BEKA BA337E, 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 BA337E or BA338E.
Terminals 2, 6 and RS2 of the BA337E and BA338E
Rate Totalisers are internally connected together. 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 500V insulation to earth.
Any Zener barrier may be used with the BA337E and
BA338E providing it's certification is for use with
apparatus in the required Zone and gas group, and
it's output parameters do not exceed the input
parameters of the Rate Totaliser terminals to which it
is connected. Only one polarity of Zener barrier i.e.
positive or negative, may be used in a Rate Totaliser
system.
Fig 2 illustrates the basic circuit that is used for all
BA337E and BA338E installations protected by Zener
barriers.
For simplicity, connections for the pulse
output and optional alarms and 4/20mA output are
shown separately in section 10 of this manual.
Fig 2 BA337E or BA338E 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 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.
When designing a system it is important to
remember that terminals 2, 6 and RS2 are
interconnected within the BA337E and the BA338E
See Fig 1.
Fig 3 BA337E or BA338E used with Zener barriers
pulse source in safe area.
4.1.1 Power supply
The BA337E and the BA338E Rate Totalisers
require a minimum of 10V between terminal 1 & 2
and consume:
10mA without optional backlight
plus 6mA when terminals 3 & 4 are linked
Any certified Zener barrier may be used to power a
BA337E or BA338E Rate Totaliser providing the
output safety parameters of the barrier are equal to
or less than the input safety parameters of terminals
1 & 2 of the Rate Totaliser.
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 15.5V and the
maximum working voltage of the Zener barrier
which, depending upon manufacturer, will be
approximately 26V.
Note:
The optional factory fitted display backlight
increase
the instrument's current
consumption and therefore increases the
minimum safe area operating voltage, see
section 10.3 for details.
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