BEKA BA384E, Manual

Page: 11 / 68

11
4. SYSTEM DESIGN FOR HAZARDOUS
AREAS.
For simplicity the pulse output is described
separately in section 8 and the optional alarms and
4/20mA output in section 11 of this manual.
4.1 Use with Zener barriers
Zener barriers are the least expensive intrinsically
safe interface between a safe and hazardous area.
However, they require a high integrity earth
connection that may be expensive to install and they
do not provide isolation. When a high integrity earth
connection is not already available, it may be less
expensive and complicated to use galvanic isolators
for the installation of a single BA384G.
Terminals 2, 6, 10 and RS2 of the BA384G Rate
Totaliser are internally connected together as shown
in Fig 1. If any of these terminals are earthed, as
shown in Figs 2 & 3, the other common terminals
should only be connected to the same earth, i.e. the
barrier busbar, or to circuits that have at least 500V
insulation to earth.
Any Zener barrier certified for the gas group in which
the BA384G is installed may be used providing the
output parameters do not exceed the input
parameters of the BA384G terminals to which it is
connected. Only one polarity of Zener barrier i.e.
positive or negative may be used in each system.
Fig 2 illustrates the basic circuit that is used for all
BA384G Rate Totaliser installations protected by
Zener barriers. For simplicity the pulse output and
the optional alarm and 4/20mA outputs are
described separately in sections 8 and 11 of this
manual.
Fig 2 BA384G used with Zener barriers
The flowmeters may also be located in the safe
area. Fig 3 shows how additional Zener barriers are
used to transfer signals to the Rate Totaliser in the
hazardous area. When more than one Zener
barrier is used in a system all must have the same
polarity. i.e. all positive or all negative barriers.
Fig 3 BA384G used with Zener barriers
flowmeters in the safe area.
4.1.1 Power supply
The BA384G Rate Totaliser requires a minimum of
10V between terminal 1 & 2 and consumes:
10mA without optional backlight
plus 6mA when terminals 3 & 4 are linked
plus 6mA when terminals 7 & 8 are linked
plus 16mA with optional backlight
Any Zener barrier certified for the gas group in which
the BA384G is installed may be used to power the
instrument providing the output safety parameters of
the barrier are equal to or less than the input safety
parameters of terminals 1 & 2.
Although this allows a wide variety of barriers to be
used, a positive polarity 28V; 93mA; 300
Ω Zener
barrier, which has an end-to-end resistance of about
340
Ω , is an industry standard device which is
frequently used. With this barrier the supply voltage
in the safe area must be between the minimum
value shown below and the maximum working
voltage of the Zener barrier which, depending upon
manufacturer, will be approximately 26V.