6
4.
SYSTEM DESIGN FOR HAZARDOUS
AREAS ATEX Certification
4.1 Flow transmitter loops
A BA358C rate totaliser may be connected
in series with almost any intrinsically safe
4/20mA flow loop and calibrated to display
the rate of flow and total flow. There are
two basic design requirements:
1. The intrinsic safety output parameters
of the 4/20mA loop, which are defined
by the Zener barrier or galvanic isolator,
must be less than:
Uo
30V dc
Io
200mA
Po
0.85W
2. The loop must be able to tolerate the
additional 1.1V required to operate the
indicator
,
increases to 1.2V at -20ºC.
Fig 2 illustrates an application in which a
BA358C indicator is connected in series with
a 2-wire flow transmitter protected by a two
channel Zener barrier.
Fig 2 BA358C in a flow loop
4.2 Remote indication
A BA358C may be driven via an intrinsically
safe interface from a 4/20mA safe area
signal to provide a remote indication of rate
and total flow within a hazardous area. The
type of interface is not critical, either a Zener
barrier or a galvanic isolator may be used,
providing that Uo, Io and Po are not
exceeded.
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 must be used.
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. A
10V 50ohm or diode return barrier may be
used for the second channel to form a safe
combination.
Again it is necessary to ensure that the
voltage capability of the 4/20mA signal is
sufficient to drive the rate totaliser plus the
voltage drop introduced by the intrinsically
safe interface.
Figs 3A, 3B and 3C show the alternative
circuits which may be used.
Fig
3A Simplest circuit with one side of
4/20mA current loop connected to
earth. Also shows optional resetting
from safe area.
