BEKA BA368E, Руководство пользователя

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4. SYSTEM DESIGN FOR HAZARDOUS AREAS
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 BA368E Counter.
Terminals 2, 6, 10 and RS2 of the BA368E Counter
are internally connected together. If any of these
terminals are earthed, as shown in Figs 2 & 3, the
other terminals should only be connected to the
same earth, i.e. the barrier busbar, or to circuits that
have 500V insulation to earth.
Any certified Zener barriers may be used with the
BA368E Counter providing their output parameters
do not exceed the input parameters of the terminals
to which they are connected. Only one polarity of
Zener barrier i.e. positive or negative, may be used
in each Counter system.
Fig 2 illustrates the basic circuit that is used for all
BA368E Counter installations protected by Zener
barriers. For simplicity, connections for the pulse
output and the optional alarms and 4/20mA output
are shown separately in sections 7 and 10 of this
manual.
Fig 2 BA368E used with Zener barriers
Alternatively the pulse source may be located in the
safe area. Fig 3 shows how an additional Zener
barrier is used to transfer the signal to the Counter 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 system it is important to
remember that terminals 2, 6, 10 and RS2 are
interconnected within the BA368E See Fig 1.
Fig 3 BA368E used with Zener barriers
pulse source in safe area.
4.1.1 Power supply
The BA368E Counter requires a minimum of 10V
between terminal 1 & 2 and consumes:
10mA without optional backlight
plus 16mA for optional backlight
plus 6mA when terminals 3 & 4 are linked
plus 6mA when terminals 7 & 8 are linked
Any certified Zener barrier may be used to power a
BA368E Counter providing the output safety
parameters of the barrier are equal to or less than
the input safety parameters of terminals 1 & 2 of the
BA368E Counter.
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, with both counter inputs energised,
must be between 17.5V and the maximum working
voltage of the Zener barrier which, depending upon
manufacturer, will be approximately 26V. The
minimum voltage increases to 23V if a display
backlight is fitted.
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