5
Fig 2 illustrates a typical application in which a
BA304C indicator is connected in series with a 2-
wire transmitter protected by a Zener barrier.
Fig 2 BA304C in a transmitter loop
4.2 Remote indication
A BA304C indicator may be driven from a 4/20mA
safe area signal via an intrinsically safe interface to
provide a remote indication within a hazardous
area. The type of interface is not critical, either a
Zener barrier or a galvanic isolator may be used,
providing the output parameters of the interface
are:
Uo
less than
30Vdc
Io
less than
200mA
Po
less than
0.85W
Note: when the hazard is a IIC gas and two single
channel Zener barriers or a two channel Zener
barrier are used, only one barrier or channel may
be a 28V 300ohm device.
Again it is necessary to ensure that the voltage
capability of the 4/20mA signal is sufficient to drive
the indicator plus the voltage drop introduced by
the intrinsically safe interface. Figs 3A, 3B and 3C
show the alternative circuits which may be used.
If one side of the 4/20mA current loop may be
earthed, a single channel Zener barrier provides
the lowest cost protection. If the 4/20mA signal is
not isolated, then two Zener barriers, a two
channel Zener barrier or a galvanic isolator should
be used. Again it is necessary to ensure that the
voltage capability of the 4/20mA signal is sufficient
to drive the indicator plus the voltage drop
intro-duced by the intrinsically safe interface. Fig 3
shows the alternative circuits which may be used.
Fig 3
Alternative circuits for remote indication in
a hazardous area