BEKA BA368C, Manual

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2.2 SYSTEM CERTIFICATES
In addition to the ATEX EC-Type Examination
Certificate, ITS have issued two system
certificates. One defines how the BA368C may be
used with Zener barriers and the other with
galvanic isolators.
Interface Certificate No
Zener barriers Ex01E2007
Galvanic isolators Ex01E2008
This section interprets these system certificates
and illustrates how to design counter 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.
2.3 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 BA368C counter
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
BA368C 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 each instrument.
To assist with system design, the Zener barrier
system certificate specifies the maximum output
parameters of the Zener barriers that may be
connected to the BA368C and defines the
maximum cable parameters. Diode return barriers
are identified by manufacturers type number,
although alternatives may be used.
The basic system used for all BA368C hazardous
area counter installations is shown in Fig 2.1
Intrinsic safety requirements are explained in
section one of this manual which should be read
before the system is designed.
Fig 2.1 Counter installation using barriers
If the total display is reset from the instrument front
panel, or from an external switch in the hazardous
area, only the power supply Zener barrier is
required.
2.3.1 Power supply
A BA368C 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 operating as a counter with two proximity
detector inputs, the BA368C consumes
approximately 20mA and requires a minimum
voltage of 10V between terminals 1 and 2. The
supply voltage in the safe area must therefore be
between 16.8V and the maximum working voltage
of the Zener barrier – usually about 26V.