or
instrument that can be switched off, is often
considered adequate and to comply with the
requirements of the standard.
9.4.2 System design
The Tachometer 4/20mA output is a passive current
sink i.e. not powered, but it is totally isolated from all
other Tachometer circuits. It is effectively a 2-wire
4/20mA transmitter requiring a minimum supply of
5V with its current being controlled by the
Tachometer speed display. Subject to complying
with Ex nA interconnection requirements, the
terminals C1 and C3 may be connected to another
instrument, which will accept a 4/20mA transmitter
input. Terminals C2 and C4 are internally linked
and may be used for joining a return 4/20mA wire.
37
Summary of Contents for BA314NG
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