BEKA BA334D, Manual

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7
4. SYSTEM DESIGN FOR HAZARDOUS
AREA
4.1 System certificates
In addition to the ATEX EC-Type Examination
Certificate, ITS have issued two system
certificates. One defines how the BA334D may
be used with Zener barriers and the other with
galvanic isolators. Both are divided into two
sections to cover input signal sources in the
hazardous and in the safe area.
Interface Certificate No
Zener barriers Ex01E2005
Galvanic isolators Ex01E2006
This section interprets these system
certificates and illustrates how to design
systems with Zener barriers and galvanic
isolators.
System certificates are Certificates of
Conformity which are primarily issued for
guidance in the UK. They do not form part of
certification to the European ATEX Directive.
4.2 Use with Zener barriers
Zener barriers are the least expensive
intrinsically safe interface between a safe and
hazardous area. However they do not provide
isolation and require a high integrity earth
connection that may be expensive to install.
For a single BA334D it may be less expensive
and complicated to use galvanic isolators when
a high integrity earth connection is not already
available.
Any certified Zener barriers may be used with
the BA334D 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 with an instrument.
To assist with system design, the Zener barrier
system certificate specifies the maximum cable
parameters for popular barriers and lists some
of the diode return barriers that may be used.
Fig 2 illustrates the basic circuit that is used for
all BA334D installations protected by Zener
barriers. For simplicity, connections for the
optional pulse output, 4/20mA output and
backlight are included in the appendix to this
manual.
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.
Fig 2 BA334D used with Zener barriers
When designing a system it is important to
remember that terminals 2, 4, 6 and 8 are
interconnected within the BA334D - see Fig 1.
4.2.1 Power supply
A BA334D is powered from the safe area via a
positive polarity 28V 300
Ω
Zener barrier which
will have an end-to-end resistance of about
340
Ω
. When used with a proximity detector
input the BA334D consumes approximately
25mA and requires a minimum voltage of 10V
between terminals 1 and 2. The supply
voltage in the safe area must therefore be
between 18.5V and the maximum working
voltage of the Zener barrier – usually about
26V.
4.2.2 Pulse input
As shown in Fig 2 the BA334D will count
pulses from a wide variety of sources in the
hazardous area, or from the safe area as
shown in Fig 3. Terminals 5 and 6 are for
pulse sources that need powering e.g. a switch
contact, an open collector or a 2-wire NAMUR
proximity detector.
Terminals 3 and 4 are for voltage pulse inputs.
Note: Only one input may be used at a time.
i.e pulses can not be counted at terminals 3 &
4 and at 5 & 6 at the same time.
No Zener barrier is required in series with the
input if the intrinsically safe pulse source is
located within the hazardous area.