5
Fig 2 BA304D in a transmitter loop
4.2 Remote indication
A BA304D 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.
Fig 3A Simplest circuit with one side of 4/20mA
current loop connected to earth
Fig 3B Two Zener barrier channels are required if
the 4/20mA current loop can not be
earthed at the barrier busbar.
Fig 3C Galvanic isolator can be used with any
4/20mA current loop.
