BEKA BA377E, Руководство пользователя

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8
3. SYSTEM DESIGN FOR HAZARDOUS
AREAS
3.1 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 BA377E.
Terminals 2, 6 and RS2 of the BA377E are internally
connected together as shown in Fig 9. If any of
these terminals are earthed, as shown in Figs 1 & 2,
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 and
Zone in which the BA377E is installed may be used,
providing the output parameters do not exceed the
input parameters of the BA377E terminals to which it
is connected. Only one polarity of Zener barrier i.e.
positive or negative may be used in each system.
Fig 1 illustrates the basic circuit that is used for all
BA377E Timer installations protected by Zener
barriers. BA377E Clock installations are the same,
except that the input terminals 5 & 6 are not used.
For simplicity the optional control outputs are
described separately in section 3.1.9 of this manual.
Fig 1 BA377E Timer used with Zener barriers
Alternatively the sensor may be located in the safe
area. Fig 2 shows how an additional Zener barrier is
used to transfer signals to the Timer 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. When designing a
Timer system it is important to remember that
terminals 2, 6 and RS2 of the BA377E are connected
together within the instrument. Similarly, terminals 2
and RS2 are internally connected together when the
BA377E is configured as Clock. See Fig 18.
Fig 2 BA377E Timer used with Zener barriers with
input sensor in the safe area.
3.1.1 Power supply
The BA377E requires a minimum of 10V between
terminal 1 & 2 and consumes:
10mA without optional backlight
plus 22mA with optional backlight
plus 6mA when terminals 3 & 4 are linked
Any Zener barrier certified for the gas group and
Zone in which the BA377E 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

Zener
barrier, which has an end-to-end resistance of about
340

, 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.
13.5V min without optional backlight
20.9V min with optional backlight
plus 2.1V when terminals 3 & 4 are linked